10-Q

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 10-Q

(Mark one)

x Quarterly report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934.

For the quarterly period ended March 31, 2006.

or

¨ Transition report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934.

For the transition period from              to             .

Commission File Number: 000-31633

Kosan Biosciences Incorporated

(Exact name of registrant as specified in its charter)

Delaware	94-3217016
(State or other jurisdiction of incorporation or organization)	(I.R.S. Employer Identification No.)

3832 Bay Center Place, Hayward, California 94545

(address of principal executive offices)

(510) 732-8400

(Registrant’s telephone number, including area code)

Indicate by check mark whether the registrant: (1) has filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that the registrant was required to file such reports), and (2) has been subject to filing requirements for the past 90 days.    Yes  x    No  ¨

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, or a non-accelerated filer. See definition of “accelerated filer and large accelerated filer” in Rule 12b-2 of the Exchange Act). (Check one):

Large accelerated filer  ¨                Accelerated filer  x                Non-accelerated filer  ¨

Indicate by check mark whether the registrant is a shell company (as defined in Rule 12b-2 of the Exchange Act).    Yes  ¨    No  x

Indicate the number of shares outstanding of each of the issuer’s classes of common stock, as of the latest practicable date: Common Stock, $.001 par value; 34,554,753 shares outstanding at April 30, 2006.

KOSAN BIOSCIENCES INCORPORATED

Form 10-Q

Quarter Ended March 31, 2006

INDEX

		Page
PART I.	FINANCIAL INFORMATION
Item 1:	Condensed Financial Statements and Notes:
	Condensed Balance Sheets as of March 31, 2006 (unaudited) and December 31, 2005	3
	Condensed Statements of Operations (unaudited) for the three months ended March 31, 2006 and 2005	4
	Condensed Statements of Cash Flows (unaudited) for the three months ended March 31, 2006 and 2005	5
	Notes to Condensed Financial Statements	6
Item 2:	Management’s Discussion and Analysis of Financial Condition and Results of Operations	17
Item 3:	Quantitative and Qualitative Disclosures About Market Risk	26
Item 4:	Controls and Procedures	26
PART II.	OTHER INFORMATION
Item 1A:	Risk Factors	27
Item 5:	Other Information	44
Item 6:	Exhibits	44
SIGNATURES		45
CERTIFICATIONS

2

PART I. FINANCIAL INFORMATION

Item 1: Condensed Financial Statements and Notes

KOSAN BIOSCIENCES INCORPORATED

CONDENSED BALANCE SHEETS

(in thousands)

	March 31, 2006			December 31, 2005 (1)
	(unaudited)
Assets
Current assets:
Cash and cash equivalents	$	14,524		$	18,750
Short-term investments		29,983			35,427
Accounts receivable		2,013			3,319
Prepaid and other current assets		1,290			1,192
Total current assets		47,810			58,688
Restricted cash		949			949
Property and equipment, net		5,617			6,061
Other assets and notes receivable from related parties		331			299
Total assets	$	54,707		$	65,997
Liabilities and Stockholders’ Equity
Current liabilities:
Accounts payable	$	1,984		$	2,639
Accrued liabilities		6,572			6,460
Current portion of deferred revenue		3,277			3,277
Current portion of equipment loans		1,733			1,854
Total current liabilities		13,566			14,230
Deferred revenue, less current portion		8,056			8,876
Equipment loans, less current portion		1,625			1,785
Stockholders’ equity:
Common stock		29			29
Additional paid-in capital		172,931			172,083
Accumulated other comprehensive income		(82	)		(169	)
Accumulated deficit		(141,418	)		(130,837	)
Total stockholders’ equity		31,460			41,106
Total liabilities and stockholders’ equity	$	54,707			$ 65,997

(1) The balance sheet data at December 31, 2005 has been derived from the audited financial statements at that date.

See accompanying notes.

3

KOSAN BIOSCIENCES INCORPORATED

CONDENSED STATEMENTS OF OPERATIONS

(unaudited)

(in thousands, except per share data)

	Three Months Ended March 31,
	2006			2005
Revenues:
Contract revenue	$	2,732		$	2,616
Grant revenue		229			259
Total revenues		2,961			2,875
Operating expenses:
Research and development (including charges for stock-based compensation of $342 and $77, respectively)		11,350			9,655
General and administrative (including charges for stock-based compensation of $193 and $39, respectively)		2,559			1,541
Total operating expenses		13,909			11,196
Loss from operations		(10,948	)		(8,321	)
Other income, net		367			306
Net loss	$	(10,581	)	$	(8,015	)
Basic and diluted net loss per common share	$	(0.36	)	$	(0.28	)
Shares used in computing basic and diluted net loss per common share		29,403			29,099

See accompanying notes.

4

KOSAN BIOSCIENCES INCORPORATED

CONDENSED STATEMENTS OF CASH FLOWS

(unaudited)

(in thousands)

	Three Months Ended March 31,
	2006			2005
Operating activities
Net loss	$	(10,581	)	$	(8,015	)
Adjustment to reconcile net loss to net cash used in operating activities:
Depreciation and amortization		603			637
Amortization of investment premiums and discounts		11			250
Amortization of stock-based compensation		535			115
Changes in assets and liabilities:
Accounts receivable		1,306			1,951
Prepaid and other current assets		(98	)		2
Other assets and notes receivable from related parties		(32	)		31
Accounts payable and accrued liabilities		(543	)		(1,273	)
Deferred revenue		(820	)		(819	)
Net cash used in operating activities		(9,619	)		(7,121	)
Investing activities
Acquisition of property and equipment		(159	)		(263	)
Purchase of investments		(6,130	)		(7,000	)
Proceeds from maturity of investments		11,650			18,614
Net cash provided by investing activities		5,361			11,351
Financing activities
Proceeds from issuance of common stock		313			316
Proceeds from equipment loans		270			944
Principal payments under equipment loans		(551	)		(533	)
Net cash provided by financing activities		32			727
Net increase (decrease) in cash and cash equivalents		(4,226	)		4,957
Cash and cash equivalents at beginning of period		18,750			13,777
Cash and cash equivalents at end of period	$	14,524		$	18,734

See accompanying notes.

5

KOSAN BIOSCIENCES INCORPORATED

NOTES TO CONDENSED FINANCIAL STATEMENTS

(unaudited)

1. Organization and Summary of Significant Accounting Policies

Overview

Kosan Biosciences Incorporated (the “Company” or “Kosan”) was incorporated under the laws of the State of California on January 6, 1995 and commenced operations in 1996. In July 2000, the Company was reincorporated under the laws of the State of Delaware.

Kosan is a biotechnology company advancing two new classes of anticancer agents through clinical development. The Company’s heat shock protein 90, or Hsp90, inhibitor, KOS-953, is in Phase I and II clinical trials, primarily for multiple myeloma and HER2 positive breast cancer. KOS-953 is the Company’s proprietary formulation of 17-AAG, a geldanamycin analog. In addition, intravenous and oral formulations of a second-generation Hsp90 inhibitor, KOS-1022, are currently in Phase I clinical trials. These compounds have a novel mechanism of action targeting multiple pathways involved in cancer cell growth and survival. The Company is collaborating with the National Cancer Institute (“NCI”), a component of the National Institutes of Health, in the development of 17-AAG and other analogs of geldanamycin for the treatment of cancer.

Kosan is also developing KOS-862 in Phase II clinical trials in breast cancer. KOS-862 is an epothilone with a mechanism of action similar to taxanes, one of the most successful classes of anti-tumor agents. Kosan’s follow-on epothilone, KOS-1584, is in Phase I clinical trials. The epothilone program is partnered with Hoffmann-La Roche, Inc. and F. Hoffmann-La Roche Ltd. (collectively “Roche”) through a global development and commercialization agreement.

Kosan also has a motilin receptor agonist program for the stimulation of gastrointestinal movement, or GI motility. The Company has selected KOS-2187 as a clinical product candidate in this program.

Kosan also has additional research programs for cancer that are undergoing preclinical evaluation.

Kosan has funded its operations primarily through sales of common stock and convertible preferred stock, contract payments under its collaboration agreements, equipment financing arrangements and government grants.

Basis of Presentation

The accompanying unaudited condensed financial statements have been prepared in accordance with accounting principles generally accepted in the United States for interim financial information. The information as of March 31, 2006, and for the three months ended March 31, 2006 and 2005, reflects all adjustments (including normal recurring adjustments) that the management of the Company believes are necessary for a fair presentation of the results for the periods presented. Results for any interim period are not necessarily indicative of results for any future interim period or for the entire year. The accompanying unaudited condensed financial statements should be read in conjunction with the financial statements and notes thereto included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2005 filed with the Securities and Exchange Commission on March 16, 2006.

6

KOSAN BIOSCIENCES INCORPORATED

NOTES TO CONDENSED FINANCIAL STATEMENTS (continued)

(unaudited)

1. Organization and Summary of Significant Accounting Policies (continued)

Use of Estimates

The preparation of financial statements in conformity with U.S. generally accepted accounting principles requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and the disclosure of contingent assets and liabilities as of the date of the financial statements and the accompanying notes. Estimates and assumptions are reviewed periodically, and the effects of revisions are reflected in the financial statements in the period they are determined. Actual results could differ from those estimates.

Cash Equivalents and Investments

The Company considers all highly liquid investments with a maturity from date of purchase of three months or less to be cash equivalents. The Company limits its concentration of risk by diversifying its investments among a variety of issuers. All investment securities are classified as available-for-sale and are recorded at fair value based on quoted market prices, with unrealized gains and losses excluded from earnings and reported in other comprehensive income/(loss). Purchase premiums and discounts are recognized in interest income using the interest method over the terms of the securities. Realized gains and losses and declines in the fair value that are deemed to be other-than-temporary are reflected in earnings. The cost of securities sold is based on the specific identification method.

The Company recognizes an impairment charge when the decline in the estimated fair value of a marketable security below the amortized cost is determined to be other-than-temporary. The Company considers various factors in determining whether to recognize an impairment charge, including the duration of time and the severity to which the fair value has been less than its amortized cost, any adverse changes in the investees’ financial condition and associated downgrades to credit ratings and the Company’s intent and ability to hold the marketable security for a period of time sufficient to allow for any anticipated recovery in market value. For the three months ended March 31, 2006 and 2005, the Company did not recognize an impairment charge related to its investment securities.

Restricted Cash

The Company held a restricted investment consisting of a certificate of deposit of approximately $949,000 at March 31, 2006 and December 31, 2005. This investment is carried at fair value and is restricted as to withdrawal under a letter of credit agreement related to a facility lease.

Property and Equipment

Property and equipment are stated at cost, net of accumulated depreciation and amortization. Property and equipment are depreciated on a straight-line basis over the estimated useful lives of three to five years. Leasehold improvements are amortized over the shorter of their estimated useful lives or the lease term.

7

KOSAN BIOSCIENCES INCORPORATED

NOTES TO CONDENSED FINANCIAL STATEMENTS (continued)

(unaudited)

1. Organization and Summary of Significant Accounting Policies (continued)

Revenue Recognition

The Company generates revenue under collaborative agreements with pharmaceutical companies and under research grants from the National Institutes of Health and the National Institute of Standards and Technology. The arrangements may include upfront non-refundable fees, reimbursement for personnel and supply costs, milestone payments for the achievement of defined collaboration objectives and royalties on potential sales of commercialized products. The Company recognizes revenue under these arrangements when (i) persuasive evidence of an arrangement exists; (ii) delivery of the services, supplies or technology license has occurred; (iii) the price is fixed and determinable; and (iv) collectability is reasonably assured.

The Company recognizes license and other upfront and initial fees pursuant to research and development collaboration agreements over the estimated research and development term of the respective agreement. These estimated terms are reviewed on a periodic basis and updated if the underlying assumptions are modified. Payments related to substantive performance milestones that are at risk at the initiation of an agreement are recognized upon successful completion of a performance milestone event.

Contract revenues related to collaborative research and development efforts are recognized as revenue as the related services are performed or delivered in accordance with contract terms. Such payments generally are made based on the number of full-time equivalent researchers assigned to the collaboration project and the related research and development expenses incurred or as other deliverables under the contracts are fulfilled. Revenues related to government grants are recognized at the time a grant is awarded and as related research expenses are incurred. Any amounts received in advance of performance are recorded as deferred revenue until earned.

Research and Development

Research and development consists of costs incurred for Company-sponsored and collaborative research and development activities. These costs consist primarily of salaries and other personnel-related expenses, including associated stock-based compensation, facility-related expenses, licensing-related expenses, depreciation of facilities and equipment, lab consumables, services performed by clinical research organizations and research institutions, external manufacturing and other outside service providers. Expenses related to clinical trials generally are accrued based on the level of patient enrollment and activity according to the protocol. The Company monitors patient enrollment level and related activity to the extent possible and adjusts estimates accordingly.

Research and development expenses under government grant awards and collaborative agreements approximated the revenue recognized, excluding milestone payments, upfront and initial fees received under such arrangements.

8

KOSAN BIOSCIENCES INCORPORATED

NOTES TO CONDENSED FINANCIAL STATEMENTS (continued)

(unaudited)

1. Organization and Summary of Significant Accounting Policies (continued)

Net Loss per Share

Basic and diluted net loss per common share has been computed using the weighted-average number of shares of common stock outstanding during the period, less shares subject to repurchase. Diluted net loss is not presented separately as the Company is in a net loss position and including potentially dilutive securities in the loss per share computation would be antidilutive.

Stock-Based Compensation

Effective January 1, 2006, the Company adopted the fair value recognition provisions of Statement of Financial Accounting Standards (“SFAS”) No. 123 (revised 2004) or “SFAS 123R”, “Share-Based Payment”, using the modified prospective transition method. SFAS 123R requires that the compensation cost relating to share-based payment transactions, including stock options and employee stock purchase plans, be recognized in the financial statements. Under this transition method, stock-based compensation expense is recognized for all stock-based compensation awards granted prior to, but not yet vested as of January 1, 2006, based on the grant date fair value estimated in accordance with the original provision of SFAS No. 123, “Accounting for Stock-Based Compensation”. Stock-based compensation expense for all stock-based compensation awards granted after December 31, 2005 is based on the grant-date fair value estimated in accordance with the provisions of SFAS 123R. The Company recognizes these compensation costs net of an expected forfeiture rate on a graded-vesting basis over the requisite service period of the award, which is generally the option vesting term of one or four years. Prior to the adoption of SFAS 123R, the Company accounted for common stock options granted to employees using the intrinsic value method and, thus, recognized compensation expense for options granted with exercise prices less than the fair value of the Company’s common stock on the date of the grant. Compensation related to the grants of stock options to non-employees is recorded in accordance with SFAS 123 and EITF 96-18. The measurement of stock-based compensation to non-employees is subject to periodic adjustment as the underlying awards vest. See Note 3 Stock-Based Compensation for further information.

2. Research and Development Agreements

Roche

Effective September 2002, the Company entered into a research and development collaboration agreement (the “Agreement”) with Roche. Under the terms of the Agreement, Roche has worldwide exclusive rights to market and sell KOS-862 and other epothilones (including KOS-1584) in the field of oncology, and the Company will co-develop and has the right to co-promote the product in the United States. The Agreement provides for the Company to receive payments for the reimbursement of research and development expenditures, funding of a back-up program, achievement of clinical, regulatory and commercial milestones, development activities and royalties on sales of collaboration products. In addition, the Company has the opportunity to increase its royalties through a buy-in at a later stage of clinical development and by co-promotion of products resulting from the collaboration. Effective July 1, 2004, the Company entered into an amendment to the Agreement that provided for the reimbursement of

9

KOSAN BIOSCIENCES INCORPORATED

NOTES TO CONDENSED FINANCIAL STATEMENTS (continued)

(unaudited)

2. Research and Development Agreements (continued)

Roche (continued)

costs that exceeded the previously stipulated amounts set forth in the Agreement. On March 27, 2006, the Company entered into a letter agreement with Roche replacing a particular at-risk milestone payment obligation in the Agreement with an obligation by Roche to reimburse the Company for certain patent expenses up to a specified amount at the time set forth in the letter agreement. The letter agreement had no effect on revenue recognized by the Company in the current quarter or historically. For the three months ended March 31, 2006 and 2005, the Company recognized revenue related to the Agreement of approximately $2.7 million and $2.6 million, respectively.

License Agreements

The Company has collaborative and license agreements with several academic, government and medical institutions. Included in research and development expenses were total payments made under these agreements of approximately $77,000 and $140,000 for the three months ended March 31, 2006 and 2005, respectively.

3. Stock-Based Compensation

At March 31, 2006, the Company had the following stock-based compensation plans:

1996 Stock Option Plan

The Company’s 1996 Stock Option Plan (the “1996 Plan”) provides for the granting of incentive stock options and nonstatutory stock options to employees, officers, directors and consultants of the Company. The maximum number of aggregate shares that may be optioned and sold under the 1996 Plan is 9,475,000 as of March 31, 2006. The 1996 Plan provides for an annual increase of shares on January 1 of each year that may be the lesser of 1,125,000 shares, 5% of the outstanding shares on such date or a lesser amount as determined by the Company’s board. Incentive stock options may be granted with exercise prices not less than fair value, and nonstatutory stock options may be granted with exercise prices not less than 85% of the fair value on the date of grant. The fair value of grants is determined by the closing market price of the Company’s common stock as listed on any established stock exchange on the date of grant. Stock options granted to a stockholder owning more than 10% of voting stock of the Company may be granted with an exercise price of not less than 110% of the fair value on the date of grant. Options expire no later than ten years from the date of the grant. The number of shares, terms and exercise period are determined by the Company’s board of directors. Options generally vest at 25% per year over a four-year period. The 1996 Plan will expire in June 2006, unless terminated in accordance with the provisions of the 1996 Plan.

10

KOSAN BIOSCIENCES INCORPORATED

NOTES TO CONDENSED FINANCIAL STATEMENTS (continued)

(unaudited)

3. Stock-Based Compensation (continued)

2000 Employee Stock Purchase Plan

The Company’s 2000 Employee Stock Purchase Plan (the “Purchase Plan”) permits eligible employees to purchase common stock at a discount through payroll deductions during defined offering 2000 Employee Stock Purchase Plan periods. The maximum number of shares available for issuance pursuant to the Purchase Plan is 750,000, as of March 31, 2006. The Purchase Plan provides for an annual increase of shares on January 1 of each year equal to the lesser of 150,000 shares, 0.75% of the outstanding shares on such date or such amount as determined by the Company’s board. The price at which the stock is purchased is equal to the lower of 85% of the fair market value of the common stock on the first day of the offering or 85% of the fair market value of the Company’s common stock on the purchase date. Each offering under the Purchase Plan is for a six-month period.

2000 Non-Employee Directors’ Plan

The Company’s 2000 Non-Employee Directors’ Stock Option Plan (the “Directors’ Plan”) provides for the granting of nonstatutory stock options to directors of the Company. The maximum number of shares of common stock available for issuance pursuant to the Directors’ Plan is 500,000, as of March 31, 2006. The Directors’ Plan provides for an annual increase of shares on January 1 of each year equal to the lesser of 150,000 shares, 0.75% of the outstanding shares on such date or a lesser amount as determined by the Company’s board. Under the Directors’ Plan, as amended, each non-employee director who becomes a director of the Company will be automatically granted a non-statutory stock option to purchase 20,000 shares of common stock on the date on which such person first becomes a director, and such option will vest over four years. Beginning with the 2002 Annual Stockholders Meeting and each year thereafter, each non-employee director will automatically be granted, one day following each year’s annual meeting of stockholders, a non-statutory option to purchase 5,000 shares of common stock, which will vest one day before the annual meeting of stockholders subsequent to the date of grant. The exercise price of options under the Directors’ Plan will be equal to the fair market value of the common stock on the date of grant. The maximum term of the options granted under the Directors’ Plan is ten years.

Prior to January 1, 2006, the Company provided pro forma disclosure amounts in accordance with SFAS No. 148, “Accounting for Stock-Based Compensation—Transition and Disclosure” (“SFAS 148”), as if the fair value method defined by SFAS 123 had been applied to its stock-based compensation. Effective January 1, 2006, the Company adopted the fair value recognition provisions of SFAS 123R, using the modified prospective transition method and therefore has not restated prior periods’ results. Under this transition method, stock-based compensation expense for the first quarter of 2006 included compensation expense for all stock-based compensation awards granted prior to, but not yet vested as of, January 1, 2006, based on the grant date fair value estimated in accordance with the original provisions of SFAS 123. Stock-based compensation expense for all share-based payment awards granted after December 31, 2005 is based on the grant-date fair value estimated in accordance with the provisions of SFAS 123R. The Company recognizes these compensation costs net of an expected forfeiture rate on a graded-vesting basis over the requisite service period of the award, which is generally the option vesting term of one or four years.

11

KOSAN BIOSCIENCES INCORPORATED

NOTES TO CONDENSED FINANCIAL STATEMENTS (continued)

(unaudited)

3. Stock-Based Compensation (continued)

As a result of adopting SFAS 123R, the Company’s net loss for the three months ended March 31, 2006 was approximately $442,000 greater than if the Company had continued to account for stock-based compensation under Accounting Principles Board Opinion No. 25. The impact on both basic and diluted earnings per share for the three months ended March 31, 2006 was $0.02 per share. Due to the Company’s lack of earnings history, no income tax benefit or cash flow effect is recognized as any resulting deferred tax asset is fully offset by a valuation allowance.

The following table presents the calculation of basic and diluted net loss per share (in thousands, except per share data) had the Company applied the fair value recognition provisions under SFAS 123 in 2005:

	Three Months Ended March 31, 2005
Net loss attributable to common stockholders, as reported	$	(8,015	)
Plus:Stock-based employee compensation expense included in reported net loss		—
Less: Stock-based employee compensation expense determined under fair value based method for all awards		(608	)
Pro forma net loss	$	(8,623	)
Basic and diluted net loss per common share:
As reported	$	(0.28	)
Pro forma	$	(0.30	)

The Company uses the Black-Scholes-Merton closed-form model to value stock-based compensation expense. Separate groups of employees that have similar historical exercise behavior are considered separately for valuation purposes. The table below presents the valuation assumptions used to determine stock-based compensation expense under the provisions of SFAS 123R. Expected term is based on the simplified method allowed under the provisions of Staff Accounting Bulletin (“SAB”) No. 107 and may only be applied through December 31, 2007. The risk-free interest rate is based on the U.S. Treasury zero coupon issues with an equivalent remaining term at the time of the option grant. Expected volatility is based on the Company’s historical volatility and other factors.

12

KOSAN BIOSCIENCES INCORPORATED

NOTES TO CONDENSED FINANCIAL STATEMENTS (continued)

(unaudited)

3. Stock-Based Compensation (continued)

	March 31, 2006		March 31, 2005
Expected term (in years)	6.25		4.00
Risk-free interest rate	4.71		4.00
Expect volatility	67	%	64	%
Expected dividends	—		—

A summary of stock option activity and changes through the period ending March 31, 2006 is as follows:

Options	Number of Shares		Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term	Aggregate Instrinsic Value
	(in thousands)				(in years)	(in thousands)
Outstanding at December 31, 2005	4,902		$	7.57
Granted	155		$	4.65
Forfeited or expired	(509	)	$	6.89
Exercised	(34	)	$	3.53
Outstanding at March 31, 2006	4,514		$	7.58	7.23	$	1,309
Vested and expected to vest at March 31, 2006	4,299		$	7.65	7.13	$	1,202
Exercisable at March 31, 2006	2,818		$	8.05	6.31	$	741

The weighted-average grant-date fair value of options granted during the quarter ending March 31, 2006 and 2005 was $3.02 and $2.42, respectively. The total intrinsic value of options exercised during the quarter ending March 31, 2006 and 2005 was approximately $73,000 and $7,000, respectively.

13

KOSAN BIOSCIENCES INCORPORATED

NOTES TO CONDENSED FINANCIAL STATEMENTS (continued)

(unaudited)

3. Stock-Based Compensation (continued)

A summary of the nonvested shares as of December 31, 2005, and changes during the period ended March 31, 2006, is presented below:

Nonvested shares	Number of Shares		Weighted- Average Grant Date Fair Value
	(in thousands)
Nonvested at December 31, 2005	2,183		$	6.98
Granted	155		$	4.65
Vested	(132	)	$	7.04
Forfeited or expired	(510	)	$	6.89
Nonvested at March 31, 2006	1,696		$	6.79

As of March 31, 2006, there was approximately $3.2 million of total unrecognized compensation cost related to nonvested share-based compensation arrangements granted under the option plans. The cost is expected to be recognized over a weighted-average period of 2.9 years.

4. Comprehensive Loss

For the three months ended March 31, 2006 and 2005, comprehensive loss was as follows (in thousands):

	Three Months Ended March 31,
	2006			2005
Net loss	$	(10,581	)	$	(8,015	)
Unrealized gain (loss) on available-for-sale securities		87			(74	)
Comprehensive loss	$	(10,494	)	$	(8,089	)

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KOSAN BIOSCIENCES INCORPORATED

NOTES TO CONDENSED FINANCIAL STATEMENTS (continued)

(unaudited)

5. Equipment Financing

The Company finances certain equipment and facility improvements under debt obligations with terms of 48 months. In April 2004, the Company entered into a $3.5 million equipment line of credit agreement, of which $2.6 million had been utilized as of March 31, 2006. The interest rates of each of the loans are fixed at the time of the draw down, with the interest rates ranging from 6.31% to 9.07%. Obligations under the loans are secured by the assets financed under the loans.

6. Accrued Liabilities

Accrued liabilities consisted of the following (in thousands):

	March 31, 2006		December 31, 2005
Research and development-related	$	3,855	$	3,115
Compensation-related		693		1,438
Professional services		1,008		899
Facilities-related		768		767
Other		248		241
	$	6,572	$	6,460

7. Line of Credit Facility

In July 2005, the Company entered into a $35.0 million secured line of credit facility with Silicon Valley Bank (“Bank”), a subsidiary of SVB Financial Group. Under the terms of the agreement, the Company has the option of making up to two draws through May 31, 2006, with a minimum initial draw of $15.0 million. In the event the Company draws down less than $35.0 million by May 31, 2006, the Company will be required to pay 0.75% of the unused amount of the credit facility. The credit facility contains financial covenants, including covenants requiring the Company to maintain: a ratio of unrestricted cash and cash equivalents, investments and eligible accounts to the aggregate principal amount outstanding under the line of credit of at least 1.3 to 1; at least six months “remaining months liquidity,” which is calculated by dividing (i) cash and cash equivalents, investments and eligible accounts by (ii) the Company’s cash utilization calculated on a rolling three-month basis; and aggregate balances in its investment and operating accounts with Bank or SVB Asset Management equal to the lesser of (a) the amount of the aggregate outstanding principal amount under the line of credit plus $5.0 million; and (b) 85% of the Company’s aggregate cash and investment account balances. As of March 31, 2006, the Company had not made any draws on the facility.

15

KOSAN BIOSCIENCES INCORPORATED

NOTES TO CONDENSED FINANCIAL STATEMENTS (continued)

(unaudited)

8. Restructuring Charge

In March 2006, the Company implemented and completed a corporate restructuring, reflecting a realignment of research priorities and corporate operations to support its clinical product candidates and pipeline opportunities. As a result, the Company reduced its workforce by 39 positions, from 119 to 80 full-time employees, primarily in research and general and administrative.

The reduction in workforce resulted in a one-time severance-related charge of approximately $0.6 million, all of which was recognized in the three months ended March 31, 2006. Approximately $50,000 of such costs were accrued at March 31, 2006.

9. Subsequent Event

In April 2006, the Company completed a public offering of 5,100,000 shares of its common stock at a public offering price of $5.00 per share. The aggregate proceeds of the offering, net of offering costs, were approximately $23.9 million.

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Item 2: Management’s Discussion and Analysis of Financial Condition and Results of Operations

Forward-Looking Statements

The following discussion of our financial condition and results of operations contains forward-looking statements within the meaning of Section 27A of the Securities Act of 1933, as amended, and within the meaning of Section 21E of the Securities Exchange Act of 1934, as amended, which are subject to the “safe harbor” created by those sections. These forward-looking statements include, but are not limited to, statements about:

• our strategy, including our plans with respect to the timing of announcements of clinical data and initiation of clinical trials;

• the progress and results of our research and development programs, including clinical testing;

• sufficiency of our cash resources;

• revenues from existing and new collaborations;

• product development;

• our research and development and other expenses; and

• our operations and legal risks.

In some cases, forward-looking statements can be identified by words such as “expect,” “anticipate,” “intend,” “believe,” “hope,” “assume,” “estimate,” “plan,” “will” and other similar words and expressions. In addition, any statements that refer to expectations, projections or other characterizations of future events or circumstances are forward-looking statements. Discussions containing these forward-looking statements may be found throughout this Form 10-Q, including the section entitled “Item 2: Management’s Discussion and Analysis of Financial Condition and Results of Operations.” These forward-looking statements involve risks and uncertainties, including the risks discussed below in Part II, Item 1A “Risk Factors” that could cause our actual results to differ materially from those in the forward-looking statements. We undertake no obligation to publicly release any revisions to the forward-looking statements or reflect events or circumstances after the date of this document. The risks discussed below in Part II, Item 1A “Risk Factors” and elsewhere in this report should be considered in evaluating our prospects and future financial performance.

Overview

We are a biotechnology company advancing two new classes of anticancer agents, Hsp90 inhibitors and epothilones, through clinical development. Following is the status of our product candidates.

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Hsp90 Inhibitors

• KOS-953. KOS-953 is our proprietary formulation of 17-AAG, a geldanamycin analog. KOS-953 is in Phase I and II clinical trials, primarily for multiple myeloma and HER2 positive breast cancer. In 2006, we expect to announce interim results and the recommended Phase II dose from our Phase Ib clinical study of KOS-953 in combination with Velcade^® in patients with multiple myeloma. We also expect to announce interim results from our Phase Ib/II clinical trial of KOS-953 in combination with Herceptin^® in patients with HER2 positive breast cancer in mid-2006. If our multiple myeloma Phase Ib results are favorable, we intend to initiate a Phase II/III registration study of KOS-953 in combination with Velcade^® in patients with relapsed refractory multiple myeloma in late 2006 or early 2007.

• KOS-1022. Intravenous and oral formulations of a second-generation Hsp90 inhibitor, KOS-1022, are currently in Phase I clinical trials. Intravenous KOS-1022 is also being studied in a Phase Ib clinical trial in combination with Herceptin^®. In mid-2006, we expect to announce interim results from the initial Phase I clinical trial of intravenous KOS-1022. If the final results from this Phase I clinical trial, and our Phase Ib/II clinical trial of KOS-953 in combination with Herceptin^® in patients with HER2 positive breast cancer, are favorable, we intend to initiate late-stage clinical trials of intravenous KOS-1022 in combination with Herceptin^® in patients with HER2 positive breast cancer in 2007. We anticipate that KOS-953 will be developed for use in multiple myeloma and KOS-1022 will be developed for other indications such as HER2 positive breast cancer.

Epothilones

• KOS-862. KOS-862 is being evaluated in a Phase II clinical trial in patients with metastatic breast cancer. In addition, KOS-862 is being evaluated in the Phase II portion of a Phase Ib/II clinical trial in combination with Herceptin^® in patients with HER2 positive locally advanced or metastatic breast cancer. We expect that interim results from the Phase Ib/II clinical trial of KOS-862 in combination with Herceptin^® will be announced in mid-2006.

• KOS-1584. KOS-1584 is our second-generation epothilone anticancer candidate that is being evaluated in two dose-escalating Phase I clinical trials in patients with solid tumors. We expect to announce interim results from our KOS-1584 Phase I clinical trials in mid-2006.

Our epothilone program is partnered with Roche through a global development and commercialization agreement. Roche is funding all of the current KOS-862 and KOS-1584 clinical trials. We anticipate that Roche will pursue late-stage clinical development of only one of our epothilone product candidates, if any. We anticipate that Roche will make a decision in this regard in 2006.

We also have a motilin receptor agonist program for gastrointestinal motility. We plan to file an Investigational New Drug, or IND, application with the Food and Drug Administration, or FDA, or an application for clinical trial authorization with regulatory authorities in Europe for our clinical candidate, KOS-2187, in 2006.

We also have additional research programs for cancer that are undergoing preclinical evaluation. These programs are also based on the use of our technology to improve the structure of known polyketides and the efficiency of large-scale production.

In March 2006, we implemented a corporate restructuring, reflecting a realignment of research priorities and corporate operations to support our clinical product candidates and pipeline opportunities.

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As a result, we reduced our workforce by 39 positions, from 119 to 80 full-time employees, primarily in research and general and administration. The reduction in workforce resulted in a one-time severance related charge of approximately $0.6 million, all of which was recognized in the three months ended March 31, 2006.

We have incurred significant losses since our inception. As of March 31, 2006, our accumulated deficit was approximately $141.4 million. We expect to incur additional operating losses over the next several years as we continue to advance our clinical product candidates into and through clinical trials.

Critical Accounting Policies

Critical accounting policies are those that require significant judgment and/or estimates by management at the time that the financial statements are prepared such that materially different results might have been reported if other assumptions had been made. We consider certain accounting policies related to revenue recognition, clinical trial accruals and stock-based compensation to be critical policies. We base our estimates on historical experience and on various other assumptions that we believe to be reasonable under the circumstances, the results of which form the basis for our making judgments about the carrying values of assets and liabilities that are not readily apparent from other sources. The basis of our current estimates or assumptions has not changed since we filed our Annual Report on Form 10-K for the year ended December 31, 2005 with the Securities and Exchange Commission on March 16, 2006. Since December 31, 2005, we have not made any significant changes to either our policies or any significant estimates or assumptions.

Stock-based Compensation

Effective January 1, 2006, we adopted the fair value recognition provisions of SFAS 123R, using the modified prospective transition method and therefore we did not restate prior periods’ results. Under this transition method, stock-based compensation expense for the first quarter of 2006 included compensation expense for all stock-based compensation awards granted prior to, but not yet vested as of January 1, 2006, based on the grant date fair value estimated in accordance with the original provisions of SFAS 123. Stock-based compensation expense for all stock-based payment awards granted after December 31, 2005 is based on the grant-date fair value estimated in accordance with the provisions of SFAS 123R. We recognize these compensation costs net of an expected forfeiture rate on a graded-vesting basis over the requisite service period of the award, which is generally the option vesting term of one or four years.

We use the Black-Scholes-Merton closed-form model to value stock-based compensation expense. Separate groups of employees that have similar historical exercise behavior are considered separately for valuation purposes. Expected term is based on the simplified method allowed under the provisions of SAB No. 107 and may only be applied through December 31, 2007. The risk-free interest rate is based on the U.S. Treasury zero coupon issues with an equivalent remaining term at the time of the option grant. Expected volatility is based on the historical volatility of our stock price and other factors.

Results of Operations

Revenues

Revenues for the quarters ended March 31, 2006 and 2005 were approximately $3.0 million and $2.9 million, respectively. Revenues in both periods consisted primarily of contract revenues recognized under our development and commercialization agreement with Roche and funded research related to government grant awards.

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	Three Months Ended March 31,				% Change
(In thousands, except percentage)	2006		2005
Contract revenue	$	2,732	$	2,616	4	%
Grant revenue		229		259	-12	%
Total revenues	$	2,961	$	2,875	3	%

The increase in revenues of approximately 3%, or $0.1 million, for the three months ended March 31, 2006, compared to the same period last year was primarily due to higher funding in KOS-1584 for the progress of Phase I trials in solid tumors and the manufacture of clinical materials, partially offset by the conclusion of certain KOS-862 Phase I trials and the KOS-862 trial in non-small cell lung cancer. We expect to receive a one-time reimbursement for certain patent expenses in 2006. If we do not maintain our agreement with Roche, our revenues will significantly decrease unless we enter into additional collaborations that provide substantial new revenues.

Research and Development Expenses

Our research and development expenses were approximately $11.4 million and $9.7 million for the three months ended March 31, 2006 and 2005, respectively. Our research and development expenses consist primarily of salaries and other personnel-related expenses, including associated stock-based compensation, facility-related expenses, licensing-related expenses, depreciation of facilities and equipment, lab consumables, services performed by clinical research organizations and research institutions, external manufacturing and other outside service providers. We group these activities into two major categories: “research and preclinical” and “clinical development.” We are unable to estimate the nature, timing or costs to complete our research and development projects, or when material net cash inflows to us could be expected to commence, if ever, due to the numerous risks and uncertainties associated with developing pharmaceutical products. These risks and uncertainties include those discussed in this report in Part II, Item 1A “Risk Factors.”

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	Three Months Ended March 31,				% Change		Inception - March 31, 2006
(In thousands, except for percentages)	2006		2005
Clinical development
Epothilones	$	2,427	$	2,203	10	%	$	51,968
Hsp90 inhibitors		4,089		2,404	70	%		19,163
Total clinical development		6,516		4,607	41	%		71,131
Research and preclinical (1)		4,834		5,048	-4	%		143,095
Total research and development	$	11,350	$	9,655	18	%	$	214,226

(1) “Research and preclinical” constitutes internal research and development costs for our early stage programs in the areas of cancer, gastrointestinal motility and infectious disease as well as technology development. Expenses for the three months ended March 31, 2006 and 2005 includes allocated personnel-related expenses of approximately $2.3 million and $2.4 million, allocated facility-related expenses of approximately $1.3 million and $1.6 million and allocated lab consumables of $204,000 and $360,000, respectively. Expenses for the period from inception through March 31, 2006 includes allocated personnel-related expenses of approximately $70.7 million, allocated facility-related expense of approximately $34.6 million and allocated lab consumables of $9.9 million.

The increase of 18%, or approximately $1.7 million, in research and development expenses for the three months ended March 31, 2006 compared to the same period in 2005 was the result of the following:

• approximately $1.9 million in increased clinical costs, primarily due to the advancement of our clinical product candidates in the Hsp90 inhibitor program, including costs associated with clinical trials of KOS-953 and KOS-1022 and the initiation of the Phase I multicenter clinical trial of the oral formulation of KOS-1022, and implementation of SFAS123R, partially offset by

• approximately $0.2 million in lower preclinical costs, primarily related to a decrease in expenses associated with the development of early stage product candidates due to the selection of KOS-2187 as a clinical product candidate and realignment of our research priorities, offset by costs associated with our March 2006 restructuring and implementation of SFAS 123R.

Our research and development employees decreased from 98 employees as of December 31, 2005 to 65 employees as of March 31, 2006, principally as a result of the March 2006 corporate restructuring. The costs associated with the corporate restructuring were recorded in the three-month period ended March 31, 2006. We expect our research and development expenses to decrease over the next several quarters primarily due to reduced employee-related expenses as a result of the corporate restructuring implemented in March 2006.

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General and Administrative Expenses

	Three Months Ended March 31,				% Change
(In thousands, except for percentages)	2006		2005
General and administrative	$	2,559	$	1,541	66	%

For the three months ended March 31, 2006, the increase of 66% or approximately $1.0 million in general and administrative expenses compared to the same period in 2005 was due to severance related payments to our former Chief Executive Officer in the first quarter of 2006, costs associated with our March 2006 corporate restructuring and the implementation of SFAS123R.

Our general and administrative employees decreased from 24 as of December 31, 2005 to 15 as of March 31, 2006, primarily as a result of our March 2006 restructuring. The costs associated with the corporate restructuring and non-recurring severance related payments to our former Chief Executive Officer were recorded in the three-month period ended March 31, 2006. We expect our general and administrative expenses to decrease over the next several quarters as we recognize reduced employee-related expenses as a result of the corporate restructuring implemented in March 2006.

Other Income, Net

	Three Months Ended March 31,						% Change
(In thousands, except for percentages)	2006			2005
Interest income	$	433		$	383		13	%
Interest expense		(66	)		(77	)	-14	%
Other income, net	$	367		$	306		20	%

Interest income increased to approximately $433,000 for the three months ended March 31, 2006, from approximately $383,000 for the same period in 2005. The increase resulted from higher returns in the current rising interest rate environment, partially offset by lower average investment balances in the first three months of 2006 compared to the same period in 2005.

Interest expense decreased to approximately $66,000 for the three months ended March 31, 2006 from approximately $77,000 for the same period last year. The decrease resulted from the lower average debt balances in the first three months of 2006, partially offset by higher average interest rates on more recent debt-financing drawdowns. We expect that our interest expense will increase in the future as a result of additional property and equipment-related debt financing combined with anticipated higher-interest rates and fees associated with our credit facility.

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Liquidity and Capital Resources

Since inception, we have financed our operations primarily through sales of our convertible preferred stock and common stock, contract payments received under our corporate collaboration agreements and government grant awards, interest income and equipment financing arrangements. As of March 31, 2006, we had received approximately $151.0 million from the sale of convertible preferred stock and common stock, approximately $106.1 million from contract payments received under our corporate collaboration agreements and government grant awards, approximately $16.3 million from interest income and approximately $14.0 million from equipment financing arrangements. As of March 31, 2006, we had approximately $45.5 million in cash and investments, compared to approximately $55.1 million as of December 31, 2005. In April 2006, we raised an additional approximately $23.9 million of net proceeds in a public offering of our common stock. Our funds are currently invested in U.S. Treasury and government agency obligations and corporate obligations.

Cash used in operating activities was approximately $9.6 million for the three months ended March 31, 2006, compared to approximately $7.1 million for the same period last year. Our net loss of approximately $10.6 million for the three months ended March 31, 2006 was partially offset by non-cash charges of approximately $1.2 million related to stock-based compensation, depreciation and amortization of investment premiums and discounts. Cash used in operating activities for the same period in 2005 was primarily used to fund the net loss of $8.0 million, partially offset by non-cash charges of approximately $1.0 million related to stock-based compensation, depreciation and amortization of investment premiums and discounts.

Cash used in investing activities, excluding changes in our investments, was approximately $159,000 for the three months ended March 31, 2006, compared to approximately $263,000 for the same period in 2005 largely due to lower capital spending for the three months ended March 31, 2006 as compared to the comparable period in 2005.

Cash provided by financing activities was approximately $32,000 for the three months ended March 31, 2006, compared to approximately $727,000 for the three months ended March 31, 2005. Financing activities for the three months ended March 31, 2006 included approximately $270,000 of equipment debt financing and approximately $313,000 in proceeds from the sale of our common stock related to stock option exercises and stock purchases made under our 2000 Employee Stock Purchase Plan, offset by approximately $551,000 of scheduled payments on new and existing debt. Financing activities for the three months ended March 31, 2005 included approximately $944,000 of equipment debt financing and approximately $316,000 in proceeds from the sale of our common stock related to stock option exercises and stock purchases made under our 2000 Employee Stock Purchase Plan, offset by approximately $533,000 of scheduled payments on new and existing debt.

In April 2004, we entered into a $3.5 million equipment line of credit agreement for facility improvements and capital purchases, which expired in April 2006. As of March 31, 2006, we had utilized approximately $2.6 million of the line of credit. We expect to obtain a new equipment line of credit for facility improvements and capital purchases.

In July 2005, we entered into a loan and security agreement with Silicon Valley Bank for a $35.0 million line of credit. Under the terms of the agreement, we have the option of making up to two draws through May 31, 2006, with a minimum initial draw of $15.0 million. In the event we draw down less than $35.0 million by May 31, 2006, we will be required to pay 0.75% of the unused amount of the credit facility. The credit facility contains financial covenants, including covenants requiring us to maintain: a ratio of unrestricted cash and cash equivalents, investments and eligible accounts to the aggregate principal amount outstanding under the line of credit of at least 1.3 to 1; at least six months “remaining months liquidity,” which is calculated by dividing (i) cash and cash equivalents, investments and eligible

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accounts by (ii) our cash utilization calculated on a rolling three-month basis; and aggregate balances in its investment and operating accounts with Bank or SVB Asset Management equal to the lesser of (a) the amount of the aggregate outstanding principal amount under the line of credit plus $5.0 million; and (b) 85% of the our aggregate cash and investment account balances. As of March 31, 2006, we had not utilized the line of credit.

On April 4, 2006, we completed a public offering of 5,100,000 shares of common stock, at a price of $5.00 per share. The aggregate proceeds of the offering, net of offering costs, were approximately $23.9 million.

We believe that our existing cash and investment securities and anticipated cash flow from our existing collaboration with Roche, including the net proceeds to us from our public offering in April 2006, will be sufficient to support our current operating plan into the second half of 2007, although we may choose to obtain additional financing from time to time. We have based this estimate on assumptions that may prove to be wrong. Our future capital uses and requirements depend on numerous forward-looking factors, including the following:

• our ability to establish any new collaborations, our rights and obligations under any new collaboration agreements and our ability to generate revenues under any new collaborations;

• the extent to which clinical and other development activities are funded by our current collaborators, Roche and the NCI;

• the progress, success and costs of preclinical testing and clinical trials of our drug candidates;

• any acceleration of our clinical development plans;

• our ability to maintain or extend our existing collaborations with Roche and the NCI;

• the progress, number and costs of our research programs;

• the costs and timing of obtaining, enforcing and defending patent and other intellectual property rights;

• any need to obtain licenses to additional patents or other intellectual property in order to use, import, manufacture, market or sell our product candidates;

• any need to expand our manufacturing capabilities; and

• expenses associated with any possible future litigation.

In addition, we review from time to time potential opportunities to expand our technologies or add to our portfolio of drug candidates. In the future, we may need further capital in order to acquire or invest in technologies, products or businesses.

We expect that additional financing will be required to fund operations. We expect to finance future cash needs through the public or private sale of equity securities, debt financings, additional collaboration or licensing arrangements, government grant awards or any combination of the foregoing or other arrangements. In September 2003, we filed a registration statement on Form S-3 to offer to sell common stock in one or more offerings up to a dollar amount of $75.0 million. In December 2003, we completed a registered direct offering of 3,115,000 shares of common stock at a price of $9.00 per share. We received approximately $26.0 million in net proceeds after placement agent fees and other offering costs. In December 2004, we filed a second registration statement on Form S-3 to offer to sell common stock and/or warrants in one or more offerings up to a dollar amount of $50.0 million. After giving effect to our April 2006 public offering, we may, pursuant to the two registration statements, sell approximately $71.5 million of additional common stock in the future.

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We have no current commitments to offer and sell any securities that may be offered or sold pursuant to the registration statements described above. Additional financing or collaboration and licensing arrangements may not be available when needed or, if available, may not be on terms favorable to us or our stockholders. Insufficient funds may require us to delay, scale back or eliminate some or all of our research or development programs, to lose rights under existing licenses or to relinquish greater or all rights to product candidates at an earlier stage of development or on less favorable terms than we would otherwise choose. Insufficient funds may preclude us from meeting the conditions required for the extension of credit and may adversely affect our ability to operate as a going concern. If additional funds are obtained by issuing equity securities, substantial dilution to existing stockholders may result. In addition, see Part II, Item 1A “Risk Factors.”

Contractual Obligations

As of March 31, 2006, our obligations and commitments to make future payments under contracts, such as debt and lease agreements, and under contingent commitments were as follows:

(In thousands)	Total		Less than 1 Year		1-3 Years		4-5 Years		After 5 Years
Equipment financing obligations	$	3,677	$	1,930	$	1,634	$	113	$	—
Operating leases		8,857		1,635		2,697		2,234		2,291
Purchase obligations		799		799		—		—		—
Total contractual cash obligations	$	13,333	$	4,364	$	4,331	$	2,347	$	2,291

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Item 3: Quantitative and Qualitative Disclosures About Market Risk

Interest Rate Risk

The primary objective of our investment activities is to preserve principal while at the same time maximize the income we receive from our investments without significantly increasing risk. Some of the securities that we invest in may have market risk. This means that a change in prevailing interest rates may cause the principal amount of the investments to fluctuate. To minimize this risk in the future, we intend to maintain our portfolio of cash equivalents and investments in a variety of securities, including commercial paper, money market funds, government and non-government debt securities and investment-grade corporate obligations. Through our money managers, we maintain risk management control systems to monitor interest rate risk. The risk management control systems use analytical techniques, including sensitivity analysis. If market interest rates were to increase by 100 basis points, or 1%, as of March 31, 2006 rates, the fair value of our portfolio would decline by approximately $88,000 on that date.

The following table represents the fair value balance of our cash, cash equivalents and short-term and long-term investments that are subject to interest rate risk by year of expected maturity and average interest rates as of March 31, 2006 (dollars in thousands):

	2006			2007
Cash and cash equivalents	$	14,524			—
Average interest rate		3.95	%		—
Short-term investments	$	27,022		$	2,961
Average interest rate		3.43	%		5.12	%
Long-term investments	$	949			—
Average interest rate		0.75	%		—

We did not hold any derivative instruments as of March 31, 2006, and we have never held such instruments in the past. In addition, we had outstanding debt, consisting of borrowings under equipment financings, of $3.4 million as of March 31, 2006, with a range of interest rates from 6.31% to 9.07%.

Item 4: Controls and Procedures

Evaluation of Disclosure Controls and Procedures. Based on their evaluation as of March 31, 2006, our chief executive officer and principal financial officer and accounting officer have concluded that the Company’s disclosure controls and procedures (as defined in Rules 13a-15(e) and 15d-15(e) under the Securities Exchange Act of 1934, as amended (the “Exchange Act”)) are effective to ensure that information required to be disclosed by us in reports that we file or submit under the Exchange Act is recorded, processed, summarized and reported within the time periods specified by the Securities and Exchange Commission’s rules and forms.

Changes in Internal Controls over Financial Reporting. There were no changes in our internal controls over financial reporting (as defined in Rule 13a-15(f) under the Exchange Act) during the quarter ended March 31, 2006 that have materially affected, or are reasonably likely to materially affect, our internal control over financial reporting.

Limitations on the Effectiveness of Controls. The Company’s management, including our chief executive officer and principal financial officer and accounting officer, does not expect that our disclosure controls and procedures or our internal controls will prevent all error and all fraud. A control system, no matter how well conceived and operated, can provide only reasonable, not absolute, assurance that the objectives of the control system are met. Further, the design of a control system must reflect the fact that there are resource constraints, and the benefits of controls must be considered relative to their costs. Because of the inherent limitations in all control systems, no evaluation of controls can provide absolute assurance that all control issues and instances of fraud, if any, within the Company have been detected.

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PART II. OTHER INFORMATION

Item 1A: Risk Factors

You should carefully consider the risks described below, together with all of the other information included in this report, in considering our business and prospects. The risks and uncertainties described below are not the only ones facing us. Additional risks and uncertainties not presently known to us or that we currently deem immaterial also may impair our business operations. The occurrence of any of the following risks could harm our business, financial condition or results of operations.

We have marked with an asterisk (*) those risk factors below that reflect substantive changes from the risk factors included in our Annual Report on Form 10-K filed with the Securities and Exchange Commission on March 16, 2006.

We have a history of net losses and may never become profitable.*

We commenced operations in 1996 and are still in an early stage of development. We have not commercialized any products, and we have incurred significant losses to date. As of March 31, 2006, we had an accumulated deficit of approximately $141.4 million. To date, our revenues have been primarily from collaborations and government grant awards. Our expenses have consisted principally of costs incurred in research and development and from general and administrative costs associated with our operations. We have incurred net losses since our inception, including a net loss of approximately $10.6 million for the three months ended March 31, 2006. We expect our expenses to increase and to continue to incur operating losses for at least the next several years as we continue our research and development efforts for our drug candidates and research programs. The amount of time necessary to successfully commercialize any of our drug candidates is long and uncertain, and successful commercialization may not occur at all. As a result, we may never become profitable.

We expect that additional financing will be required, and an inability to obtain the capital necessary to fund our operations on acceptable terms or at all would threaten the continued operation of our business.*

We expect that additional financing will be required to fund operations. We do not know whether additional financing will be available when needed, or that, if available, we will obtain financing on favorable terms. We have consumed substantial amounts of cash to date and expect to incur significant operating expenditures over the next several years as we continue to advance our clinical product candidates into and through clinical trials.

We may raise additional financing through public or private equity offerings, debt financings, additional collaboration or licensing arrangements, government grant awards or any combination of the foregoing or other arrangements. To the extent we raise additional capital by issuing equity securities, our stockholders may experience dilution.

In July 2005, we entered into a $35.0 million line of credit facility. However, we may not be able to borrow funds under this credit facility if we are not able to meet various conditions, covenants and representations, including covenants requiring us to maintain:

• a ratio of unrestricted cash and cash equivalents, investments and eligible accounts to the aggregate principal amount outstanding under the line of credit of at least 1.3 to 1;

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• at least six months “remaining months liquidity,” which is calculated by dividing (a) cash and cash equivalents, investments and eligible accounts by (b) our cash burn calculated on a rolling three-month basis; and

• aggregate balances in our investment and operating accounts with Silicon Valley Bank or SVB Asset Management equal to the lesser of (a) the amount of the aggregate outstanding principal amount under the line of credit plus $5.0 million; and (b) 85% of our aggregate cash and investment account balances.

We may need to raise additional capital in order to stay in compliance with these covenants. Further debt financing, if available, may subject us to similar or additional restrictive covenants and significant interest costs.

To the extent that we raise additional funds through collaboration and licensing arrangements, it may be necessary to relinquish some rights to our technologies, product candidates or marketing territories.

If we are unable to raise sufficient funds when needed, we may be required to delay, scale back or eliminate some or all of our research or development programs; lose rights under existing licenses; or relinquish greater or all rights to product candidates at an earlier stage of development or on less favorable terms than we would otherwise choose. Insufficient funds may adversely affect our ability to operate as a going concern. See “Part I, Item 2. Management’s Discussion and Analysis of Financial Condition and Result of Operations - Liquidity and Capital Resources.”

We believe that our existing cash and investment securities and anticipated cash flow from our existing collaboration with Roche, including the net proceeds from our public offering in April 2006, will be sufficient to support our current operating plan into at least the second half of 2007, although we may choose to obtain additional financing from time to time. We have based this estimate on assumptions that may prove to be wrong. Our future capital uses and requirements depend on numerous forward-looking factors, including the following:

• our ability to establish any new collaborations, our rights and obligations under any new collaboration agreements and our ability to generate revenues under any new collaborations;

• the extent to which clinical and other development activities are funded by our current collaborators, Roche and the NCI;

• the progress, success and costs of preclinical testing and clinical trials of our drug candidates;

• any acceleration of our clinical development plans;

• our ability to maintain or extend our existing collaborations with Roche and the NCI;

• the progress, number and costs of our research programs;

• the costs and timing of obtaining, enforcing and defending patent and other intellectual property rights;

• any need to obtain licenses to additional patents or other intellectual property in order to use, import, manufacture, market or sell our product candidates;

• any need to expand our manufacturing capabilities; and

• expenses associated with any possible future litigation.

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Our workforce reduction announced in March 2006 may have an adverse impact on our ability to make significant progress on our clinical and research programs or otherwise harm our business.*

In March 2006, we announced a workforce reduction of 39 employees to reflect a realignment of our research priorities and corporate operations to support our clinical product candidates and pipeline opportunities. Workforce and expense reductions have resulted in a reduction in the scope of our research programs, and further reductions could result in the reduced scope of, and progress on, our clinical and research programs. In addition, employees, whether or not directly affected by a reduction, may seek alternative employment. Although our employees are required to sign a confidentiality agreement at the time of hire, the confidential nature of certain proprietary information may not be maintained in the course of any such future employment. We may face litigation from former employees who were affected by the workforce reduction. In addition, the implementation of expense reduction programs has resulted in, and may in the future result in, the diversion of efforts of our executive management team and other key employees, which could adversely affect our business.

If we are unable to recruit and retain skilled employees and consultants, we may not be able to successfully operate our business.*

Retaining our current management and other employees and recruiting qualified scientific personnel to perform future research, manufacturing and development work, as well as key management personnel with expertise in clinical testing and finance, among other areas, will be critical to our success. None of our employees have employment commitments for any fixed period of time and could leave our employment at will. In the past, we have experienced turnover among our management, including the recent resignations of our former Chief Executive Officer and Chief Financial Officer. In addition, in March 2006, we experienced a restructuring that resulted in a workforce reduction. We may have difficulty attracting required personnel as a result of a perceived risk of future workforce and expense reductions, or otherwise. Competition is intense among biotechnology, pharmaceutical and healthcare companies, universities and non-profit research institutions for experienced scientists and other personnel, and we may not be able to retain or recruit sufficient skilled personnel on acceptable terms to allow us to pursue collaborations and develop our product candidates and research programs, which would likely have an adverse effect on our business.

Our 1996 Stock Option Plan will expire on June 12, 2006. Regulations implemented by the NASDAQ National Market require stockholder approval for all stock option plans, and regulations implemented by the New York Stock Exchange, or NYSE, prohibit NYSE member organizations from giving a proxy to vote on equity-compensation plans unless the beneficial owner of the shares has given voting instructions. As a result of these regulations, we will not be able to grant stock options to employees after our 1996 Stock Option Plan expires unless our stockholders approve a new plan. We have asked stockholders to approve the 2006 Equity Incentive Plan at our 2006 annual meeting of stockholders. If the 2006 Equity Incentive Plan is not approved, we may incur increased compensation costs, be required to change our equity compensation strategy or find it difficult to attract, retain and motivate employees, each of which could materially and adversely affect our business.

If our current collaborations are unsuccessful or if conflicts develop with our collaborators, our research and development efforts could be delayed, curtailed or terminated, our revenues could significantly decrease and our operations may be adversely affected.

We have a corporate research and commercialization collaboration with Roche in the field of epothilones. We also have collaborations with, or have licenses to technology and compounds from, several research groups, including Sloan-Kettering in the field of epothilones, the NCI in the field of geldanamycin analogs and Stanford University in the field of polyketide technology. The agreements permit our collaborators or licensors to terminate the agreements under certain circumstances. Our collaboration agreements with the NCI are currently set to expire in October 2006. We may not be able to maintain or extend these collaborations or license agreements on acceptable terms, if at all. If we do not

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maintain, extend or replace our corporate collaboration with Roche, our research and development efforts could be delayed, our revenues would significantly decrease and our operations could be adversely affected. If we are unable to maintain our research collaborations, including our collaboration agreements with the NCI, or if our license agreements are terminated, our research and development efforts could be delayed, curtailed or terminated or we could lose our rights to use the licensed technology and compounds.

We control neither the amount or timing of resources that our collaborators devote to our programs or potential products, nor the scope, content and timing of the efforts that they conduct or permit under the collaborations. As a result, we do not know if our collaborators will dedicate sufficient resources or if the development or commercialization efforts by our corporate partners will be successful. We also do not know if the development or commercialization efforts by our collaborators will be the same as those we would choose to devote if we solely controlled the development and commercialization of our programs and product candidates. In particular, in our collaboration with Roche, we do not control the amount and timing of resources that Roche devotes to the epothilone program beyond limited funding for certain Kosan activities specified under the contract, and we do not control the scope, content and timing of the preclinical studies, clinical trials and other development efforts that Roche conducts or permits under the program. For example, we anticipate that Roche will pursue late-stage clinical development of only one of our epothilone product candidates, if any. We anticipate that Roche will make a decision in this regard in 2006. Roche may elect to cease development of KOS-862, our lead epothilone compound, in favor of further development of KOS-1584, which is currently in Phase I clinical trials. In our collaboration with the NCI, we do not control the selection, conduct, timing and resources provided to clinical trials of geldanamycin analogs sponsored by the NCI. For these reasons, we may choose to undertake product development efforts that are within the scope of our collaborations at our own expense. We also do not know whether our current collaborative partners or future collaborative partners, if any, might pursue alternative technologies or develop alternative products either on their own or in collaboration with others, including our competitors, as a means for developing treatments for the diseases targeted by collaborative arrangements with us. In addition, business combinations or significant changes in a collaborator’s business strategy may adversely affect a collaborator’s willingness or ability to continue the collaboration with us.

Failure by our corporate partners to develop or commercialize a compound or product for which they have rights from us could materially harm our business, financial condition and results of operations. For example, if Roche does not successfully develop and commercialize a product from our epothilone program, we may not receive any future milestone payments and will not receive any royalties under our collaboration with Roche.

If our collaborators fail to conduct the collaborative activities successfully and in a timely manner or if they or our licensors breach or terminate their agreements with us, the development or commercialization of the affected product candidates, technology or research program could be delayed or terminated. If any of our existing collaboration agreements are terminated, we may be required to seek new collaborators or to undertake product development and commercialization at our own expense. This may limit the number of product candidates we will be able to develop and commercialize, significantly increase our capital requirements and reduce the likelihood of successful product introduction. Disputes might also arise with collaborators or licensors concerning rights to particular compounds or technologies. If we are unable to resolve these disputes in our favor, we could lose our rights to use those compounds or technologies.

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If we fail to enter into new collaborative agreements in the future, our business and operations would be negatively impacted.

Our strategy depends upon the formation and sustainability of multiple collaborative arrangements and license agreements with third parties. We expect to rely on these arrangements for not only financial resources, but also for expertise that we expect to need in the future relating to clinical trials, manufacturing, sales and marketing, and for license and technology rights. Although we have established collaborative arrangements and various license agreements, we do not know if we will be able to establish additional arrangements on favorable terms, or whether current or any future collaborative arrangements will ultimately be successful. There have been, and may continue to be, a significant number of business combinations among large pharmaceutical companies that have resulted, and may continue to result, in a reduced number of potential future corporate collaborators, which may limit our ability to find partners who will work with us in developing and commercializing our drug candidates. If we do not enter into new collaborative agreements, we may be required to curtail, suspend or terminate research and development programs and therefore our ability to generate revenues from these programs will be adversely affected. Our ability to start new research and development programs may also be materially harmed.

Our potential products are in an early stage of development, and substantial additional effort and expense will be necessary for development.

Our drug candidates are in early stages of research and development. We may not be able to develop products that prove to be safe and effective, meet applicable regulatory standards, are capable of being manufactured at reasonable costs or can be marketed successfully. All of the potential products that we are currently developing will require significant development and investment, including extensive clinical testing, before we can submit any application for regulatory approval.

Our products must satisfy rigorous standards of safety and efficacy before they can be approved by the FDA and international regulatory authorities for commercial use. We will need to conduct significant additional research and clinical trials before we can determine if our products are sufficiently safe and effective to file with the FDA and other regulatory agencies for product approval. Clinical trials are expensive, and therefore, significant amounts of money will need to be spent testing our products.

In addition, significant time and investment will be required to try to develop manufacturing processes for our products so that they are economical to manufacture on a commercial scale and satisfactory to the FDA and other governmental authorities.

The progress and results of our animal and human testing are highly uncertain.

We must provide the FDA and foreign regulatory authorities with clinical data that demonstrate the safety and efficacy of our products before they can be approved for commercial sale. As a result, commercialization of our product candidates depends upon successful completion of preclinical and clinical trials. Preclinical testing and clinical development are long, expensive and uncertain processes. It may take us a number of years to complete our testing, and failure can occur at any stage of testing. For example, in October 2005, we discontinued a Phase II clinical study of KOS-862 in hormone-refractory prostate cancer because the study did not meet the primary objective of tumor marker response in the first stage of a two stage clinical trial, and we previously discontinued clinical studies of KOS-862 in patients with non-small cell lung cancer and colorectal cancer. We could experience similar failures in other current or future clinical testing of our product candidates.

Success in preclinical testing and early clinical trials does not ensure that later clinical trials will be successful, and interim results of trials do not necessarily predict final results. A number of companies in the pharmaceutical and biotechnology industry, including Kosan, have suffered significant setbacks in advanced clinical trials, even after promising results in earlier trials. Also, preclinical and clinical data can be interpreted in different ways, which could delay, limit or prevent further testing or regulatory approval.

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We do not know whether clinical trials of our product candidates (including ongoing and anticipated clinical trials of KOS-953, KOS-1022, KOS-862, KOS-1584, KOS-2187 or other product candidates) will begin on time or whether any of our clinical trials will be completed on schedule, or at all. We also do not know whether clinical trials will indicate that an earlier-stage compound, such as KOS-1584 or KOS-1022, will be more appropriate for commercial development than a related compound that is at a later stage of clinical development, such as KOS-862 or KOS-953, respectively. Negative or inconclusive results or adverse medical events during a clinical trial could cause a clinical trial to be suspended, repeated or terminated. Certain of the clinical trials of our product candidates are or may in the future be designed to include two stages, with the decision whether to proceed to the second stage dependent on results obtained in the first stage. Failure to achieve predetermined response rates as defined in the protocol may result in the decision not to proceed into the second stage of the trial.

We have multiple product candidates in human clinical trials for the treatment of cancer, KOS-953, KOS-1022, KOS-862 and KOS-1584. Anticancer drugs frequently have a narrow therapeutic window between efficacy and toxicity. If unacceptable toxicity is observed in a clinical trial, the trial may be terminated at an early stage. For example, in June 2004, we discontinued a Phase II clinical study of KOS-862 in colorectal cancer due to unanticipated cumulative drug toxicities in patients who had previously been treated with the cancer treatment oxaliplatin. We also observed a higher incidence of adverse events resulting in patient withdrawal in our Phase II clinical study of KOS-862 in hormone-refractory prostate cancer than in our Phase II non-small cell lung cancer trial. We cannot predict whether future trials of KOS-862, or other compounds, will demonstrate toxicity issues or adverse events resulting in a significant patient withdrawal.

Completion of clinical trials may take several years or more. The length of time generally varies substantially according to the type, complexity, novelty and intended use of the drug candidate. Our clinical trials may be suspended at any time if we, our collaborators, the FDA, or other regulatory authorities believe the patients participating in our studies are exposed to unacceptable health risks or that study protocols or patient informed consents should be amended to reflect additional health risks, additional testing procedures or other changes. For example, in September 2005, we temporarily suspended enrollment in our KOS-953 and KOS-1022 clinical trials in connection with a request by the FDA to amend the protocols and patient informed consents for those trials. We provided amended protocols and informed consents for those clinical trials to the FDA in response to certain of its requested changes, and we resumed enrollment in the clinical trials. While the FDA has not agreed or objected to these changes, the FDA may require further changes to these protocols and informed consents, which may have a material adverse effect on the timing of, and our ability to conduct, the KOS-953 and KOS-1022 clinical trials.

Our ability to commence or timely complete clinical trials may be adversely affected by many factors, including:

• ineffectiveness of the study compound, or perceptions by physicians that the compound is not effective for a particular indication;

• inability to manufacture sufficient quantities of compound for use in clinical trials;

• a failure to obtain approval from the FDA, other regulatory authorities or an investigational site’s institutional review board to conduct a clinical trial;

• inability to reach agreement with a sufficient number of investigational sites to conduct a study;

• the number of patients required, slower than expected rate of patient recruitment or inability to recruit a sufficient number of patients;

• adverse medical events or the death of patients during a clinical trial, even if caused by the advanced status of their disease or medical problems that are not related to our product candidates;

• inconclusive or negative results from the clinical trial;

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• competing clinical trials in the same or similar indication;

• third-party clinical investigators failing to perform our clinical trials on our anticipated schedule or consistent with a clinical trial protocol, and other third-party organizations not performing data collection and analysis in a timely or accurate manner; and

• a decision by the FDA or other governmental authorities to require suspension or modification of a clinical trial.

Our product development costs will increase if we have delays in testing or approvals, if we need to perform more or larger clinical trials than planned or if our clinical trials include more expensive testing or other procedures than planned. If the delays are significant, our financial results and the commercial prospects for our products will be harmed, and our ability to become profitable will be adversely affected. If any clinical trials of our product candidates are not successful, our business, financial condition and results of operations will be harmed.

If we are not able to obtain required regulatory approvals, we will not be able to commercialize our product candidates, and our ability to generate revenue will be materially impaired.

Our product candidates and the activities associated with their development, manufacture and commercialization are subject to comprehensive regulation by the FDA and other regulatory agencies in the United States, and by comparable authorities in other countries. Our products may not be commercialized unless and until our collaborators or we obtain regulatory approval from the FDA or foreign governmental authorities to do so. The process of obtaining regulatory approvals is expensive, often takes many years, if approval is obtained at all, and can vary substantially based upon the type, complexity, novelty, safety and efficacy of the product candidates involved. We have not received regulatory approval to market any of our product candidates in any jurisdiction and, although our personnel have experience from working at other companies, we as a company have no experience in preparing and filing the applications necessary to gain regulatory approvals to commercialize our products. This lack of experience may impede our ability to obtain FDA or other foreign regulatory approvals to commercialize our products in a timely manner, if at all.

Changes in the regulatory approval policy during the development period, changes in or the enactment of additional regulations or statutes, or changes in regulatory review for each submitted product application, may cause delays in the approval or rejection of an application for regulatory approval. Furthermore, the approval procedure and the time required to obtain approval varies among countries and can involve additional testing beyond that required by the FDA. Approval by one regulatory authority does not ensure approval by regulatory authorities in other jurisdictions.

The FDA and other regulatory authorities have substantial discretion in the approval process and may refuse to accept any application or may decide that our data is insufficient for approval and require additional preclinical, clinical or other studies or modifications to the manufacturing processes or facilities or quality control procedures for our products. Any clinical trial may fail to produce results satisfactory to the FDA or other regulatory authorities. For example, we currently conduct, and expect to conduct in the future, clinical trials for our product candidates in countries outside of the United States. The FDA or other regulatory authorities may reject data from clinical trials conducted in other countries if they are not conducted in accordance with applicable regulatory standards and procedures.

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We do not know whether clinical trials for our product candidates (including ongoing and anticipated clinical trials of KOS-953, KOS-1022, KOS-862, KOS-1584 or other product candidates) will demonstrate safety and efficacy sufficient to obtain the requisite regulatory approvals or will result in marketable products. Our failure to adequately demonstrate the safety and efficacy of our products under development will prevent receipt of FDA and foreign approvals and, ultimately, commercialization of our products.

We rely on third parties to conduct our clinical trials, and those third parties may not perform satisfactorily.

We do not have the ability to independently conduct clinical trials for our products, and we rely on third parties such as contract research organizations, medical institutions and clinical investigators to perform this function. We also rely on Roche to conduct certain clinical trials for KOS-862 and potentially KOS-1584 and the NCI to conduct certain clinical trials for 17-AAG and KOS-1022. We may rely on future collaborators to conduct clinical trials for our product candidates. If any of these third parties do not successfully carry out their obligations or meet expected deadlines, clinical trials may be extended, delayed, suspended or terminated, and our product candidates may not receive regulatory approval or be successfully commercialized.

We may not be able to obtain or maintain orphan drug exclusivity for our product candidates.

Some jurisdictions, including Europe and the United States, may designate drugs for relatively small patient populations as orphan drugs. The FDA and the European Medicines Agency have granted orphan drug status to 17-AAG for the treatment of multiple myeloma and chronic myelogenous leukemia. Orphan drug designation does not convey any advantage in, or shorten the duration of, the regulatory review and approval process, but does make the product eligible for orphan drug exclusivity and, in the United States, specific tax credits. Generally, if a company receives the first marketing approval for a product with an orphan drug designation in the clinical indication for which it has such designation, the product is entitled to orphan drug exclusivity. Orphan drug exclusivity means that another application to market the same drug for the same indication may not be approved, except in limited circumstances, for a period of up to ten years in Europe (reviewable after six years), and for a period of seven years in the United States. This exclusivity, however, could block the approval of KOS-953 if a competitor obtains approval before us of a product containing 17-AAG for the treatment of multiple myeloma or chronic myelogenous leukemia in the United States or Europe. Even if we obtain orphan drug exclusivity for any of our product candidates, we may not be able to maintain it. For example, if a competitive product is shown to be clinically superior to our product, any orphan drug exclusivity we have obtained will not block the approval of such competitive product.

Even if any of our product candidates receives regulatory approval, we may still face significant development and regulatory difficulties.

Even if the FDA or other regulatory authorities approves a product candidate, the approval may impose significant restrictions on the indicated uses, conditions for use, labeling, advertising, promotion, marketing and/or production of such product, and may impose ongoing requirements for post-approval studies, including additional research and development and clinical trials. In addition, regulatory agencies subject a product, its manufacturer and the manufacturer’s facilities to continual review and periodic inspections. If a regulatory agency discovers previously unknown problems with a product, including adverse events of unanticipated severity or frequency, or problems with the facility where the product is manufactured, a regulatory agency may impose restrictions on that product, our collaborators or us, including requiring withdrawal of the product from the market.

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If any of our approved products, our collaborators or we fail to comply with applicable regulatory requirements, a regulatory authority may take various actions, including:

• issuing warning letters;

• imposing civil or criminal penalties;

• suspending regulatory approval;

• refusing to approve pending applications or supplements to approved applications filed by us or our collaborators;

• imposing restrictions on operations, including costly new manufacturing requirements; or

• seizing or detaining products or requiring a product recall.

Any inability to protect our proprietary technologies could significantly harm our business and ability to successfully commercialize product candidates.

Our commercial success will depend in part on our ability to obtain patents and maintain adequate protection of other intellectual property for our technologies and products in the United States and other countries and prevent others from infringing our proprietary rights. If we are unable to adequately protect our intellectual property, competitors may be able to use our technologies and erode or negate any competitive advantage we may have. Intellectual property laws vary from country to country, and the laws of a particular country may afford less intellectual property protection than another country.

We will be able to protect our proprietary rights from unauthorized use by third parties only to the extent that our proprietary technologies and products are covered by valid and enforceable patents or are effectively maintained as trade secrets. However, the patent positions of biotechnology companies, including our patent position, involve complex legal and factual questions, and, therefore, we cannot predict with certainty whether our patent applications will be allowed or any resulting patents will be valid and enforceable. Further, our patents or patent applications or those of our licensors could be placed into interference, and we may lose our rights in such patents or applications. In particular, two of our exclusively licensed patents, which cover KOS-862 and its use, are the subject of an interference proceeding with a patent application concerning epothilones C, D, E, and F filed by Gesellschaft für Biotechnologische Forschung, or GBF. This patent application is licensed to BMS. Patents may be challenged, held unenforceable, invalidated or circumvented. Thus, any patents that we own or license from third parties may not provide protection against competitors.

The degree of future protection for our proprietary rights is uncertain, and we cannot ensure that:

• we or our licensors were the first to make the inventions covered by each of our patents or pending patent applications;

• we or our licensors were the first to file patent applications for these inventions;

• others will not independently develop similar or alternative technologies or duplicate any of our technologies;

• any of our or our licensors’ pending patent applications will result in issued patents;

• any of our or our licensors’ patents will be valid or enforceable;

• any patents issued to us or our licensors and collaborators will provide a basis for commercially viable products, will provide us with any competitive advantages or will not be challenged by third parties;

• we will develop additional proprietary technologies that are patentable; or

• the patents of others will not have an adverse effect on our business.

We apply for patents covering our technologies, drug candidates, formulations and uses thereof, as we deem appropriate. However, we may fail to apply for patents on important technologies or products in a

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timely fashion or at all. Our existing patents and any future patents we obtain may not be sufficiently broad to prevent others from practicing our technologies or from developing competing products and technologies. For example, 17-AAG, the active pharmaceutical ingredient in the most advanced product candidate in our Hsp90 inhibitor program, KOS-953, was originally disclosed in a now-expired third party patent. Consequently, others can develop products containing 17-AAG. We are aware of at least two other companies that have been developing product candidates containing or based on 17-AAG, and these companies have filed patent applications relating to their products in development. Other competitors may currently be developing, or may in the future develop, products containing or based on 17-AAG. In addition, we generally are unable to control the patent prosecution of technology that we license from others to the same degree as we would for our own technology.

In addition to patents, we rely on trade secrets and proprietary know-how. We have taken measures to protect our confidential information and trade secrets. However, these measures may not provide adequate protection. We seek to protect our confidential information and trade secrets by entering into confidentiality agreements with employees, collaborators, consultants and others. Nevertheless, parties may breach these agreements or competitors may otherwise obtain or independently develop our trade secrets.

Patents related to one of our product candidates, KOS-862, are the subject of an interference in the U.S. involving patent rights licensed to us. Oppositions have been filed in Europe against patents granted to a third party that purportedly cover this compound. These proceedings or any other proceedings are costly, and an unfavorable outcome could prevent us from commercializing this compound.*

We are aware of a significant number of patents and patent applications relating to aspects of our technologies and compounds filed by, and issued to, other parties. Others have filed patent applications or have been granted patents claiming inventions also claimed or licensed by us, and we may have to participate in an interference or other proceeding before a patent agency or court to determine priority of invention or which party was first to invent and, thus, has the right to a patent for these inventions. Two of our exclusively licensed patents, which cover KOS-862 and its use, are the subject of an interference proceeding with a patent application concerning epothilones C, D, E and F filed by GBF. GBF has licensed this patent application to BMS, one of our competitors. In connection with the anticipated settlement of the interference proceeding, we have entered into a cross-license agreement with BMS under which both parties have agreed to grant the other a co-exclusive, worldwide license to epothilone D, or KOS-862. Further, we believe one or more interferences may be declared between patents and applications we own or have exclusively licensed and patents and applications owned by Novartis AG relating to epothilone biosynthetic genes; and patents and applications owned by Abbott Laboratories or Biotica Technologies Ltd. relating to erythromycin PKS genes, methods for altering PKS genes, and erythromycin analogs. In April 2005, a hearing was held at the European Patent Office to address an opposition filed by Biotica to one of our exclusively licensed patents related to the recombinant production of polyketides. Although the written decision has not yet been issued, at the hearing, the patent was maintained (upheld), but with narrowing amendments; the time for Biotica to appeal the ruling has not yet expired. In addition, the European Patent Office has recently granted patents to GBF, which have been licensed to BMS and which, if valid in individual European countries, would cover KOS-862 in those countries. Formal oppositions to these patents have been filed with the European Patent Office.

A proceeding or a lawsuit involving an interference or opposition could result in substantial cost to us even if the outcome is favorable, and if the outcome is unfavorable, we could be required to license the other party’s rights, on terms that may be unfavorable to us, or cease using the technology. An interference or opposition may also result in loss of claims based on patentability grounds raised in the interference or opposition. Although patent and intellectual property disputes in the biotechnology area

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are often settled through licensing or similar arrangements, costs associated with these arrangements may be substantial and could include ongoing royalties. Furthermore, we cannot be certain that a license would be available to us on satisfactory terms, if at all. Companies and others developing products that could compete with our product candidates, such as Novartis AG in the area of potential epothilone products, may be particularly unwilling to grant us a license at any price. If we are not able to obtain necessary licenses, we may not be able to manufacture or commercialize, which could materially harm our business, financial condition and results of operations.

Claims by third parties of intellectual property infringement would require us to spend time and money and could deprive us of valuable rights needed to develop or commercialize our products.

Our commercial success depends significantly on not infringing the patents and proprietary rights of other parties and not breaching any licenses that we have entered into with regard to our technologies and products. Other parties may currently or in the future possess intellectual property rights covering drug candidates that we are developing or desire to develop; methods of treatment or administration involving our drug candidates; formulations of our drug candidates; and genes, gene fragments, cell lines, compounds and other technologies we use or may wish to use. Any infringement of patent rights or violation of other proprietary rights may require us to obtain a license from another party, forego product development or commercialization or face lawsuits or other claims.

The biotechnology industry is characterized by extensive litigation regarding patents and other intellectual property rights. We are aware of patents and published patent applications that, if valid, and if we are unsuccessful in circumventing or acquiring the rights to these patents, may block our ability to commercialize products based on the drug candidates that we are developing or pursue our polyketide synthase, or PKS, gene manipulation and production technologies. We cannot be sure that other parties have not filed for or obtained relevant patents that could affect our ability to obtain patents or operate our business. Others may challenge our patent or other intellectual property rights or sue us for patent infringement, misappropriation of their intellectual property rights or breach of license agreements. We may be required to commence legal proceedings to resolve our patent or other intellectual property rights. An adverse determination in any litigation or administrative proceeding to which we may become a party could subject us to significant liabilities, result in our patents being deemed invalid, unenforceable or revoked, require us to license disputed rights from others or to cease using the disputed technology. In addition, our involvement in any of these proceedings may cause us to incur substantial costs and result in diversion of management and technical personnel.

Other parties may obtain patents in the future and claim that our products or the use of our technologies infringes these patents or that we are employing their proprietary technology without authorization. We could incur substantial costs and diversion of management and technical personnel in defending ourselves against any claims that the use of our technologies infringes any patents, defending ourselves against any claim that we are employing any proprietary technology without authorization or enforcing our patents against others. Furthermore, parties making claims against us may be able to obtain injunctive or other equitable relief that could effectively block our ability to develop, commercialize and sell products, and could result in the award of substantial damages against us. In the event of a successful claim of infringement against us, we may be required to:

• pay substantial damages;

• stop producing certain products and using certain methods;

• develop non-infringing products and methods; and

• obtain one or more licenses from other parties.

We may not be able to obtain licenses from other parties at a reasonable cost, or at all. If we are not

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able to obtain necessary licenses at a reasonable cost or at all, we could encounter substantial delays in product introductions while we attempt to develop alternative methods and products, which we may not be able to accomplish. Litigation or the failure to obtain licenses could prevent us from manufacturing or commercializing products and could materially harm our business, financial condition and results of operation.

Manufacturing difficulties could delay or preclude commercialization of our products and substantially increase our expenses.

We currently use two manufacturers to make 17-AAG, which is the active pharmaceutical ingredient in KOS-953, and KOS-1022. We formulate the final drug product for KOS-953 at our own facility and through contract manufacturers. We formulate the final drug product for KOS-1022 through contract manufacturers. The NCI currently formulates drug product for the formulation of 17-AAG being studied by it under our cooperative research and development agreement, or CRADA. We maintain a limited inventory of KOS-953 and KOS-1022 at our facilities in Hayward, California, and we also maintain a limited inventory at the facilities of an outside contractor. The NCI is not obligated to maintain an inventory of either the active pharmaceutical ingredient or formulated drug product for the formulation of 17-AAG or KOS-1022 being studied by the NCI under our CRADA. In our epothilone program, we are the sole manufacturer of the active pharmaceutical ingredient for KOS-862, and we rely on contract manufacturers for the active pharmaceutical ingredient for KOS-1584. We use a single outside contractor to formulate drug product for KOS-862, and drug product for KOS-1584 is formulated at our facilities and by an outside contractor. We maintain limited inventories of formulated drug product for KOS-862 and KOS-1584 at our facilities in Hayward, California and at the facilities of an outside contractor. Limited inventories of formulated drug product for KOS-862 are also maintained by Roche.

If any of our or our contract manufacturers’ manufacturing or inventory facilities encounter delays, are destroyed or otherwise become unavailable to us, then the clinical development of our product candidates or submissions for their regulatory approval, and therefore commercialization, could be delayed or precluded. Adverse effects would be particularly acute if problems arise with our sole sourcing or inventory relationships. Alternative qualified production capacity may not be available on a timely basis or at all because manufacturing processes for our product candidates are complex and may be subject to a lengthy regulatory approval process.

A number of factors could cause prolonged interruptions in the manufacturing and supply of our products, including:

• the failure of a supplier to provide raw materials or key intermediates used for manufacture of our products;

• equipment malfunctions or failures;

• the failure to manufacture in accordance with current good manufacturing practices or other regulatory requirements;

• the delay of product shipments due to U.S. custom regulations or third-party carriers used to transport our products, and damage to our products while they are in transit;

• changes in FDA or other regulatory authority requirements or standards that require modifications to the manufacturing processes or facilities used in the production of our products;

• action by the FDA or other regulatory authorities to suspend production of one or more of our products; or

• difficulties in scaling-up production of our products for large clinical trials or commercial supply.

While our manufacturing personnel have extensive experience from working at other companies, we as a company have no experience manufacturing products for commercial sale. We may encounter

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difficulties in scaling-up our manufacturing processes and equipment. We may not be able to achieve such scale-up in a timely manner or at a commercially reasonable cost, if at all. In addition, our facilities in Hayward, California are located within the San Francisco Bay Area, an area where earthquakes periodically occur. Our access to any key intermediates, active pharmaceutical ingredient or formulated drug product for our product candidates sourced or inventoried solely through our facilities in Hayward, California may be subject to interruption in the event of an earthquake.

As discussed above, we rely upon outside contractors to manufacture and supply to us key intermediates, active pharmaceutical ingredients and formulated drug product for our product candidates. Our dependence upon others for the manufacture of our product candidates and components thereof may adversely affect our ability to continue in a timely manner clinical development of our product candidates and may adversely affect any future profit margins and our ability to commercialize any products that we may develop on a timely and competitive basis. Dependence on contract manufacturers involves a number of additional risks, many of which are outside of our control, including:

• failure of a contract manufacturer to manufacture products to our specifications or to deliver products in the quantities or timeframe that we require;

• a decision by the FDA or other regulatory authorities not to approve our use of a particular contract manufacturer to supply our products;

• intellectual property rights to any improvements in a manufacturing process or new manufacturing processes being owned by or shared with a contract manufacturer;

• termination of an agreement with a contract manufacturer or increased prices charged by a contract manufacturer; or

• a contract manufacturer declaring bankruptcy or otherwise going out of business.

Any of these factors could cause us to delay or suspend clinical trials, regulatory submissions or commercialization of our products and could result in significantly increased costs.

In addition, our future contract manufacturers may not be in the United States, and we currently utilize contract manufacturers located outside the United States. Consequently, we may face additional manufacturing difficulties due to a number of potential factors, including importation and customs issues, political uncertainties and a potentially limited ability to enforce our contractual rights against parties not located within the United States.

We face intense competition from large pharmaceutical companies, biotechnology companies and academic groups.*

We face, and will continue to face, intense competition from organizations such as large biotechnology and pharmaceutical companies, as well as academic and research institutions and government agencies, that are pursuing competing technologies and products. These organizations may develop or currently possess technologies or products that are superior alternatives to ours. For example, companies with competing Hsp90 inhibitors include Conforma Therapeutics, which has initiated Phase I clinical trials with its formulation of 17-AAG in solid tumors and chronic myelogenous leukemia and has announced that it expects to be acquired by Biogen Idec Inc. in the second quarter of 2006, Infinity Pharmaceuticals, which has initiated Phase I clinical trials of its Hsp90 inhibitor in multiple myeloma and gastrointestinal stromal tumors, and Vernalis plc, which has announced plans to enter clinical trials of its Hsp90 inhibitor in the second half of 2006 in collaboration with Novartis AG, as well as other companies reported to be pursuing Hsp90 inhibitors. Competing epothilones in clinical development include those being developed by BMS (reported to be in Phase III clinical trials), Novartis AG (reported to be in Phase III clinical trials) and Schering AG (reported to be in Phase II clinical trials). Gastrointestinal motility competitors include Chugai Pharmaceuticals, whose motilide agonist is reported to be in Phase II clinical

39

trials. Further, our competitors in the polyketide gene-engineering field may be more effective at implementing their technologies to develop commercial products or may hold or develop patents or other proprietary rights that may prevent us from practicing our technologies and pursuing our programs. Some of these competitors have entered into collaborations with leading companies within our target markets to produce polyketides for commercial purposes.

Any products that we develop through our technologies will compete in multiple, highly competitive markets. Development of pharmaceutical products requires significant investment and resources. Many of the organizations competing with us in the markets for such products have greater capital resources, research and development and marketing staffs, facilities and capabilities, and greater experience in discovery and developing drugs, obtaining regulatory approvals and product manufacturing and marketing. Accordingly, our competitors may succeed in more rapidly developing and marketing technologies and products that are more effective than our technologies and products or that would render our products or technologies obsolete or noncompetitive.

We believe that our ability to successfully compete will depend on, among other things:

• our ability to develop novel compounds with attractive pharmaceutical properties and to secure and protect intellectual property rights based on our innovations;

• the efficacy, safety and reliability of our product candidates;

• the speed at which we develop our product candidates;

• our ability to design and successfully execute appropriate clinical trials;

• the timing and scope of regulatory approvals;

• our ability to manufacture and sell commercial quantities of future products to the market;

• acceptance of future products by physicians and other healthcare providers; and

• the development of effective pricing and reimbursement strategies.

If we face product liability claims, these claims will divert our management’s time and we will incur litigation costs, and if we are held liable, our business, financial condition and results of operation may be materially harmed.

We face an inherent business risk of liability claims in the event that the use of our potential products in clinical trials or otherwise, or any other products manufactured in our facility, results in personal injury or death. Even though we have obtained product liability insurance, it may not be sufficient to cover claims that may be made against us. Product liability insurance is expensive, difficult to obtain and may not be available in the future on acceptable terms, if at all. Any claims against us, regardless of their merit, could materially and adversely affect our business, financial condition and results of operation, because litigation related to these claims would strain our financial resources in addition to consuming the time and attention of our management. If we are sued for any injuries caused by our products or products manufactured at our facility, our liability could exceed our total assets.

We use hazardous chemicals and radioactive and biological materials in our business. Any claims relating to improper handling, storage or disposal of these materials could be time consuming and costly.

Our research and development processes involve the controlled use of hazardous materials, including hazardous chemicals and radioactive and biological materials. Some of these materials may be novel, including bacteria with novel properties and bacteria that produce biologically active compounds. Our operations also produce hazardous waste products. We cannot eliminate the risk of accidental contamination or discharge and any resultant injury from these materials. Federal, state and local laws and regulations govern the use, manufacture, storage, handling and disposal of these materials. We could be subject to civil damages in the event of an improper or unauthorized release of, or exposure of individuals

40

to, hazardous materials. In addition, we could be sued for injury or contamination that results from our use or the use by third parties or our collaborators of these materials, and our liability may exceed our total assets. Compliance with environmental laws and regulations may be expensive, and current or future environmental regulations may impair our research, development or commercialization efforts.

We have a stockholders rights plan and anti-takeover provisions in our corporate charter documents that may result in outcomes with which you do not agree.

Our board of directors has the authority to issue up to 10,000,000 shares of preferred stock and to determine the rights, preferences, privileges and restrictions of those shares without further vote or action by our stockholders. The rights of the holders of any preferred stock that may be issued in the future may adversely affect the rights of the holders of common stock. The issuance of preferred stock could make it more difficult for third parties to acquire a majority of our outstanding voting stock.

Our certificate of incorporation provides for staggered terms for the members of the board of directors and prevents our stockholders from acting by written consent. These provisions and other provisions of our bylaws and of Delaware law applicable to us could delay or make more difficult a merger, tender offer or proxy contest involving us. This could reduce the price that investors might be willing to pay for shares of our common stock and result in the market price being lower than it would be without these provisions. In addition, these provisions may frustrate or prevent any attempts by our stockholders to replace or remove our current management by making it more difficult for stockholders to replace members of our board of directors. This is because our board of directors is responsible for appointing the members of our management team.

We have adopted a rights agreement under which all stockholders have the right to purchase shares of a new series of preferred stock at an exercise price of $70.00 per one one-hundredth of a share, if a person acquires more than 20% of our common stock. The rights plan could make it more difficult for a person to acquire a majority of our outstanding voting stock. The rights plan could also reduce the price that investors might be willing to pay for shares of our common stock and result in the market price being lower than it would be without the rights plan. In addition, the existence of the rights plan itself may deter a potential acquirer from acquiring us. As a result, either by operation of the rights plan or by its potential deterrent effect, mergers and acquisitions of us that our stockholders may consider in their best interests may not occur.

Some of our existing stockholders can exert control over us and may not make decisions that are in the best interest of all stockholders.*

Our officers, directors and their affiliates together controlled approximately 13% of our outstanding common stock as of March 31, 2006. As a result, these stockholders, if they act together, are able to exert a significant degree of influence over our management and affairs and over matters requiring stockholder approval, including the election of directors and approval of significant corporate transactions. This concentration of ownership may delay or prevent a change in control of us and might affect the market price of our common stock, even when a change may be in the best interests of all stockholders. In addition, the interests of this concentration of ownership may not always coincide with our interests or the interests of other stockholders, and accordingly, they could cause us to enter into transactions or agreements, which we would not otherwise consider.

Our stock price has been, and may continue to be, extremely volatile.

The trading price of our common stock has been, and is likely to continue to be, highly volatile. During the period from April 1, 2005 through March 31, 2006, our common stock traded between $3.96

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and $9.77 on the NASDAQ National Market. The trading price of our common stock could be subject to wide fluctuations in price in response to various factors, many of which are beyond our control, including:

• announcements of technological developments in research by us or our competitors;

• delay or failure in initiating, conducting, completing or analyzing clinical trials or unsatisfactory design or results of these trials by our collaborators or us;

• developments in clinical trials for potentially competitive product candidates;

• changes in the United States or foreign health care systems or regulations;

• regulatory approvals for competitive product candidates or delays or failures by our collaborators or us in obtaining regulatory approvals for our product candidates;

• new products or services introduced or announced by us or our competitors;

• published reports by securities analysts;

• announcements of expirations, terminations or amendments of collaborations, licenses or government research grants, or announcements that we have entered into new collaboration, licensing or similar arrangements;

• departures of key personnel;

• developments or disputes as to patent or other proprietary rights;

• litigation or an unfavorable outcome in litigation;

• sales of our common stock, including sales by previous executive officers and other former employees or consultants;

• announcements of, and actual or anticipated fluctuations in, our financial results; and

• economic and other external factors, disasters or crises.

In addition, the stock market in general, and the NASDAQ National Market and the market for biotechnology companies in particular, has experienced extreme price and volume fluctuations that have often been unrelated or disproportionate to the operating performance of those companies. These broad market and industry factors may seriously harm the market price of our common stock, regardless of our operating performance. In the past, following periods of volatility in the market price of a company’s securities, securities class-action litigation has often been instituted against that company. If this type of litigation were instituted against us, we would be faced with substantial costs and management’s attention and resources would be diverted, which could in turn seriously harm our business, financial condition and results of operations.

We expect that our quarterly results of operations will fluctuate, and this fluctuation could cause our stock price to decline, creating investor losses.

Our quarterly operating results have fluctuated in the past and are likely to do so in the future. These fluctuations could cause our stock price to fluctuate significantly or decline. Some of the factors that could cause our operating results to fluctuate include:

• expiration or termination of research contracts with collaborators or government research grants, which may not be renewed or replaced;

• the success rate of our efforts leading to milestone payments and royalties under our collaboration agreement with Roche or any future collaboration or license agreements;

• the timing and willingness of collaborators to develop and commercialize our products;

• general and industry specific economic conditions, which may affect our collaborators’ research and development expenditures; and

• costs and expenses related to any litigation or administrative proceedings in which we may be involved.

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If our revenues decline or do not grow due to expiration, termination or amendment of current or future collaboration agreements, licenses or government research grants, failure to obtain new contracts or other factors, we may not be able to reduce our operating expenses correspondingly. In addition, we expect operating expenses to continue to increase. Failure to achieve anticipated levels of revenues could therefore significantly harm our operating results for a particular fiscal period.

Due to the possibility of fluctuations in our revenues and expenses, we believe that quarter-to-quarter comparisons of our operating results are not a good indication of our future performance. Our operating results in some quarters may not meet the expectations of stock market analysts and investors. In that case, our stock price would probably decline.

Changes in the accounting treatment of stock options will continue to adversely affect our results of operations.*

We adopted SFAS 123R on January 1, 2006, under which we are required to record additional compensation expense related to stock options and other share-based payments in 2006 and beyond. The impact on our earnings resulting from this new standard will have a negative impact on our reported results of operations compared to the results we have reported under prior accounting standards on stock options and other share-based payments.

If we are unable to favorably assess the effectiveness of internal controls over financial reporting, or if our independent auditors are unable to provide an unqualified attestation report on our assessment, our stock price could be adversely affected.

Pursuant to Section 404 of the Sarbanes-Oxley Act of 2002, on an annual basis, our management is required to report on, and our independent auditors to attest to, the effectiveness of our internal controls over financial reporting. The rules governing the standards that must be met for management to make its annual assessment are complex and require significant documentation and testing. While our internal controls over financial reporting were deemed effective by both our management and our independent auditors as of December 31, 2005, there may be changes in our systems, processes or operations that will effect the effectiveness of internal controls in the future. Because of its inherent limitations, internal control over financial reporting may not prevent or detect misstatements. Our future assessments of internal controls may continue to result in increased expenses and the devotion of significant management resources. If we cannot favorably assess the effectiveness of our internal controls over financial reporting in the future, or if our independent auditors are unable to provide an unqualified attestation report on our assessment, investor confidence and our stock price could be adversely affected.

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Item 5: Other Information

None.

Item 6: Exhibits

Exhibit No.
3.1	Amended and Restated Certificate of Incorporation. (1)
3.2	Amended and Restated Bylaws of Registrant. (2)
4.1	Form of Specimen Common Stock Certificate. (3)
4.2	Third Amended and Restated Registration Rights Agreement, dated March 30, 2000, between Registrant and certain stockholders. (3)
4.3	Registrant’s Certificate of Designation of Series A Junior Preferred Stock. (4)
10.28	Consulting Agreement between Registrant and Susan M. Kanaya dated January 1, 2006.(5)*
10.29	Form of Level I Change of Control and Severance Benefit Agreement. (5)*
10.30	Form of Level II Change of Control and Severance Benefit Agreement. (5)*
10.31	First Amendment to Loan and Security Agreement between Registrant and Silicon Valley Bank dated as of February 21, 2006. (5)
10.32	Employment Agreement between Registrant and Petrus Timmermans, Ph.D. dated December 6, 2004.(5)*
10.33	Compensation Information for Executive Officers. (5)*
10.34	Separation and Consulting Agreement between Registrant and Daniel V. Santi, M.D., Ph.D. dated March 20, 2006.*
10.35	Letter agreement among Registrant, Hoffmann-La Roche Inc. and F. Hoffmann-La Roche Ltd. dated March 27, 2006.+
31.1	Certification required by Rule 13a-14(a) or Rule 15d-14(a)
31.2	Certification required by Rule 13a-14(a) or Rule 15d-14(a)
32.1	Certification by the Chief Executive Officer and the Chief Financial Officer of Kosan Biosciences Incorporated, as required by Rule 13a-14(b) or Rule 15d-14(b) and Section 1350 of Chapter 63 of Title 18 of the United Stated Code (18 U.S.C. 1350). (6)

(1) Incorporated herein by reference to an exhibit of our quarterly report on Form 10-Q for the period ended June 30, 2001.

(2) Incorporated herein by reference to an exhibit of our quarterly report on Form 10-Q for the period ended June 30, 2003.

(3) Incorporated herein by reference to an exhibit of our Registration Statement on Form S-1, Registration No. 333-33732.

(4) Incorporated by reference to an exhibit of our current report on Form 8-K filed on October 15, 2001.

(5) Incorporated by reference to an exhibit of our annual report on Form 10-K for the year ended December 31, 2005.

(6) This certification “accompanies” the Quarterly Report on Form 10-Q to which it relates, pursuant to Section 906 of the Sarbanes-Oxley Act of 2002, and is not deemed filed with the Securities and Exchange Commission and is not to be incorporated by reference into any filing of Kosan Biosciences Incorporated under the Securities Act or the Exchange Act (whether made before or after the date of the Quarterly Report on Form 10-Q), irrespective of any general incorporation language contained in such filing.

+ Confidential treatment has been requested for certain portions of this exhibit. Omitted portions have been filed separately with the Securities and Exchange Commission.

* Represents a management or director compensation plan.

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SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the Registrant has duly caused this report to be signed on its behalf by the undersigned, thereunto duly authorized.

	Kosan Biosciences Incorporated
May 10, 2006	By:	/s/ Robert G. Johnson, Jr.
		Robert G. Johnson, Jr., M.D., Ph.D.
		Chief Executive Officer
May 10, 2006	By:	/s/ David L. Johnson
		David L. Johnson
		Principal Accounting Officer and Principal Financial Officer

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EXHIBIT INDEX

Exhibit No.
3.1	Amended and Restated Certificate of Incorporation. (1)
3.2	Amended and Restated Bylaws of Registrant. (2)
4.1	Form of Specimen Common Stock Certificate. (3)
4.2	Third Amended and Restated Registration Rights Agreement, dated March 30, 2000, between Registrant and certain stockholders. (3)
4.3	Registrant’s Certificate of Designation of Series A Junior Preferred Stock. (4)
10.28	Consulting Agreement between Registrant and Susan M. Kanaya dated January 1, 2006.(5)*
10.29	Form of Level I Change of Control and Severance Benefit Agreement. (5)*
10.30	Form of Level II Change of Control and Severance Benefit Agreement. (5)*
10.31	First Amendment to Loan and Security Agreement between Registrant and Silicon Valley Bank dated as of February 21, 2006. (5)
10.32	Employment Agreement between Registrant and Petrus Timmermans, Ph.D. dated December 6, 2004.(5)*
10.33	Compensation Information for Executive Officers.(5)*
10.34	Separation and Consulting Agreement between Registrant and Daniel V. Santi, M.D., Ph.D. dated March 20, 2006.*
10.35	Letter agreement among Registrant, Hoffmann-La Roche Inc. and F. Hoffmann-La Roche Ltd. dated March 27, 2006.+
31.1	Certification required by Rule 13a-14(a) or Rule 15d-14(a)
31.2	Certification required by Rule 13a-14(a) or Rule 15d-14(a)
32.1	Certification by the Chief Executive Officer and the Chief Financial Officer of Kosan Biosciences Incorporated, as required by Rule 13a-14(b) or Rule 15d-14(b) and Section 1350 of Chapter 63 of Title 18 of the United Stated Code (18 U.S.C. 1350). (6)

(1) Incorporated herein by reference to an exhibit of our quarterly report on Form 10-Q for the period ended June 30, 2001.

(2) Incorporated herein by reference to an exhibit of our Quarterly Report on Form 10-Q for the period ended June 30, 2003.

(3) Incorporated herein by reference to an exhibit of our Registration Statement on Form S-1, Registration No. 333-33732.(7) Incorporated by reference to an exhibit of our current report on Form 8-K filed on October 15, 2001.

(5) Incorporated by reference to an exhibit of our annual report on Form 10-K for the year ended December 31, 2005.

(6) This certification “accompanies” the Quarterly Report on Form 10-Q to which it relates, pursuant to Section 906 of the Sarbanes Oxley Act of 2002, and is not deemed filed with the Securities and Exchange Commission and is not to be incorporated by reference into any filing of Kosan Biosciences Incorporated under the Securities Act or the Exchange Act (whether made before or after the date of the Quarterly Report on Form 10-Q), irrespective of any general incorporation language contained in such filing.

+ Confidential treatment has been requested for certain portions of this exhibit. Omitted portions have been filed separately with the Securities and Exchange Commission.

* Represents a management or director compensation plan.

EX-10.34

Exhibit 10.34

Kosan Biosciences Incorporated

3832 Bay Center Place

Hayward, CA 94545

February 13, 2006

VIA HAND DELIVERY

Daniel V. Santi, M.D., Ph.D.

Kosan Biosciences Incorporated

3832 Bay Center Place

Hayward, CA 94545

Re: Separation and Consulting Agreement

Dear Dan:

As discussed, this letter sets forth the substance of the separation and consulting agreement (the “Agreement”) that Kosan Biosciences, Inc. (the “Company”) is offering to you to aid in your employment transition.

1. Resignation of Employment and Chairman of Board Positions. Pursuant to the signed resignation letter dated and effective as of February 13, 2006 (the “Resignation Date”), you resigned from your position of Chief Executive Officer (“CEO”) of the Company, and from any other employment or officer positions with the Company and all of its affiliated entities (the “Affiliates”), and as Chairman of the Company’s Board of Directors (the “Board”). In addition, pursuant to the signed resignation letter dated and effective as of February 22, 2006, you resigned as a member of the Board. Your signed resignation letters are attached hereto as Exhibit A.

2. Final Pay. The Company will pay you all accrued salary and all accrued and unused vacation (if any) earned by you through the Resignation Date, less applicable withholdings and deductions, in accordance with applicable law.

3. Severance Benefits.

(a) Severance Pay. Provided that you sign this Agreement, return it to the Company and allow it to become effective, the Company will provide you a severance payment of $690,605.70 (the “Severance”). The Severance will be subject to applicable withholdings and deductions and will be paid within ten (10) business days after the Effective Date of this Agreement (defined in Section 19 (ADEA Waiver)).

(b) Benefits Payments. Your group health insurance coverage will terminate on the Resignation Date. To the extent provided by the federal COBRA law or applicable state insurance laws (collectively, “COBRA”), and by the Company’s current group health insurance policies, you then will be eligible to continue your group health insurance benefits at your own

expense. Later, you may be able to convert to an individual policy through the provider of the Company’s health insurance, if you wish. You will be provided with a separate notice more specifically describing your rights and obligations to continuing health insurance coverage under applicable state and/or federal insurance laws and the terms of the applicable health insurance plans on or after the Resignation Date. If you enter into this Agreement and abide by the terms set forth herein, and you timely elect continued health insurance coverage, the Company agrees, through the earlier of August 31, 2007 or the date on which you become eligible for insurance coverage with another employer, to pay your health insurance premiums sufficient to continue your coverage at the same level in effect as of the Resignation Date (including dependent coverage, if any) (the “Benefits Payments”) to the extent such coverage is available. You agree to notify the Company in writing immediately upon commencing other employment that provides health insurance benefits.

(c) Deferred Compensation. In the event that the Company determines that any payments hereunder (including but not limited to payments pursuant to Sections 3(a) (Severance Pay)), or continued insurance coverage or Benefits Payments provided under Section 3(b) (Benefits Payments), fail to satisfy the distribution requirement of Section 409A(a)(2)(A) of the Internal Revenue Code (the “Code”) as a result of Section 409A(a)(2)(B)(i) of the Code, then the payment of such benefits shall not be made pursuant to the payment schedules provided herein and instead the payment of such benefits shall be delayed or otherwise restructured to the minimum extent necessary so that such benefits are not subject to the provisions of Section 409A(a)(1) of the Code.

4. Final Expense Reimbursements. No later than thirty (30) days after the Resignation Date, you must submit your final documented expense reimbursement statement reflecting all business expenses you incurred through the Resignation Date for which you seek reimbursement. The Company will reimburse you for such expenses pursuant to its regular business practice.

5. Return of Company Property. On the Resignation Date, you shall return to the Company all documents (and all copies thereof) and other property belonging to the Company that you have in your possession or control, with the exception of any property that the Company authorizes you in writing to retain in connection with your consulting Services hereunder (defined in Section 6(b) (Consulting Duties)), which property shall be returned promptly upon the request of the Company. The documents and property to be returned by you include, but are not limited to, all files, correspondence, email, memoranda, notes, notebooks, drawings, records, plans, forecasts, reports, studies, analyses, compilations of data, proposals, agreements, financial information, research and development information, sales and marketing information, operational and personnel information, specifications, code, software, databases, computer-recorded information, tangible property and equipment (including, but not limited to, computers, facsimile machines, mobile telephones, and servers), credit cards, entry cards, identification badges and keys; and any materials of any kind which contain or embody any proprietary or confidential information of the Company or any of its Affiliates (and all reproductions thereof in whole or in part). You agree to make a diligent search to locate any such documents, property and information. You further agree, on a mutually agreeable day and time, to provide the Company’s representative access to any personally owned computer, server, or e-mail system that you have used to receive, store, review, prepare or transmit any Company confidential or

2

proprietary data, materials or information, so that the Company can make a computer-useable copy of all such information and the Company can also permanently delete and expunge such confidential or proprietary information from those systems. Your eligibility to receive Severance and Benefits Payments is conditioned upon your compliance with the provisions set forth in this paragraph.

6. Consulting Relationship. If you sign this Agreement, return it to the Company, and allow it to become effective, then the Company will engage you as a consultant under the terms and conditions specified below (the “Consulting Relationship”).

(a) Consulting Period. The Company will engage you as a consultant for the period (the “Consulting Period”) commencing on the day immediately following the Resignation Date and continuing until the earlier of: (i) the date your consulting relationship is terminated by the Company due to material breach of this Agreement or your Proprietary Information Obligations (defined in Section 8 (Proprietary Information Obligations)); (ii) the date on which the Company terminates the consulting relationship at its discretion, which may not occur prior to February 13, 2008; (iii) the date on which you terminate the consulting relationship at your discretion; (iv) the date that you and the Company mutually agree to terminate the consulting relationship; or (v) the date that you revoke this Agreement pursuant to Section 19 hereof.

(b) Consulting Duties. During the Consulting Period, you agree to provide at least eight (8) hours per month of consulting services (the “Services”), to and at the request of the CEO or any Company officer or vice president, or the Board, in any area within your expertise. You shall exercise the highest degree of professionalism and utilize your expertise and creative talents in performing the Services. The Company shall not require you to perform the Services in a manner that would unreasonably interfere with your performance of your other professional duties.

(c) Consulting Fees. During the Consulting Period, the Company will pay you consulting fees at a rate of $325.00 per hour, prorated for any partial hours of Services (which shall include consulting discussions, associated research, telephone time, and necessary travel time) (the “Consulting Fees”). You must submit monthly invoices to the Company for the Consulting Fees on or shortly after the end of the month, providing documentation to support the amount of Consulting Fees billed, and the Company will provide payment within thirty (30) days after receipt of the invoice. Because you will perform the Services as an independent contractor, the Company will not withhold from the Consulting Fees any amount for taxes, social security or other payroll deductions. The Company will report your Consulting Fees on an IRS Form 1099. You acknowledge that you will be entirely responsible for payment of any taxes which may be due with regard to the Consulting Fees or continued vesting of the Equity Awards (discussed below), and you hereby indemnify and save harmless the Company from any liability for any taxes, penalties or interest that may be assessed by any taxing authority with respect thereto, with the exception of the employer’s share of social security, if any.

(d) Equity Award Vesting. To the extent consistent with and subject to the terms of your stock option grants provided to you in connection with your employment (collectively, the “Equity Awards”), and the terms of the applicable equity incentive plans, your Equity Awards will continue to vest during the Consulting Period. You will be able to exercise

3

any vested shares subject to the Equity Awards for thirty (30) days after the termination of the Consulting Period in accordance with the terms of your Equity Awards and the applicable equity incentive plans, or, if the Consulting Period terminates due to your death or Disability (as defined in the applicable equity incentive plans), you (or your estate, as applicable) will be able to exercise any vested shares subject to the Equity Awards within the applicable time periods specified in your Equity Awards and the governing equity incentive plans.

(e) Protection of Information. You agree that, during the Consulting Period and thereafter, you will not use or disclose any confidential or proprietary information or materials of the Company that you obtain or develop in the course of performing the Services, except with permission of a duly-authorized Company officer. Any and all work product you create in the course of performing the Services will be the sole and exclusive property of the Company. You hereby assign to the Company all right, title, and interest in all inventions, techniques, processes, materials, and other intellectual property developed in the course of performing the Services.

(f) Expenses. The Company will reimburse you for reasonable, documented business expenses incurred in performing the Services pursuant to its regular business practice, provided that these expenses have been pre-approved by the Company in writing.

(g) Other Work Activities. During the Consulting Period, you will not carry on any business or activity (whether directly or indirectly, as a partner, stockholder, principal, agent, director, affiliate, employee or consultant) that is competitive in any manner with the business of the Company in developing, licensing, marketing or otherwise exploiting its work on: (i) Hsp90 inhibitors, (ii) tubulin inhibitors, (iii) agonists of the motilin receptor, or (iv) nuclear export inhibitors, nor, except as otherwise specifically provided below, engage in other activities that conflict with your obligations to the Company. Notwithstanding the above restrictions in this Section 6(g), you shall not be prohibited from being a passive shareholder of up to 1% of the public stock of an entity that competes with the Company in any of the above listed areas, and you are not prohibited from engaging in teaching or research activities in any of the above listed areas so long as such teaching or research activities are in connection with your engagement by or services to a college or university. During the Consulting Period, you may engage in any form of employment, consulting, research, teaching, business activity or combination of any of the same which is not prohibited by this paragraph, provided that it shall not unreasonably interfere with your ability to perform the Services for the Company.

7. Other Compensation or Benefits. You acknowledge that, except as expressly provided in this Agreement, you have not earned and will not receive from the Company any additional compensation (including base salary, bonus, incentive compensation, or equity), severance, or benefits on or after the Resignation Date, with the exception of any vested right you may have under the express terms of a written ERISA-qualified benefit plan (e.g., 401(k) account) or a vested Equity Award.

8. Proprietary Information Obligations. You hereby acknowledge your continuing obligations (the “Proprietary Information Obligations”) as provided in your Employee Proprietary Information and Invention Assignment Agreement signed December 16, 1998 (the “Proprietary Information Agreement”) (attached hereto as Exhibit B).

4

9. Disclosure. You hereby acknowledge and agree that this Agreement and a description of the terms set forth herein will be filed by the Company with the Securities and Exchange Commission pursuant to its obligations as a reporting company under the Securities Exchange Act of 1934, as amended and the rules and regulations promulgated thereunder (the “Exchange Act”), and consequently shall be publicly available.

10. Nondisparagement. You agree not to disparage the Company and its officers, directors, employees, shareholders and agents, in any manner likely to be harmful to them or their business, business reputation or personal reputation; and the Company (through its executive officers and directors) agrees not to disparage you in any manner likely to be harmful to you or your business, business reputation or personal reputation. Notwithstanding anything else in this paragraph, both the Company and you may respond accurately and fully to any inquiry or request for information if required by legal process.

11. Job Reference Inquiries. The Company agrees to refer requests for job references concerning you from prospective employers to Jean Deleage, Margaret (Peg) Horn, Pieter Timmermans, or any other individual you propose who is acceptable to the Company.

12. Nonsolicitation. During the later of either the Consulting Period or two (2) years after the Resignation Date, except with the Company’s advance written consent, you will not, directly or indirectly, recruit, solicit, entice, induce or encourage any employee, independent contractor or consultant of the Company to terminate a relationship with the Company in order to become an employee, independent contractor or consultant for any other person or entity. By way of example, but not limitation, during the above-referenced timeframe, except with the Company’s advance written consent, you are prohibited from participating in any manner in the recruitment, targeting or hiring process with respect to any employee, independent contractor or consultant of the Company, and you are prohibited from providing any entity with any information related to the selection, recruitment, or hiring of any such individual.

13. No Admissions. The promises and payments in consideration of this Agreement shall not be construed to be an admission of any liability or obligation by either party to the other party, and neither party makes any such admission.

14. No Voluntary Adverse Action. You agree that you will not voluntarily assist any person in preparing, bringing, or pursuing any litigation, arbitration, administrative claim or other formal proceeding against the Company, its parents, subsidiaries, Affiliates, distributors, officers, directors, employees or agents, unless pursuant to subpoena or other compulsion of law.

15. Cooperation. You agree to cooperate fully with the Company in connection with its actual or contemplated defense, prosecution, or investigation of any claims, demands, audits, government or regulatory inquiries, or other matters arising from events, acts, or failures to act that occurred during the time period in which you were employed by the Company. Such cooperation includes, without limitation, making yourself available upon reasonable notice, without subpoena, to provide accurate and complete information to the Company and making yourself available for truthful and accurate interviews, depositions, and trial testimony. The Company will reimburse you for reasonable out-of-pocket expenses you incur in connection with any such cooperation (excluding foregone wages, salary, or other compensation), and will make reasonable efforts to accommodate your scheduling needs.

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16. Acts Necessary To Effect This Agreement. You and the Company agree to timely execute any instruments or perform any other acts that are or may be necessary or appropriate to effect and carry out the transactions contemplated by this Agreement.

17. Common Stock of the Company. You agree that, for the longer of either (a) twelve (12) months after the Separation Date, or (b) the date upon which your total beneficial ownership of the Company’s common stock constitutes less than ten percent (10%) of the outstanding common stock of the Company, any shares of the Company’s common stock beneficially owned by you will be subject to lock-up restrictions with respect to the sale, transfer, assignment or other disposal of shares of the Company’s common stock to the same extent as those to which shares owned by the directors and officers of the Company are subject in connection with any public or other offering of the Company’s common stock, and you hereby agree to promptly execute any and all agreements to that effect with the underwriter(s) or placement agent(s) of any proposed public or other offering, such agreement to be in substantially the same form as that signed by the Company’s directors and officers, provided that (i) no such lock-up shall exceed in duration ninety (90) days after the date of execution of an underwriting or other agreement in connection with the sale of shares in a public or other offering, nor shall your shares of the Company’s common stock be subject to more than one lock-up in any period of one hundred eighty (180) consecutive days and (ii) no such lock-up shall restrict the sale of shares subject to a plan adopted under Rule 10b5-1 of the Exchange Act that is in effect prior to the date that any such lock-up agreement is requested. The Company agrees to provide you with advance notice of any such lock-up restrictions to the extent such advance notice is practicable, and the Company agrees to process your requests for 10b5-1 applications promptly before the lock-up period begins to the extent practicable. You agree that you will not sell or otherwise transfer any shares of the Company’s common stock beneficially owned by you to a person or entity who or which is not a member of your family or controlled by you or a member of your family, including pursuant to a plan adopted under Rule 10b5-1 of the Exchange Act, prior to April 14, 2006; provided, however, that you may adopt a plan under Rule 10b5-1 of the Exchange Act prior to such date so long as no sales of shares are made thereunder prior to April 14, 2006. For the longer of either (a) twelve (12) months after the Separation Date, or (b) the date upon which your total beneficial ownership of the Company’s common stock constitutes less than ten percent (10%) of the outstanding common stock of the Company, in the event that you determine to sell or otherwise transfer, or adopt a plan under Rule 10b5-1 of the Exchange Act to sell or otherwise transfer, more than 10,000 shares of the Company’s common stock beneficially owned by you in a single transaction or series of related transactions to a person or entity who or which is not a member of your family or controlled by you or a member of your family, you shall advise the Company at least five (5) business days prior to any such sale or other transfer or the adoption of any such plan and, if requested by the Company, consult with the Company in connection with the disposition of such shares. You acknowledge that you have continuing obligations, and are subject to continuing restrictions, under the federal securities laws, including Section 16 of the Exchange Act and Rule 144 under the Securities Act of 1933, as amended, and that you should consult with your counsel in connection with any proposed transaction involving the Company’s common stock. Until such time as you first cease to be a “Section 16 person” (or such later date as may be subsequently agreed with the

6

Company), the Company shall continue to provide to you the same assistance as it has heretofore provided you in complying with your obligations under Section 16 of the Exchange Act; provided, however, that the Company shall have no obligations to make any filings on your behalf within less than 36 hours of notice of a transaction requiring any such filing.

18. Santi’s Release. In exchange for the consideration under this Agreement to which you would not otherwise be entitled, except as provided in this Agreement, you hereby generally and completely release the Company and its directors, officers, employees, shareholders, partners, agents, attorneys, predecessors, successors, parent and subsidiary entities, insurers, Affiliates, and assigns from any and all claims, liabilities and obligations, both known and unknown, that arise out of or are in any way related to events, acts, conduct, or omissions occurring at any time prior to or at the time you sign this Agreement. This general release includes, but is not limited to: (a) all claims arising out of or in any way related to your employment with the Company or the termination of that employment; (b) all claims related to your compensation or benefits from the Company, including salary, bonuses, commissions, vacation pay, expense reimbursements, severance pay, fringe benefits, stock, stock options, or any other ownership or equity interests in the Company; (c) all claims for breach of contract, wrongful termination, and breach of the implied covenant of good faith and fair dealing, including, but not limited to, claims based on or arising from the Employment Agreement between you and the Company dated November 1, 1998 (attached as Exhibit C); (d) all tort claims, including claims for fraud, defamation, emotional distress, and discharge in violation of public policy; and (e) all federal, state, and local statutory claims, including claims for discrimination, harassment, retaliation, attorneys’ fees, or other claims arising under the federal Civil Rights Act of 1964 (as amended), the federal Americans with Disabilities Act of 1990, the federal Age Discrimination in Employment Act (as amended) (“ADEA”), the federal Family and Medical Leave Act, the California Family Rights Act, and the California Fair Employment and Housing Act (as amended). Notwithstanding the foregoing, you are not releasing the Company hereby from any obligation to indemnify you pursuant to the articles and bylaws of the Company, the Indemnification Agreement dated January 12, 1995 (attached as Exhibit D), and applicable law. You represent that you have no lawsuits, claims or actions pending in your name, or on behalf of any other person or entity, against the Company or any other person or entity subject to the release granted in this paragraph.

19. ADEA Waiver. You acknowledge that you are knowingly and voluntarily waiving and releasing any rights you may have under the ADEA, and that the consideration given for the waiver and release in the preceding paragraph hereof is in addition to anything of value to which you are already entitled. You further acknowledge that you have been advised, as required by the ADEA, that: (a) your waiver and release do not apply to any rights or claims that may arise after the date that you sign this Agreement; (b) you should consult with an attorney prior to signing this Agreement (although you may choose voluntarily not to do so); (c) you have at least twenty-one (21) days from the date you receive this Agreement to consider this Agreement (although you may choose voluntarily to sign it earlier); (d) you have seven (7) days following the date you sign this Agreement to revoke the Agreement by providing written notice of your revocation to the Board; and (e) this Agreement will not be effective until the date upon which the revocation period has expired, which will be the eighth day after the date that this Agreement is signed by you (the “Effective Date”). Notwithstanding the preceding, the Consulting Relationship shall be effective as of the day immediately following the Resignation

7

Date, and your revocation of this Agreement, if it occurs, will result in immediate termination of the Consulting Relationship. Additionally, since you asked for, and received, material changes in this Agreement in your favor, you agree to waive an additional twenty-one day period in which to consider this Agreement, you agree that the required twenty-one (21)-day consideration period for this Agreement will be deemed to have started on February 13, 2006 (the date that you received the Company’s original offer), and you and the Company agree that you will have until March 20, 2006 in which to consider this Agreement.

20. Company’s Release. Except as provided in this Agreement, the Company hereby generally and completely releases you and your agents, successors, assigns, attorneys and affiliates, from any and all claims, liabilities and obligations, both known and unknown, that arise out of or are in any way related to events, acts, conduct, or omissions, within the course and scope of your employment, occurring at any time prior to or at the time the Company signs this Agreement. Notwithstanding the foregoing, the Company is not releasing you hereby from any claims, liabilities and obligations arising under or based on: (a) criminal or fraudulent acts, conduct or omissions; or (b) your obligations to protect the Company’s proprietary information, including without limitation any claims arising from your obligations under the Proprietary Information Agreement or under the California Uniform Trade Secrets Act. The Company represents that it has no lawsuits, claims or actions pending in its name, or on behalf of any other person or entity, against you or any other person or entity subject to the release granted in this paragraph.

21. Section 1542 Waiver. In giving the releases set forth in this Agreement, which include claims which may be unknown to you and the Company at present, the parties acknowledge that they have read and understand Section 1542 of the California Civil Code which reads as follows: “A general release does not extend to claims which the creditor does not know or suspect to exist in his favor at the time of executing the release, which if known by him must have materially affected his settlement with the debtor.” You and the Company hereby expressly waive and relinquish all rights and benefits under that section and any law or legal principle of similar effect in any jurisdiction with respect to the releases of claims herein, including but not limited to the releases of unknown and unsuspected claims.

22. Dispute Resolution. To ensure rapid and economical resolution of any disputes regarding this Agreement, the parties hereby agree that any and all claims, disputes or controversies of any nature whatsoever arising out of, or relating to, this Agreement, or its interpretation, enforcement, breach, performance or execution, your employment with the Company, or the termination of such employment, shall be resolved, to the fullest extent permitted by law, by final, binding and confidential arbitration in San Francisco, California conducted before a single arbitrator by JAMS, Inc. (“JAMS”) or its successor, under the then applicable JAMS arbitration rules. The parties each acknowledge that by agreeing to this arbitration procedure, they waive the right to resolve any such dispute, claim or demand through a trial by jury or judge or by administrative proceeding. You will have the right to be represented by legal counsel at any arbitration proceeding. The arbitrator shall: (a) have the authority to compel adequate discovery for the resolution of the dispute and to award such relief as would otherwise be available under applicable law in a court proceeding; and (b) issue a written statement signed by the arbitrator regarding the disposition of each claim and the relief, if any, awarded as to each claim, the reasons for the award, and the arbitrator’s essential findings

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and conclusions on which the award is based. The arbitrator, and not a court, shall also be authorized to determine whether the provisions of this paragraph apply to a dispute, controversy, or claim sought to be resolved in accordance with these arbitration procedures. Nothing in this Agreement is intended to prevent either you or the Company from obtaining injunctive relief in court to prevent irreparable harm pending the conclusion of any arbitration.

23. Miscellaneous. This Agreement, including Exhibits A, B, C, and E, constitutes the complete, final and exclusive embodiment of the entire agreement between you and the Company with regard to the subject matter hereof. It is entered into without reliance on any promise or representation, written or oral, other than those expressly contained herein, and it supersedes any other agreements, promises, warranties or representations concerning its subject matter. This Agreement may not be modified or amended except in a writing signed by both you and a duly authorized officer of the Company. This Agreement will bind the heirs, personal representatives, successors and assigns of both you and the Company, and inure to the benefit of both you and the Company, their heirs, successors and assigns. If any provision of this Agreement is determined to be invalid or unenforceable, in whole or in part, this determination shall not affect any other provision of this Agreement and the provision in question shall be modified so as to be rendered enforceable in a manner consistent with the intent of the parties insofar as possible under applicable law. This Agreement shall be construed and enforced in accordance with the laws of the State of California without regard to conflicts of law principles. Any ambiguity in this Agreement shall not be construed against either party as the drafter. Any waiver of a breach of this Agreement, or rights hereunder, shall be in writing and shall not be deemed to be a waiver of any successive breach or rights hereunder. This Agreement may be executed in counterparts which shall be deemed to be part of one original, and facsimile signatures shall be equivalent to original signatures.

If this Agreement is acceptable to you, please sign below on or before March 20, 2006 and return the original to me. If I do not receive the fully executed Agreement from you by such date, the Company’s offer contained herein will expire.

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We look forward to continuing to work with you.

Sincerely,

KOSAN BIOSCIENCES INCORPORATED
By:	/s/ Peter Davis
	Peter Davis
	Lead Independent Director
	Board of Directors,
	on behalf of the Board of Directors

Exhibit A – Signed Resignation Letters

Exhibit B – Employee Proprietary Information and Invention Assignment Agreement

Exhibit C – Employment Agreement

Exhibit D – Indemnification Agreement

UNDERSTOOD AND AGREED:
/s/ Daniel V. Santi	March 20, 2006
Daniel V. Santi, M.D., Ph.D.	Date

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EXHIBIT A

SIGNED RESIGNATION LETTERS

11

February 13, 2006

Board of Directors

Kosan Biosciences Incorporated

3832 Bay Center Place

Hayward, CA 94545

Re: Resignation of CEO and Chairman of Board Positions

To the Board of Directors of Kosan Biosciences, Inc.:

Effective immediately, I hereby submit my resignation as Chief Executive Officer of Kosan Biosciences, Inc. (the “Company”), and I hereby resign from any other employment or officer positions with the Company and all its affiliated entities, and as Chairman of the Company’s Board of Directors. I reserve any rights to severance benefits that I may have under the terms of my employment agreement dated as of November 1, 1998.

I wish the best for the continued success of the Company.

Sincerely,

/s/ Daniel V. Santi

Daniel V. Santi, M.D., Ph.D.

February 22, 2006

Peg Horn

Secretary, Kosan Board of Directors & Chief Counsel

Kosan Biosciences

3832 Bay Center Place

Hayward, CA 94545

Dear Peg,

I hereby resign as a member of the Kosan Biosciences, Inc. Board of Directors.

/s/ Daniel V. Santi

Daniel V. Santi

cc: Suzanne Hooper

EXHIBIT B

EMPLOYEE PROPRIETARY INFORMATION AND INVENTION ASSIGNMENT AGREEMENT

12

KOSAN BIOSCIENCES INCORPORATED

EMPLOYEE PROPRIETARY INFORMATION AND

INVENTION ASSIGNMENT AGREEMENT

As an employee of Kosan Biosciences Incorporated, a California corporation, its subsidiary or its affiliate (together, the “Company”), in consideration of my employment with the Company as previously agree by me in my employment letter with the Company and inconsideration of the compensation now and hereafter paid to me, I agree to the following:

1. Maintaining Confidential Information

a. Company Information. I agree at all times during the term of my employment and thereafter to hold in strictest confidence, and not to use, except for the benefit of the Company, or to disclose to any person, firm or corporation without written authorization of the Company, any trade secrets, confidential knowledge, data or other proprietary information relating to products, processes, know-how, designs, formulas, developmental or experimental work, computer programs, data bases, other original works of authorship, customer lists, business plans, financial information or other subject matter pertaining to any business of the Company or any of its clients, consultants, or licensees.

b. Former Employer Information. I agree that I will not, during my employment with the Company, improperly use or disclose any proprietary information or trade secrets of my former or concurrent employers or companies, or any other person, and that I will not bring onto the premises of the Company any unpublished document or any property belonging to my former or concurrent employers or companies, or any other person, unless consented to in writing by said employers, companies, or other person.

c. Third-Party Information. I recognize that the Company has received and in the future will receive from third parties their confidential or proprietary information subject to a duty on the Company’s part to maintain the confidentiality of such information and to use it only for certain limited purposes. I agree that I owe the Company and such third parties, during the term of my employment and thereafter, a duty to hold all such confidential or proprietary information in the strictest confidence and not to disclose it to any person, firm or corporation (except as necessary in carrying out my work for the Company consistent with the Company’s agreement with such third party) or to use it for the benefit of anyone other than for the Company or such third party (consistent with the Company’s agreement with such third party) without the express written authorization of the Company.

2. Retaining and Assigning Inventions and Original Works

a. Inventions and Original Works Retained by Me. I have listed in Section 7 hereof descriptions of any and all inventions, original works of authorship, developments, improvements, and trade secrets belonging to me which were made by me prior to my employment with the Company, which relate to the Company’s proposed business and products, and which are not assigned to the Company (collectively, “Prior Inventions”).

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b. Inventions and Original Works Assigned to the Company. I agree that I will promptly make full written disclosure to the Company, will hold in trust for the sole right and benefit of the Company, and will assign to the Company all my right, title, and interest in and to any and all inventions, original works of authorship, developments, improvements or trade secrets which I may solely or jointly conceive or develop or reduce to practice, or cause to be conceived or developed or reduced to practice, during the period of time I am in the employ of the Company (collectively, “Company Inventions”). I recognize, however, that assignment to the Company under this provision of any invention is subject to Section 2870 of the California Labor Code, which reads as follows:

“(a) Any provision in an employment agreement which provides that an employee shall assign, or offer to assign, any of his or her rights in an invention to his or her employer shall not apply to an invention that the employee developed entirely on his or her own time without using the employer’s equipment, supplies, facilities, or trade secret information except for those inventions that either:

(1) Relate at the time of conception or reduction to practice of the invention to the employer’s business, or actual or demonstrably anticipated research or development of the employer.

(2) Result from any work performed by the employee for the employer.

(b) To the extent a provision in an employment agreement purports to require an employee to assign an invention otherwise excluded from being required to be assigned under subdivision (a), the provision is against the public policy of this state and unenforceable.”

I agree to grant the Company or its designees a royalty free, irrevocable, worldwide license (with rights to sublicense through multiple tiers of distribution) to practice all applicable patent, copyright and other intellectual property rights relating to any Prior Inventions that I incorporate, or permit to be incorporated, in any Company Invention. Notwithstanding the foregoing, I agree that I will not incorporate, or permit to be incorporated, with Prior Invention in any Company Invention without the Company’s prior written consent.

I acknowledge that all original works of authorship which are made by me (solely or jointly with others) within the scope of my employment and which are protectable by copyright are “works made for hire,” as the term is defined in the United States Copyright Act (17 USCA, Section 101).

c. Maintenance of Records. I agree to keep and maintain adequate and current written records of all inventions and original works of authorship made by me (solely or jointly with others) during the term of my employment with the Company. The

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records will be in the form of notes, sketches, drawings, and any other format that may be specified by the Company. The records will be available to and remain the sole property of the Company at all times.

d. Inventions Assigned to the United States. I agree to assign to the United States government all my right, title, and interest in and to any and all inventions, original works of authorship, developments, improvements or trade secrets whenever such full title is required to be in the United States by a contract between the Company and the United states or any of its agencies.

e. Obtaining Letters Patent, Copyrights, and Mask Work Rights. I agree that my obligation to assist the Company to obtain United States or foreign letters patent, copyrights, or mask work rights covering inventions, works of authorship, and mask works, respectively, assigned hereunder to the Company shall continue beyond the termination of my employment, but the Company shall compensate me at a reasonable rate for time actually spent by me at the Company’s request on such assistance. If the Company is unable because of my mental or physical incapacity or for any other reason to secure my signature to apply for or to pursue any application for any United States or foreign letters patent, copyrights, or mask work rights covering inventions or other rights assigned to the Company as above, then I hereby irrevocably designate and appoint the Company and its duly authorized officers and agents as my agent and attorney in fact, to act for and in my behalf and stead to execute and file any such applications and to do all other lawfully permitted acts to further the prosecution and issuance of letters patent, copyrights, and mask work rights with the same legal force and effect as if executed by me. I hereby waive and quitclaim to the Company any and all claims, of any nature whatsoever, which I now or may hereafter have, of infringement of any patents, copyrights, or mask work rights resulting from any such application assigned hereunder to the Company.

f. Exception to Assignments. I understand that the provisions of this agreement requiring assignment to the Company do not apply to any invention which qualifies fully under the provisions of Section 2870 of the California Labor Code. I will advise the company promptly in writing of any inventions, original works of authorship, developments, improvements or trade secrets that I believe are exempt from assignment to the Company based upon the application of Section 2870 of the California Labor Code; and I will at that time provide to the Company in writing all evidence necessary to substantiate that belief. I understand that the Company will keep in confidence and will not disclose to third parties without my consent any confidential information disclosed in writing to the Company relating to inventions that qualify fully under the provisions of section 2870 of the California Labor Code.

3. Conflicting Employment. I agree that, during the term of my employment with the Company, I will not engage in any other employment, occupation, consulting or other business activity directly related to the business in which the Company is now involved or becomes involved during the term of my employment, nor will I engage in any other activities that conflict with my obligations to the Company.

4. Company Documents and Property. I agree that, at the time of leaving the employ of the Company, I will deliver to the Company (and will not keep in my possession

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or deliver to anyone else) any and all devices, records, data, notes, reports, proposals, lists, correspondence, specifications, drawings, blueprints, sketches, materials, equipment, other documents or property, or reproductions of any aforementioned items belong to the Company, its successors or assigns. I further agree that any property situated on the Company’s premises and owned by the Company, including desks, filing cabinets, or other storage or work areas, is subject to inspection by Company personnel at any time with or without notice.

5. Representations. I agree to execute any proper oath or verify any proper document required to carry out the terms of this agreement. I represent that my performance of all the terms of this agreement will not breach any agreement to keep in confidence proprietary information acquired by me in confidence or in trust prior to my employment by the Company. I have not entered into, and I agree I will not enter into, any oral or written agreement in conflict herewith.

6. General Provisions

a. Governing Law. This agreement will be governed by and construed in accordance with the laws of the State of California as such laws are applied to agreements entered into and to be performed entirely within California by California residents.

b. Entire Agreement; Amendment. This agreement sets forth the entire agreement and understanding between the Company and me relating to the subject matter herein and merges all prior discussions between us. No modification of or amendment to this agreement, nor any waiver of any rights under this agreement, will be effective unless in writing signed by the party to be charged. Any subsequent change or changes in my duties, salary or compensation will not affect the validity or scope of this agreement.

c. Severability. If one or more of the provisions in this agreement are deemed void by law, then the remaining provisions will continue in full force and effect.

d. Successors and Assigns. This agreement will be binding upon my heirs, executors, administrators and other legal representatives and will be for the benefit of the Company, its successors, and its assigns.

e. Survival. The provisions of this agreement shall survive the termination of my employment and the assignment of this agreement by the Company to any successor in interest or other assignee. This agreement is binding upon my heirs and legal representatives.

f. Employment. As used herein, my employment includes any time during which I may be retained by the Company as a consultant. I agree and understand that nothing in this agreement shall confer any right with respect to continuation of employment by the Company, nor shall it interfere in any way with my right or the Company’s right to terminate my employment at any time, with or without cause.

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g. No Solicitation. During the term of my employment with the Company and for a period of two years thereafter, I will not solicit, encourage, or cause others to solicit or encourage any employees of the Company to terminate their employment with the Company.

h. Injunctive Relief. I agree that a breach of any of the representations, warranties or covenants contained in this agreement will result in irreparable and continuing damage to the Company for which there will be no adequate remedy at law, and that consequently the Company will be entitled to injunctive relief and/or a decree for specific performance and such other relief as may be proper (including monetary damages if appropriate).

i. Waiver. The waiver by the Company of a breach of any provision of this agreement by me will not operate or be construed as a waiver of any other or subsequent breach by me.

7. List of Inventions. Pursuant to Section 2(a) of this agreement, set forth below is a list of my prior inventions and original works of authorship:

Title Date Description

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IF NO PRIOR INVENTIONS OR ORIGINAL WORKS OF AUTHORSHIP ARE LISTED IN THIS SECTION 7, I HEREBY AFFIRM THAT THERE ARE NO SUCH INVENTIONS OR ORIGINAL WORKS OF AUTHORSHIP.

KOSAN BIOSCIENCES INCORPORATED
By:	/s/ Daniel V. Santi
	Daniel V. Santi, M.D., Ph.D.,
	Chief Executive Officer & President
Dated: 12-16-98

ACCEPTED AND AGREED:

Dan Santi

/s/ Daniel V. Santi

Sign Name

Print Name

Dated: 12-16-98

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EXHIBIT C

EMPLOYMENT AGREEMENT

13

KOSAN BIOSCIENCES, INC.

EMPLOYMENT AGREEMENT

This Agreement is entered into by and between Kosan Biosciences, Inc., a California corporation (the “Company”), and Daniel V. Santi (“Executive”), as of November 1, 1998.

WHEREAS, Executive is currently employed as a professor with the University of California at San Francisco, (“UCSF”) and works as a consultant for the Company pursuant to the terms of the Amended and Restated Consulting Agreement by and between Executive and the Company, dated March 29, 1996 (the “Consulting Agreement”);

WHEREAS, Executive shall take a leave of absence from UCSF to become an employee of the Company;

WHEREAS, as of the date Executive receives a leave of absence from UCSF, the Company desires to employ the Executive as the Chief Executive Officer and President of the Company, reporting to the Board of Directors of the Company (the “Board”);

WHEREAS, the parties desire and agree to enter into an employment relationship by means of this Agreement; and

NOW THEREFORE, in consideration of the promises and mutual covenants herein contained, and other good and valuable consideration, the receipt and sufficiency of which is hereby acknowledged, it is mutually covenanted and agreed by and among the parties as follows:

1. Duties and Scope of Employment.

(a) Position: Employment Commencement Date. As of the date Executive receives a leave of absence from UCSF that, at a minimum, exceeds nine (9) months (“Leave of Absence”), the Executive shall be employed as the Chief Executive Officer and President of the Company reporting to the Board (“Commencement Date”).

(b) Obligations. Executive shall devote his full business efforts and time to the Company. As Chief Executive Officer and President of the Company, Executive shall have the duties and responsibilities customarily associated with such positions, including senior management powers and responsibilities for the Company’s business and affairs. During the term of this Agreement, Executive agrees not to actively engage in any other employment, occupation or consulting activity for any direct or indirect remuneration that creates an actual or potential conflict of interest with the Company without the prior approval of the Board; provided, however, that Executive may engage in activities that do not materially interfere with his duties and obligations under this Agreement or create an actual or potential conflict of interest with the Company for up to four hours per week. Executive shall report the nature and extent of such activities, if any, to the Board every six months.

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2. At-Will Employment. Executive and the Company understand and acknowledge that Executive’s employment with the Company constitutes “at-will” employment. Executive and the Company acknowledge that this employment relationship may be terminated at any time, with or without cause or for any or no cause, at the option either of the Company or Executive.

3. Compensation, Fringe Benefits and Stock Options.

(a) Base Salary. While employed by the Company pursuant to this Agreement, the Company shall pay the Executive as compensation for his services a base salary at the annualized rate of $250,000 (the “Base Salary”). Such salary shall be paid periodically in accordance with normal Company payroll practices and subject to the usual, required withholding. Executive’s Base Salary shall be adjusted annually by a percentage equal to the percent change set forth in the U.S. Department of Labor and Bureau of Labor Statistics’ Consumer Price Index for U.S. Cities. Executive understands and agrees that neither his job performance nor promotions, commendations, bonuses or the like from the Company give rise to or in any way serve as the basis for modification, amendment, or extension, by implication or otherwise, of this Agreement.

(b) Discretionary Bonus. The performance of Executive and the Company may be reviewed by the Board periodically, and, on that basis, the Board may, in its discretion, award the Executive a bonus. Any such bonus shall be subject to applicable withholding.

(c) Executive Benefits. During his employment hereunder, Executive shall be eligible to participate in the employee benefit plans currently and hereafter maintained by the Company of general applicability to other key executives of the Company, including, without limitation, group health, disability, and life insurance benefits and participation in any Company profit-sharing, retirement or pension plan, and vacation consistent with the vacation policies of the Company.

(d) Stock Option. As of the Commencement Date, Executive shall be granted a nonstatutory stock option which shall consist of 250,000 shares of the Company’s then issued and outstanding shares of Common Stock at an exercise price equal to the fair market value of the Common Stock on the date of grant. Subject to the acceleration of vesting provisions in this Section 3 and Section 5 of This Agreement, the Option shall commence vesting on October 1, 1998, and shall vest and become exercisable as to  ¹/48^th of the shares subject to the Option per month, so as to be fully vested on October 1, 2002, subject to Executive continuing to render services to the Company as President and Chief Executive Officer. The Option shall be in all respects subject to the terms, definitions and provisions of the Company’s 1996 Stock Option Plan (the “Option Plan”) and the stock option agreement by and between Executive and the Company (the “Option Agreement”), all of which documents are incorporated herein by reference.

Notwithstanding the above, Executive shall fully vest in and have the right to exercise the Option as to all of the shares subject to the Option, including shares as to which it would not otherwise by vested or exercisable, in the event that (i) the Company enters into a merger or other reorganization (as defined in Section 181 of the California Corporations Code)

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with or into another corporation or entity (except where California Corporations Code Section 1201(b) does not require the approval of the outstanding shares of the Company with respect to such merger or other reorganization), (ii) the Company sells all or substantially all of its assets, (iii) a person or entity makes a tender or exchange offer for and acquires 50% or more of the issued, and outstanding voting securities of the Company, or (iv) any person within the meaning of Section 3(a)(9) or Section 13(d)(3) of the Securities Exchange Act of 1934, as amended, acquires more than 50% of the Company’s issued and outstanding voting securities of the Company.

4. Expenses. The company will pay or reimburse Executive for reasonable travel, entertainment or other expenses incurred by Executive in the furtherance of or in connection with the performance of Executive’s duties hereunder in accordance with the Company’s established policies.

5. Cancellation of Consulting Agreement. Upon the Commencement Date, Executive and Company agree to cancel and forego their rights, if any, under the Consulting Agreement.

6. Severance Benefits.

(a) Termination without Cause during Leave of Absence. If Executive’s employment with the Company terminates other than voluntarily by the Executive or for “Cause” (as defined herein) at any time during Executive’s Leave of Absence, then (i) Executive shall become a consultant of the Company and enter into an agreement with the Company containing the terms of the Consulting Agreement; provided, however, that Executive’s compensation level shall equal that of Dr. Khosla, (ii) vesting of the Option will immediately cease and Executive shall have the right to exercise any vested portion of the Option for three (3) months following such termination, and (iii) Executive shall only be eligible for severance benefits in accordance with the Company’s established policies as then in effect.

(b) Voluntary Termination During Leave of Absence. If Executive’s employment with the Company terminates voluntarily by the Executive at any time during Executive’s Leave of Absence, then (i) Executive shall become a consultant of the Company and enter into an agreement with the Company containing the terms of the Consulting Agreement; provided, however, that Executive’s compensation level shall equal that of Dr. Khosla, (ii) the vesting of the Option will immediately cease and Executive shall have thirty (30) days to exercise vested shares, if any, subject to the Option, and (iii) Executive shall only be eligible for severance benefits in accordance with the Company’s established policies as then in effect.

(c) Termination without Cause after Leave of Absence. If Executive’s employment with the Company terminates other than voluntarily by the Executive or for “Cause” (as defined herein”) at any time after Executive’s Leave of Absence, then (i) Executive shall be entitled to receive a lump sum severance payment (less applicable withholding taxes) in an amount equal to eighteen (18) months of his Base Salary, as then in effect; and (ii) an additional eighteen (18) months of the shares subject to the Option shall vest as of the date of such termination and Executive have the right to exercise, for three (3) months following termination, the vested and exercisable shares subject to the Option.

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(d) Voluntary Termination after Leave of Absence. If Executive’s employment with the Company terminates voluntarily by Executive at any time after the Leave of Absence, then Executive shall only be eligible for severance benefits in accordance with the Company’s established policies as then in effect.

(e) Termination for Cause. If Executive’s employment with the Company terminates for “Cause” (as defined herein) by the Company, then Executive shall only be eligible for severance benefits in accordance with the Company’s established policies as then in effect. For this purpose, “Cause” is defined as (i) an act of dishonesty made by Executive in connection with Executive’s responsibilities as an employee, (ii) Executive’s conviction of, or plea of nolo contendere to, a felony, (iii) Executive’s gross misconduct, or (iv) Executive’s failure to perform his employment duties.

7. Enforcement. In the event of any action to enforce the terms of this Agreement, the prevailing party in such action shall be entitled to such party’s reasonable costs and expenses of enforcement including, without limitation, reasonable attorneys’ fees.

8. Assignment. This Agreement shall be binding upon and inure to the benefit of (a) the heirs, executors and legal representatives of Executive upon Executive’s death and (b) any successor of the Company. Any such successor of the Company shall be deemed substituted for the Company under the terms of this Agreement for all purposes. As used herein, “successor” shall include any person, firm, corporation or other business entity which at any time, whether by purchase, merger or otherwise, directly or indirectly, acquires all or substantially all of the assets or business of the Company. None of the rights of Executive to receive any form of compensation payable pursuant to this Agreement shall be assignable or transferable except through a testamentary disposition or by the laws of descent and distribution upon the death of Executive following termination without cause. Any attempted assignment, transfer, conveyance or other disposition (other than as aforesaid) of any interest in the rights of Executive to receive any form of compensation hereunder shall be null and void.

9. Notices. All notices, requests, demands and other communications called for hereunder shall be in writing and shall be deemed given if delivered personally, one (1) day after mailing via Federal Express overnight or a similar overnight delivery service, or three (3) days after being mailed by registered or certified mail, return receipt requested, prepaid and addressed to the parties or their successors in interest at the following addresses, or at such other addresses as the parties may designate by written notice in the manner aforesaid:

If to the Company:	Kosan Biosciences, Inc.
	1450 Rollins Road
	Burlingame, CA 94010
If to Executive:	Daniel V. Santi
	at the last residential address known by Company.

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10. Severability. In the event that any provision hereof becomes or is declared by a court of competent jurisdiction to be illegal, unenforceable or void, this Agreement shall continue in full force and effect without said provision.

11. Entire Agreement. This Agreement, the Option Agreement and the Confidential Information and Invention Assignment Agreement dated November 1, 1998 represent the entire agreement and understanding between the Company and Executive concerning Executive’s employment relationship with the Company, and supersede and replace any and all prior agreements and understandings concerning Executive’s employment relationship with the Company.

12. No Oral Modification, Cancellation or Discharge. This Agreement may only be amended, canceled or discharged in writing signed by Executive and the Company.

13. Governing Law. This Agreement shall be governed by the internal substantive laws, but not the choice of law rules, of the State of California.

14. Effective Date. This Agreement is effective immediately after it has been signed.

15. Acknowledgment. Executive acknowledges that he has had the opportunity to discuss this matter with and obtain advice from his private attorney, has had sufficient time to, and has carefully read and fully understands all the provisions of this Agreement, and is knowingly and voluntarily entering into this Agreement.

IN WITNESS WHEREOF, the undersigned have executed this Agreement on the respective dates set forth below.

KOSAN BIOSCIENCES, INC.
By:	/s/ Michael S. Ostrach
Name:	Michael S. Ostrach
Title:	VP and COO
DANIEL V. SANTI
/s/ Daniel V. Santi
Signature

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EXHIBIT D

INDEMNIFICATION AGREEMENT

14

KHOSLA AND SANTI, INC.

INDEMNIFICATION AGREEMENT

This Indemnification Agreement (“Agreement”) is made as of this 12^th day of January, 1995, by and between Khosla and Santi, Inc. a California corporation (the “Company”), and Daniel V. Santi (Indemnitee”).

WHEREAS, the Company and Indemnitee recognize the increasing difficulty in obtaining directors’ and officers’ liability insurance, the significant increases in the cost of such insurance and the general reductions in the coverage of such insurance;

WHEREAS, the Company and Indemnitee further recognize the substantial increase in corporate litigation in general, subjecting officers and directors to expensive litigation risks at the same time as the availability and coverage of liability insurance has been severely limited;

WHEREAS, Indemnitee does not regard the current protection available as adequate under the present circumstances, and Indemnitee and other officers and directors of the Company may not be willing to continue to serve as officers and directors without additional protection; and

WHEREAS, the Company desires to attract and retain the services of highly qualified individuals, such as Indemnitee, to serve as officers and directors of the Company and to indemnify its officers and directors so as to provide them with the maximum protection permitted by law.

NOW, THEREFORE, the Company and Indemnitee hereby agree as follows:

1. Indemnification.

(a) Third Party Proceedings. The Company shall indemnify Indemnitee if Indemnitee is or was a party or is threatened to be made a party to any threatened, pending or completed action or proceeding, whether civil, criminal, administrative or investigative (other than an action by or in the right of the Company) by reason of the fact that Indemnitee is or was a director, officer, employee or agent of the Company, or any subsidiary of the Company, by reason of any action or inaction on the part of Indemnitee while an officer or director or by reason of the fact that Indemnitee is or was serving at the request of the Company as a director, officer, employee or agent of another corporation, partnership, joint venture, trust or other enterprise, against expenses (including attorneys’ fees), judgments, fines and amounts paid in settlement (if such settlement is approved in advance by the Company, which approval shall not be unreasonably withheld) actually and reasonably incurred by Indemnitee in connection with such action or proceeding if Indemnitee acted in good faith and in a manner Indemnitee believed to be in the best interests of the Company, and, with respect to any criminal action or proceeding, had no reasonable cause to believe Indemnitee’s conduct was unlawful. The termination of any

action or proceeding by judgment, order, settlement, conviction, or upon a plea of nolo contendere or its equivalent, shall not, of itself, create a presumption that (i) Indemnitee did not act in good faith and in a manner which Indemnitee reasonably believed to be in the best interests of the Company, or (ii) with respect to any criminal action or proceeding, Indemnitee had reasonable cause to believe that Indemnitee’s conduct was unlawful.

(b) Proceedings By or in the Right of the Company. The Company shall indemnify Indemnitee if Indemnitee was or is a party or is threatened to be made a party to any threatened, pending or completed action or proceeding by or in the right of the Company or any subsidiary of the Company to procure a judgment in its favor by reason of the fact that Indemnitee is or was a director, officer, employee or agent of the Company, or any subsidiary of the Company, by reason of any action or inaction on the part of Indemnitee while an officer or director or by reason of the fact that Indemnitee is or was serving at the request of the Company as a director, officer employee or agent of another corporation, partnership, joint venture, trust or other enterprise, against expenses (including attorneys’ fees) and, to the fullest extent permitted by law, amounts paid in settlement, in each case to the extent actually and reasonably incurred by Indemnitee in connection with the defense or settlement of such action or proceeding if Indemnitee acted in good faith and in a manner Indemnitee believed to be in the best interests of the Company and its shareholders.

2. Expenses: Indemnification Procedure.

(a) Advancement of Expenses. The Company shall advance all expenses incurred by Indemnitee in connection with the investigation, defense, settlement or appeal of any civil or criminal action or proceeding referenced in Section 1(a) or (b) hereof (but not amounts actually paid in settlement of any such action or proceeding). Indemnitee hereby undertakes to repay such amounts advanced only if, and to the extent that, it shall ultimately be determined that Indemnitee is not entitled to be indemnified by the Company as authorized hereby. The advances to be made hereunder shall be paid by the Company to Indemnitee within twenty (20) days following delivery of a written request therefore by Indemnitee to the Company.

(b) Notice/Cooperation by Indemnitee. Indemnitee shall, as a condition precedent to his right to be indemnified under this Agreement, give the Company notice in writing as soon as practicable of any claim made against Indemnitee for which indemnification will or could be sought under this Agreement. Notice to the Company shall be directed to the Chief Executive Officer of the Company at the address shown on the signature page of this Agreement (or such other address as the Company shall designate in writing to Indemnitee). Notice shall be deemed received three business days after the date postmarked if sent by domestic certified or registered mail, properly addressed; otherwise notice shall be deemed received when such notice shall actually be received by the Company. In addition, Indemnitee shall give the Company such information and cooperation as it may reasonably require and as shall be within Indemnitee’s power.

(c) Procedure. Any indemnification provided for in Section 1 shall be made no later than forty-five (45) days after receipt of the written request of Indemnitee. If a claim under this Agreement, under any statute, or under any provision of the Company’s Articles of

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Incorporation or Bylaws providing for indemnification, is not paid in full by the Company within forty-five (45) days after a written request for payment thereof has first been received by the Company, Indemnitee may, but need not, at any time thereafter bring an action against the Company to recover the unpaid amount of the claim, and subject to Section 13 of this Agreement, Indemnitee shall also be entitled to be paid for the expenses (including attorneys’ fees) of bringing such action. It shall be a defense to any such action (other than an action brought to enforce a claim for expenses incurred in connection with any action or proceeding in advance of its final disposition) that Indemnitee has not met the standards of conduct which make it permissible under applicable law for the Company to indemnify Indemnitee for the amount claimed, and Indemnitee shall be entitled to receive interim payments of expenses pursuant to Subsection 2(a) unless and until such defense may be finally adjudicated by court order or judgment from which no further right of appeal exists. It is the parties’ intention that if the Company contests Indemnitee’s right to indemnification, the question of Indemnitee’s right to indemnification shall be for the court to decide, and neither the failure of the Company (including its Board of Directors, any committee or subgroup of the Board of Directors, independent legal counsel, or its shareholders) to have made a determination that indemnification of Indemnitee is proper in the circumstances because Indemnitee has met the applicable standard of conduct required by applicable law, nor an actual determination by the Company (including its Board of Directors, any committee or subgroup of the Board of Directors, independent legal counsel, or its shareholders) that Indemnitee has not met such applicable standard of conduct, shall create a presumption that Indemnitee has or has not met the applicable standard of conduct.

(d) Notice to Insurers. If, at the time of the receipt of a notice of a claim pursuant to Section 2(b) hereof, the Company has director and officer liability insurance in effect, the Company shall give prompt notice of the commencement of such proceeding to the insurers in accordance with the procedures set forth in the respective policies. The Company shall thereafter take all necessary or desirable action to cause such insurers to pay, on behalf of the Indemnitee, all amounts payable as a result of such proceeding in accordance with the terms of such policies.

(e) Selection of Counsel. In the event the Company shall be obligated under Section 2(a) hereof to pay the expenses of any proceeding against Indemnitee, the Company, if appropriate, shall be entitled to assume the defense of such proceeding, with counsel approved by Indemnitee, which approval shall not be unreasonably withheld, upon the delivery to Indemnitee of written notice of its election so to do. After delivery of such notice, approval of such counsel by Indemnitee and the retention of such counsel by the Company, the Company will not be liable to Indemnitee under this Agreement for any fees of counsel subsequently incurred by Indemnitee with respect to the same proceeding, provided that (i) Indemnitee shall have the right to employ his counsel in any such proceeding at Indemnitee’s expense; and (ii) if (A) the employment of counsel by Indemnitee has been previously authorized by the Company, (B) Indemnitee shall have reasonably concluded that there may be a conflict of interest between the Company and Indemnitee in the conduct of any such defense or (C) the Company shall not, in fact, have employed counsel to assume the defense of such proceeding, then the fees and expenses of Indemnitee’s counsel shall be at the expense of the Company.

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3. Additional Indemnification Rights; Nonexclusivity.

(a) Scope. Notwithstanding any other provision of this Agreement, the Company hereby agrees to indemnify the Indemnitee to the fullest extent permitted by law, notwithstanding that such indemnification is not specifically authorized by the other provisions of this Agreement, the Company’s Article of Incorporation, the Company’s Bylaws or by statute. In the event of any change, after the date of this Agreement, in any applicable law, statute or rule which expands the right of a California corporation to indemnify a member of its board of directors, an officer or other corporate agent, such changes shall be, ipso facto, within the purview of Indemnitee’s rights and Company’s obligations, under this Agreement. In the event of any change in any applicable law, statute or rule which narrows the right of a California corporation to indemnify a member of its Board of Directors, an officer or other corporate agent, such changes, to the extent required by such law, statute or rule to be applied to this Agreement, shall have the effect on this Agreement and the parties’ rights and obligations hereunder as is required by such law, statute or rule.

(b) Nonexclusivity. The indemnification provided by this Agreement shall not be deemed exclusive by any rights to which Indemnitee may be entitled under the Company’s Articles of Incorporation, its Bylaws, any agreement, any vote of shareholders or disinterested directors, the California General Corporation Law, or otherwise, both as to action in Indemnitee’s official capacity and as to action in another capacity while holding such office. The indemnification provided under this Agreement shall continue as to Indemnitee for any action taken or not taken while serving in an indemnified capacity even though he may have ceased to serve in such capacity at the time of any action or other covered proceeding.

4. Partial Indemnification. If Indemnitee is entitled under any provision of this Agreement to indemnification by the Company for some or a portion of the expenses, judgments, fines or penalties actually or reasonably incurred by him in the investigation, defense, appeal or settlement of any civil or criminal action or proceeding, but not, however, for the total amount thereof, the Company shall nevertheless indemnify Indemnitee for the portion of such expenses, judgments, fines or penalties to which Indemnitee is entitled.

5. Mutual Acknowledgement. Both the Company and Indemnitee acknowledge that in certain instances, Federal law or applicable public policy may prohibit the Company from indemnifying its directors and officers under this Agreement or otherwise. Indemnitee understands and acknowledges that the Company has undertaken or may be required in the future to undertake with the Securities and Exchange Commission to submit the question of indemnification to a court in certain circumstances for a determination of the Company’s right under public policy to indemnify Indemnitee.

6. Directors’ and Officers’ Liability Insurance. The Company shall, from time to time, make the good faith determination whether or not it is practicable for the Company to obtain and maintain a policy or policies of insurance with reputable insurance companies providing the officers and directors of the Company with coverage for losses from wrongful acts, or to ensure the Company’s performance of its indemnification obligations under this Agreement. Among other considerations, the Company will weigh the costs of obtaining such

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insurance coverage against the protection afforded by such coverage. In all policies of directors’ and officers’ liability insurance, Indemnitee shall be named as an insured in such a manner as to provide Indemnitee the same rights and benefits as are accorded to the most favorably insured of the Company’s directors, if Indemnitee is a director; or of the Company’s officers, if Indemnitee is not a director of the Company but is an officer; or of the Company’s key employees, if Indemnitee is not an officer or director but is a key employee. Notwithstanding the foregoing, the Company shall have no obligation to obtain or maintain such insurance if the Company determines in good faith that such insurance is not reason-ably available, if the premium costs for such insurance are disproportionate to the amount of coverage provided, if the coverage provided by such insurance is limited by exclusions so as to provide an insufficient benefit, or if Indemnitee is covered by similar insurance maintained by a subsidiary or parent of the Company.

7. Severability. Nothing in this Agreement is intended to require or shall be construed as requiring the Company to do or fail to do any act in violation of applicable law. The Company’s inability, pursuant to court order, to perform its obligations under this Agreement shall not constitute a breach of this Agreement. The provisions of this Agreement shall be severable as provided in this Section 7. If this Agreement or any portion hereof shall be invalidated on any ground by any court of competent jurisdiction, then the Company shall nevertheless indemnify Indemnitee to the full extent permitted by any applicable portion of this Agreement that shall not have been invalidated, and the balance of this Agreement not so invalidated shall be enforceable in accordance with its terms.

8. Exceptions. Any other provisions herein to the contrary notwithstanding, the Company shall not be obligated pursuant to the terms of this Agreement:

(a) Excluded Acts. To indemnify Indemnitee for any acts or omissions or transactions from which a director may not be relieved of liability under the California General Corporation Law.

(b) Claims Initiated by Indemnitee. To indemnify or advance expenses to Indemnitee with respect to proceedings or claims initiated or brought voluntarily by Indemnitee and not by way of defense, except with respect to proceedings brought to establish or enforce a right to indemnification under this Agreement or any other statute or law or otherwise as required under Section 317 of the California General Corporation Law, but such indemnification or advancement of expenses may be provided by the Company in specific cases if the Board of Directors has approved the initiation or bringing of such suit; or

(c) Lack of Good Faith. To indemnify Indemnitee for any expenses incurred by the Indemnitee with respect to any proceeding instituted by Indemnitee to enforce or interpret this Agreement, if a court of competent jurisdiction determines that each of the material assertions made by the Indemnitee in such proceeding was not made in good faith or was frivolous; or

(d) Insured Claims. To indemnify Indemnitee for expenses or liabilities of any type whatsoever (including, but not limited to, judgments, fines, ERISA excise taxes or penalties, and amounts paid in settlement) which have been paid directly to Indemnitee by an insurance carrier under a policy of directors’ and officers’ liability insurance maintained by the Company; or

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(e) Claims Under Section 16(b). To indemnify Indemnitee for expenses and the payment of profits arising from the purchase and sale by Indemnitee of securities in violation of Section 16(b) of the Securities Exchange Act of 1934, as amended, or any similar successor statute.

9. Effectiveness of Agreement. To the extent that the indemnification permitted under the terms of certain provisions of this Agreement exceeds the scope of the indemnification expressly permitted by Section 317 of the California General Corporation Law, such provisions shall not be effective unless and until the Company’s Articles of Incorporation authorize such additional rights of indemnification. In all other respects, the balance of this Agreement shall be effective as of the date set forth on the first page and may apply to acts or omissions of Indemnitee which occurred prior to such date if Indemnitee was an officer, director, employee or other agent of the Company, or was serving at the request of the Company as a director, officer, employee or agent of another corporation, partnership, joint venture, trust or other enterprise, at the time such act or omission occurred.

10. Construction of Certain Phrases.

(a) For purposes of this Agreement, references to the “Company” shall also include, in addition to the resulting corporation, any constituent corporation (including any constituent of a constituent) absorbed in consolidation or merger which, if its separate existence had continued, would have had power and authority to indemnify its directors, officers, employees or agents, so that if Indemnitee is or was a director, officer, employee or agent of such constituent corporation, or is or was serving at the request of such constituent corporation as a director, officer, employee or agent of another corporation, partnership, joint venture, trust or other enterprise, Indemnitee shall stand in the same position under the provisions of this Agreement with respect to the resulting or surviving corporation as Indemnitee would have with respect to such constituent corporation if its separate existence had continued.

(b) For purposes of this Agreement, references to “other enterprises” shall include employee benefit plans; references to “fines” shall include any excise taxes assessed on Indemnitee with respect to an employee benefit plan; and references to “serving at the request of the Company” shall include any service as a director, officer, employee or agent of the Company which imposes duties on, or involves services by, such director, officer, employee or agent with respect to an employee benefit plan, its participants, or beneficiaries.

11. Counterparts. This Agreement may be executed in one or more counterparts, each of which shall constitute an original.

12. Successors and Assigns. This Agreement shall be binding upon the Company and its successors and assigns, and shall inure to the benefit of Indemnitee and Indemnitee’s estate, heirs, legal representatives and assigns.

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13. Attorneys’ Fees. In the event that any action is instituted by Indemnitee under this Agreement to enforce or interpret any of the terms hereof, Indemnitee shall be entitled to be paid all costs and expenses, including reasonable attorneys’ fees, incurred by Indemnitee with respect to such action, unless as a part of such action, a court of competent jurisdiction determines that each of the material assertions made by Indemnitee as a basis for such action were not made in good faith or were frivolous. In the event of an action instituted by or in the name of the Company under this Agreement or to enforce or interpret any of the terms of this Agreement, Indemnitee shall be entitled to be paid all costs and expenses, including reasonable attorneys’ fees, incurred by Indemnitee in defense of such action (including with respect to Indemnitee’s counterclaims and cross-claims made in such action), unless as a part of such action the court determines that each of Indemnitee’s material defenses to such action were made in bad faith or were frivolous.

14. Notice. All notices, requests, demands and other communications under this Agreement shall be in writing and shall be deemed duly given (i) if delivered by hand and receipted for by the party addressee, on the date of such receipt, or (ii) if mailed by domestic certified or registered mail with postage prepaid, on the third business day after the date postmarked. Addresses for notice to either party are as shown on the signature page of this Agreement, or as subsequently modified by written notice.

15. Consent to Jurisdiction. The Company and Indemnitee each hereby irrevocably consent to the jurisdiction of the courts of the State of California for all purposes in connection with any action or proceeding which arises out of or relates to this Agreement and agree that any action instituted under this Agreement shall be brought only in the state courts of the State of California.

16. Choice of Law. This Agreement shall be governed by and its provisions construed in accordance with the laws of the State of California as applied to contracts between California residents entered into and to be performed entirely within California.

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IN WITNESS WHEREOF, the parties hereto have executed this Agreement as of the date first above written.

KHOSLA AND SANTI, INC.,
a California corporation
By:	/s/ Daniel V. Santi
Title:	President

AGREED TO AND ACCEPTED:

INDEMNITEE:

Daniel V. Santi

/s/ Daniel V. Santi

(Signature)

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EX-10.35

Exhibit 10.35

March 27, 2006

Hoffmann-La Roche Inc.

F. Hoffmann-La Roche Ltd

Mr. Dennis E. Burns

Hoffmann-La Roche Inc.

340 Kingsland Street

Nutley, New Jersey 07110

Re: Cross License Agreement with Bristol-Myers Squibb Company

Dear Mr. Burns:

This letter confirms our mutual understanding and agreement regarding that certain Cross License Agreement between Kosan Biosciences Incorporated (“Kosan”) and Bristol-Myers Squibb Company (“BMS”), effective as of March 24, 2006 (“BMS Agreement”) as follows. Capitalized terms used but not defined in this letter have the meanings given in that certain Collaborative Research, Development and Commercialization Agreement between Kosan and Roche dated as of September 19, 2002 (“Roche Agreement”). Notwithstanding the foregoing, as used herein Net Sales (with respect to Roche’s, its Affiliates’ or Sublicensees’, Net Sales under the BMS Agreement and Roche Agreement are identical), Licensed Product and Cancer Field shall have the meaning set forth in the BMS Agreement.

1. Royalty Payments.

a. Royalty Payments to BMS. As between Roche and Kosan:

(i) Kosan shall be obligated to pay [*] of the royalty obligations due to BMS based on Roche’s, its Affiliates’ or Sublicensees’ Net Sales of Licensed Product in the Cancer Field; and

(ii) Roche shall be obligated to pay [*] of the royalty obligations due to BMS based on Roche’s, its Affiliates’ or Sublicensees’ Net Sales of Licensed Product in the Cancer Field;

to the extent required to be paid pursuant to the BMS Agreement.

CERTAIN CONFIDENTIAL INFORMATION CONTAINED IN THIS DOCUMENT, MARKED BY BRACKETS, HAS BEEN OMITTED AND FILED SEPARATELY WITH THE SECURITIES AND EXCHANGE COMMISSION PURSUANT TO RULE 24B-2 OF THE SECURITIES EXCHANGE ACT OF 1934, AS AMENDED.

Mr. Dennis E. Burns

March 27, 2006

Page 2

b. Royalty Payments from BMS. As between Roche and Kosan:

(i) Kosan shall receive a [*] share of the royalty payments due to Kosan based on BMS’, its Affiliates’ or Sublicensees’ Net Sales of Licensed Product in the Cancer Field; and

(ii) Roche shall receive a [*] share of the royalty obligations due to Kosan based on BMS, its Affiliates’ or Sublicensees’ Net Sales of Licensed Product in the Cancer Field.

(iii) Roche shall not receive any percentage of royalty payments made by BMS based on Net Sales of Licensed Products by BMS, its Affiliates or Sublicensees outside the Cancer Field.

c. Payments to Sloan-Kettering Institute for Cancer Research (“SKI”). If any payments must be made to SKI as a result of the BMS Agreement, then Kosan shall be solely responsible for making any such payments out of its share. In no event shall any payments to SKI impact upon Roche’s right to receive its [*] share of the royalties paid by BMS to Kosan.

2. Payment Process.

a. Payments to BMS. The parties may agree to modify the final details of the payment process, however, it is currently the parties’ intent that for payment due to BMS based on Net Sales of Licensed Product by Roche, its Affiliates or Sublicensees in the Cancer Field, after receipt of Roche’s royalty report and royalty payment pursuant to the Roche Agreement, Kosan will transmit relevant information from Roche’s royalty report (pursuant to Section 4.4 of the BMS Agreement) to BMS together with payment of one hundred percent (100%) of the royalty owed to BMS under the BMS Agreement. Thereafter, Kosan will invoice Roche for Roche’s [*] share of the royalty payment to BMS. Within thirty (30) days after receipt of an invoice from Kosan, Roche shall be obligated to reimburse Kosan in Dollars for Roche’s [*] share of royalties paid to BMS.

b. Payments from BMS. The parties may agree to modify the final details of the process for royalty payments paid by BMS, however, it is currently the parties’ intent that for all payments due to Kosan under the BMS Agreement, based on Net Sales of Licensed Product by BMS, its Affiliates or Sublicensees in the Cancer Field, Kosan will receive one hundred percent (100%) of the payments from BMS. Within thirty (30) days of receipt of such payment from BMS, Kosan will transmit to Roche the relevant information from the BMS royalty report together with [*] of the royalty payments paid by BMS based on Net Sales of Licensed Product by BMS, its Affiliates or Sublicensees in the Cancer Field.

3. [*] Milestone. Section 4.5 of the Roche Agreement is hereby deleted and shall be replaced with the following:

a. The BMS Agreement contemplates a settlement process to finally resolve U.S. Interference No. 105,298 (the “Interference”). Following final resolution of the Interference, regardless of the outcome, Roche will reimburse Kosan for Kosan’s Expenses associated with the Interference up to [*] U.S. Dollars [*] by paying Kosan a single, one-time, lump sum payment. Specifically, within thirty (30)

CERTAIN CONFIDENTIAL INFORMATION CONTAINED IN THIS DOCUMENT, MARKED BY BRACKETS, HAS BEEN OMITTED AND FILED SEPARATELY WITH THE SECURITIES AND EXCHANGE COMMISSION PURSUANT TO RULE 24B-2 OF THE SECURITIES EXCHANGE ACT OF 1934, AS AMENDED.

Mr. Dennis E. Burns

March 27, 2006

Page 3

days of receipt of an invoice from Kosan, Roche shall reimburse Kosan in Dollars for its Expenses associated with the Interference. “Expenses” shall mean documented out of pocket costs actually paid by Kosan to third parties in connection with the Interference, including but not limited to outside counsel fees, USPTO fees, expert witness fees, fees associated with expert review of priority documents in the settlement process, and the like.

4. General. In the event of any conflict, inconsistency or incongruity between any provision of this letter agreement and the Roche Agreement, this letter agreement shall govern and control. The signatories hereto represent that they have the requisite power and authority to enter into this letter agreement on behalf of the respective parties hereto. This letter agreement may be executed in any number of counterparts, each of which shall be deemed an original, but all of which together shall constitute one and the same document.

This letter is being provided in triplicate originals. If you are in agreement with the foregoing, please sign and date where indicated below and return one signed original to Margaret A. Horn of Kosan. The other two originals are for your corporate records.

Very truly yours,

/s/ Robert G. Johnson, Jr.

Robert G. Johnson, M.D., Ph.D.

Chief Executive Officer

Accepted and Agreed:

Hoffmann-La Roche Inc.
By:	/s/ Dennis E. Burns
Name:	Dennis E. Burns
Title:	Vice President, Global Head
	of Business Development

F. Hoffmann-La Roche Ltd		F. Hoffmann-La Roche Ltd
By:	/s/ Robin Breckenridge	By:	/s/ Melanie Frey Wick
Name:	Dr. Robin Breckenridge	Name:	Melanie Frey Wick
Title:	Pharma Partnering	Title:	Authorized Signatory

CERTAIN CONFIDENTIAL INFORMATION CONTAINED IN THIS DOCUMENT, MARKED BY BRACKETS, HAS BEEN OMITTED AND FILED SEPARATELY WITH THE SECURITIES AND EXCHANGE COMMISSION PURSUANT TO RULE 24B-2 OF THE SECURITIES EXCHANGE ACT OF 1934, AS AMENDED.

EX-31.1

EXHIBIT 31.1

I, Robert G. Johnson, Jr., M.D., Ph.D. certify that:

1. I have reviewed this quarterly report on Form 10-Q of Kosan Biosciences Incorporated;

2. Based on my knowledge, this report does not contain any untrue statement of a material fact or omit to state a material fact necessary to make the statements made, in light of the circumstances under which such statements were made, not misleading with respect to the period covered by this report;

3. Based on my knowledge, the financial statements, and other financial information included in this report, fairly present in all material respects the financial condition, results of operations and cash flows of the registrant as of, and for, the periods presented in this report;

4. The registrant’s other certifying officer(s) and I are responsible for establishing and maintaining disclosure controls and procedures (as defined in Exchange Act Rules 13a-15(e) and 15d-15(e)) and internal control over financial reporting (as defined in Exchange Act Rules 13a-15(f) and 15d-15(f)) for the registrant and have:

(a) Designed such disclosure controls and procedures, or caused such disclosure controls and procedures to be designed under our supervision, to ensure that material information relating to the registrant, including its consolidated subsidiaries, is made known to us by others within those entities, particularly during the period in which this report is being prepared;

(b) Designed such internal control over financial reporting, or caused such internal control over financial reporting to be designed under our supervision, to provide reasonable assurance regarding the reliability of financial reporting and the preparation of financial statements for external purposes in accordance with generally accepted accounting principles;

(c) Evaluated the effectiveness of the registrant’s disclosure controls and procedures and presented in this report our conclusions about the effectiveness of the disclosure controls and procedures, as of the end of the period covered by this report based on such evaluation; and

(d) Disclosed in this report any change in the registrant’s internal control over financial reporting that occurred during the registrant’s most recent fiscal quarter (the registrant’s fourth fiscal quarter in the case of an annual report) that has materially affected, or is reasonably likely to materially affect, the registrant’s internal control over financial reporting; and

5. The registrants other certifying officer(s) and I have disclosed, based on our most recent evaluation of internal control over financial reporting, to the registrant’s auditors and the audit committee of the registrant’s board of directors (or persons performing the equivalent functions):

(a) All significant deficiencies and material weaknesses in the design or operation of internal control over financial reporting which are reasonably likely to adversely affect the registrant’s ability to record, process, summarize and report financial information; and

(b) Any fraud, whether or not material, that involves management or other employees who have a significant role in the registrant’s internal control over financial reporting.

Date: May 10, 2006

/s/ Robert G. Johnson, Jr.

Robert G. Johnson, Jr., M.D., Ph.D.

Chief Executive Officer

EX-31.2

EXHIBIT 31.2

I, David L. Johnson, certify that:

1. I have reviewed this quarterly report on Form 10-Q of Kosan Biosciences Incorporated;

2. Based on my knowledge, this report does not contain any untrue statement of a material fact or omit to state a material fact necessary to make the statements made, in light of the circumstances under which such statements were made, not misleading with respect to the period covered by this report;

3. Based on my knowledge, the financial statements, and other financial information included in this report, fairly present in all material respects the financial condition, results of operations and cash flows of the registrant as of, and for, the periods presented in this report;

4. The registrant’s other certifying officer(s) and I are responsible for establishing and maintaining disclosure controls and procedures (as defined in Exchange Act Rules 13a-15(e) and 15d-15(e)) and internal control over financial reporting (as defined in Exchange Act Rules 13a-15(f) and 15d-15(f)) for the registrant and have:

(a) Designed such disclosure controls and procedures, or caused such disclosure controls and procedures to be designed under our supervision, to ensure that material information relating to the registrant, including its consolidated subsidiaries, is made known to us by others within those entities, particularly during the period in which this report is being prepared;

(b) Designed such internal control over financial reporting, or caused such internal control over financial reporting to be designed under our supervision, to provide reasonable assurance regarding the reliability of financial reporting and the preparation of financial statements for external purposes in accordance with generally accepted accounting principles;

(c) Evaluated the effectiveness of the registrant’s disclosure controls and procedures and presented in this report our conclusions about the effectiveness of the disclosure controls and procedures, as of the end of the period covered by this report based on such evaluation; and

(d) Disclosed in this report any change in the registrant’s internal control over financial reporting that occurred during the registrant’s most recent fiscal quarter (the registrant’s fourth fiscal quarter in the case of an annual report) that has materially affected, or is reasonably likely to materially affect, the registrant’s internal control over financial reporting; and

5. The registrants other certifying officer(s) and I have disclosed, based on our most recent evaluation of internal control over financial reporting, to the registrant’s auditors and the audit committee of the registrant’s board of directors (or persons performing the equivalent functions):

(a) All significant deficiencies and material weaknesses in the design or operation of internal control over financial reporting which are reasonably likely to adversely affect the registrant’s ability to record, process, summarize and report financial information; and

(b) Any fraud, whether or not material, that involves management or other employees who have a significant role in the registrant’s internal control over financial reporting.

Date: May 10, 2006

/s/ David L. Johnson

David L. Johnson

Principal Accounting Officer and Principal Financial Officer

EX-32.1

EXHIBIT 32.1

CERTIFICATION¹

Pursuant to the requirement set forth in Rule 13a-14(b) of the Securities Exchange Act of 1934, as amended (the “Exchange Act”), and Section 1350 of Chapter 63 of Title 18 of the United States Code (18 U.S.C. § 1350), Robert G. Johnson, Jr., M.D., Ph.D., Chief Executive Officer of Kosan Biosciences Incorporated (the “Company”), and David L. Johnson, Principal Accounting Officer and Principal Financial Officer of the Company, each hereby certifies that, to the best of his or her knowledge:

1. The Company’s Quarterly Report on Form 10-Q for the period ended March 31, 2006, to which this Certification is attached as Exhibit 32.1 (the “Periodic Report”), fully complies with the requirements of Section 13(a) or Section 15(d) of the Exchange Act, and

2. The information contained in the Periodic Report fairly presents, in all material respects, the financial condition and results of operations of the Company.

In Witness Whereof, the undersigned have set their hands hereto as of May 10, 2006.

/s/ Robert G. Johnson, Jr.

Robert G. Johnson, Jr., M.D., Ph.D.

Chief Executive Officer

/s/ David L. Johnson

David L. Johnson

Principal Accounting Officer and Principal Financial Officer

¹ This certification accompanies the Form 10-Q to which it relates, is not deemed filed with the Securities and Exchange Commission and is not to be incorporated by reference into any filing of Kosan Biosciences Incorporated under the Securities Act of 1933, as amended, or the Securities Exchange Act of 1934, as amended (whether made before or after the date of the Form 10-Q), irrespective of any general incorporation language contained in such filing.

A signed original of this written statement required by Section 906 has been provided to Kosan Biosciences Incorporated and will be retained by Kosan Biosciences Incorporated and furnished to the Securities and Exchange Commission or its staff upon request.