Underwriting Agreement

1. Introductory. Kosan Biosciences Incorporated, a Delaware corporation (“Company”), proposes to issue and sell 5,000,000 shares (“Firm Securities”) of its common stock, par value $0.001 per share (“Securities”), and also proposes to issue and sell to the Underwriters, at the option of the Underwriters, an aggregate of not more than 750,000 additional shares (“Optional Securities”) of its Securities as set forth below. The Firm Securities and the Optional Securities are herein collectively called the “Offered Securities”. The Company hereby agrees with the several Underwriters named in Schedule A hereto (“Underwriters”) as follows:

2. Representations and Warranties of the Company. The Company represents and warrants to, and agrees with, the several Underwriters that:

(a) A registration statement (No. 333-121597) (“initial registration statement”) relating to the Offered Securities, including a form of prospectus, has been filed with the Securities and Exchange Commission (“Commission”) and an additional registration statement (“additional registration statement”) relating to the Offered Securities may have been or may be filed with the Commission pursuant to Rule 462(b) (“Rule 462(b)”) under the Securities Act of 1933, as amended (“Act”). “Initial Registration Statement” as of any time means the initial registration statement, in the form then filed with the Commission, including all material then incorporated by reference therein, all information contained in the additional registration statement (if any) and then deemed to be a part of the initial registration statement pursuant to the General Instructions of the Form on which it is filed and all information (if any) included in a prospectus then deemed to be a part of the initial registration statement pursuant to Rule 430B (“Rule 430B”) or Rule 430C (“Rule 430C”) under the Act or retroactively deemed to be a part of the initial registration statement pursuant to Rule 430A(b) (“Rule 430A(b)”) under the Act and that in any case has not then been superseded or modified. “Additional Registration Statement” as of any time means the additional registration statement, in the form then filed with the Commission, including the contents of the Initial Registration Statement incorporated by reference therein and including all information (if any) included in a prospectus then deemed to be a part of the additional registration

statement pursuant to Rule 430B or Rule 430C or retroactively deemed to be a part of the additional registration statement pursuant to Rule 430A(b) and that in any case has not then been superseded or modified. The Initial Registration Statement and the Additional Registration Statement are herein referred to collectively as the “Registration Statements” and individually as a “Registration Statement”. “Registration Statement” as of any time means the Initial Registration Statement and any Additional Registration Statement as of such time. For purposes of the foregoing definitions, information contained in a form of prospectus that is deemed retroactively to be a part of a Registration Statement pursuant to Rule 430A or Rule 430B shall be considered to be included in such Registration Statement as of the time specified in Rule 430A or Rule 430B. As of the time of execution and delivery of this Agreement, the Initial Registration Statement has been declared effective under the Act and is not proposed to be amended. Any Additional Registration Statement has or will become effective upon filing with the Commission pursuant to Rule 462(b) and is not proposed to be amended. The Offered Securities all have been or will be duly registered under the Act pursuant to the Initial Registration Statement and, if applicable, the Additional Registration Statement. For purposes of this Agreement, “Effective Time” with respect to the Initial Registration Statement or, if filed prior to the execution and delivery of this Agreement, the Additional Registration Statement means the date and time as of which such Registration Statement was declared effective by the Commission or has become effective upon filing pursuant to Rule 462(c) (“Rule 462(c)”) under the Act. If an Additional Registration Statement has not been filed prior to the execution and delivery of this Agreement but the Company has advised the Representatives that it proposes to file one, “Effective Time” with respect to such Additional Registration Statement means the date and time as of which such Registration Statement is filed and becomes effective pursuant to Rule 462(b). “Effective Date” with respect to the Initial Registration Statement or the Additional Registration Statement (if any) means the date of the Effective Time thereof. A “Registration Statement” without reference to a time means such Registration Statement as of its Effective Time. For purposes of this definition, information contained in a form of prospectus or prospectus supplement that is deemed retroactively to be a part of the Registration Statement pursuant to Rule 430B shall be considered to be included in the Registration Statement as of the time specified in Rule 430B. “Statutory Prospectus” as of any time means the prospectus included in a Registration Statement immediately prior to that time, including any document incorporated by reference therein and any information in a prospectus deemed to be a part thereof pursuant to Rule 430A, Rule 430B or 430C that has not been superseded or modified. For purposes of the preceding sentence, information contained in a form of prospectus that is deemed retroactively to be a part of a Registration Statement pursuant to Rule 430A or Rule 430B shall be considered to be included in the Statutory Prospectus as of the actual time that form of prospectus is filed with the Commission pursuant to Rule 424(b) (“Rule 424(b)”) under the Act. “Prospectus” means the Statutory Prospectus that discloses the public offering price and other final terms of the Offered Securities and otherwise satisfies Section 10(a) of the Act. “Issuer Free Writing Prospectus” means any “issuer free writing prospectus,” as defined in Rule 433, relating to the Offered Securities in the form filed or required to be filed with the Commission or, if not required to be filed, in the form retained in the Company’s records pursuant to Rule 433(g). “General Use Issuer Free Writing Prospectus” means any Issuer Free Writing Prospectus that is intended for general distribution to prospective investors, as specified in Schedule B. “Limited Use Issuer Free Writing Prospectus” means any Issuer Free Writing Prospectus that is not a General Use Issuer Free Writing Prospectus. “Applicable Time” means 9:15 a.m (Eastern time) on the date of this Agreement.

(b) On the Effective Date of the Initial Registration Statement, the Initial Registration Statement conformed in all material respects to the requirements of the Act and the rules and regulations of the Commission (“Rules and Regulations”) and did not include any untrue statement of a material fact or omit to state any material fact required to be stated therein or necessary to make the statements therein not misleading, (ii) on the Effective Date of the Additional Registration Statement (if any), each Registration Statement conformed, or will

conform, in all respects to the requirements of the Act and the Rules and Regulations and did not include, or will not include, any untrue statement of a material fact and did not omit, or will not omit, to state any material fact required to be stated therein or necessary to make the statements therein not misleading and (iii) on the date of this Agreement, the Initial Registration Statement and, if the Effective Time of the Additional Registration Statement is prior to the execution and delivery of this Agreement, the Additional Registration Statement each conforms, and at the time of filing of the Prospectus pursuant to Rule 424(b) or (if no such filing is required) at the Effective Date of the Additional Registration Statement in which the Prospectus is included, each Registration Statement and the Prospectus will conform, in all material respects to the requirements of the Act and the Rules and Regulations, and neither of such documents includes, or will include, any untrue statement of a material fact or omits, or will omit, to state any material fact required to be stated therein or necessary to make the statements therein not misleading. The preceding sentence does not apply to statements in or omissions from a Registration Statement or the Prospectus based upon written information furnished to the Company by any Underwriter through the Representatives specifically for use therein, it being understood and agreed that the only such information is that described as such in Section 8(b) hereof.

(c) (i) At the earliest time after the filing of the Registration Statement that the Company or another offering participant made a bona fide offer (within the meaning of Rule 164(h)(2) under the Act) of the Offered Securities and (ii) at the date of this Agreement, the Company was not and is not an “ineligible issuer,” as defined in Rule 405 (“Rule 405”) under the Act, including (x) the Company or any other subsidiary in the preceding three years not having been convicted of a felony or misdemeanor or having been made the subject of a judicial or administrative decree or order as described in Rule 405 and (y) the Company in the preceding three years not having been the subject of a bankruptcy petition or insolvency or similar proceeding, not having had a registration statement be the subject of a proceeding under Section 8 of the Act and not being the subject of a proceeding under Section 8A of the Act in connection with the offering of the Offered Securities, all as described in Rule 405.

(d) As of the Applicable Time, neither (i) the General Use Issuer Free Writing Prospectus(es) issued at or prior to the Applicable Time, the preliminary prospectus, dated March 29, 2006 (which is the most recent Statutory Prospectus distributed to investors generally) and the information set forth in Schedule C, all considered together (collectively, the “General Disclosure Package”), nor (ii) any individual Limited Use Issuer Free Writing Prospectus, when considered together with the General Disclosure Package, included any untrue statement of a material fact or omitted to state any material fact necessary in order to make the statements therein, in the light of the circumstances under which they were made, not misleading. The preceding sentence does not apply to statements in or omissions from any prospectus included in the Registration Statement or any Issuer Free Writing Prospectus in reliance upon and in conformity with written information furnished to the Company by any Underwriter through the Representatives specifically for use therein, it being understood and agreed that the only such information furnished by any Underwriter consists of the information described as such in Section 8(b) hereof.

(e) Each Issuer Free Writing Prospectus, as of its issue date and at all subsequent times through the completion of the public offer and sale of the Offered Securities or until any earlier date that the Company notified or notifies Credit Suisse Securities (USA) LLC (“Credit Suisse”) as described in the next sentence, did not, does not and will not include any information that conflicted, conflicts or will conflict with the information then contained in the Registration Statement. If at any time following issuance of an Issuer Free Writing Prospectus there occurred or occurs an event or development as a result of which such Issuer Free Writing Prospectus conflicted or would conflict with the information then contained in the Registration Statement or included or would include an untrue statement of a material fact or omitted or would omit to state a material fact necessary in order to make the statements therein, in the light of the circumstances

prevailing at that subsequent time, not misleading, (i) the Company has promptly notified or will promptly notify Credit Suisse and (ii) the Company has promptly amended or will promptly amend or supplement such Issuer Free Writing Prospectus to eliminate or correct such conflict, untrue statement or omission. The foregoing two sentences do not apply to statements in or omissions from any Issuer Free Writing Prospectus in reliance upon and in conformity with written information furnished to the Company by any Underwriter through the Representatives specifically for use therein, it being understood and agreed that the only such information furnished by any Underwriter consists of the information described as such in Section 8(b) hereof.

(f) The Company has been duly incorporated and is an existing corporation in good standing under the laws of the State of Delaware, with power and authority (corporate and other) to own its properties and conduct its business as described in the Prospectus; and the Company is duly qualified to do business as a foreign corporation in good standing in all other jurisdictions in which its ownership or lease of property or the conduct of its business requires such qualification except for such jurisdictions where the failure to so qualify or be in good standing would not, individually or in the aggregate, have a material adverse effect on the condition (financial or other), business, properties or results of operations of the Company (a “Material Adverse Effect”).

(g) The Company does not have any subsidiaries (as such term is defined in Rule 405 under the Act).

(h) The Offered Securities and all other outstanding shares of capital stock of the Company have been duly authorized; all outstanding shares of capital stock of the Company are, and, when the Offered Securities have been delivered and paid for in accordance with this Agreement on each Closing Date (as defined below), such Offered Securities will have been, validly issued, fully paid and nonassessable, will be consistent with the information in the General Disclosure Package, and all outstanding shares of capital stock of the Company, and the Offered Securities will, conform to the description thereof contained in the Prospectus under the heading “Description of Capital Stock”; and the stockholders of the Company have no preemptive or similar rights with respect to the Securities.

(i) Except as disclosed in the General Disclosure Package, there are no contracts, agreements or understandings between the Company and any person that would give rise to a valid claim against the Company or any Underwriter for a brokerage commission, finder’s fee or other like payment in connection with this offering.

(j) Except as disclosed in the General Disclosure Package, there are no contracts, agreements or understandings between the Company and any person granting such person the right to require the Company to file a registration statement under the Act with respect to any securities of the Company owned or to be owned by such person or to require the Company to include such securities in the securities registered pursuant to a Registration Statement or in any securities being registered pursuant to any other registration statement filed by the Company under the Act.

(k) The Offered Securities are approved for quotation on the Nasdaq Stock Market’s National Market.

(l) No consent, approval, authorization, or order of, or filing with, any governmental agency or body or any court is required to be obtained or made by the Company for the consummation of the transactions contemplated by this Agreement in connection with the issuance and sale of the Offered Securities by the Company, except such as have been obtained and made under the Act and such as may be required under state securities laws.

(m) The execution, delivery and performance of this Agreement by the Company, and the issuance and sale of the Offered Securities will not result in a breach or violation of any of the terms and provisions of, or constitute a default under, any statute, any rule, regulation or order of any governmental agency or body or any court, domestic or foreign, having jurisdiction over the Company any of its properties, or any agreement or instrument to which the Company is a party or by which the Company is bound or to which any of the properties of the Company is subject, or the charter or by-laws of the Company, and the Company has full corporate power and authority to authorize, issue and sell the Offered Securities as contemplated by this Agreement.

(n) This Agreement has been duly authorized, executed and delivered by the Company.

(o) Except as disclosed in the General Disclosure Package, the Company has good and marketable title to all real properties and all other properties and assets owned by it, in each case free from liens, encumbrances and defects that would materially affect the value thereof or materially interfere with the use made or to be made thereof by it; and except as disclosed in the General Disclosure Package, the Company holds any leased real or personal property under valid and enforceable leases with no exceptions that would materially interfere with the use made or to be made thereof by it.

(p) The Company possesses adequate certificates, authorities or permits issued by appropriate governmental agencies or bodies necessary to conduct the business now operated by it except where the lack thereof would not individually or in the aggregate have a Material Adverse Effect, including without limitation all such certificates, authorities or permits required by the United States Food and Drug Administration (“FDA”) or any other federal, state or foreign agencies or bodies engaged in the regulation of pharmaceuticals and biohazardous materials, and has not received any notice of proceedings relating to the revocation or modification of any such certificate, authority or permit that, if determined adversely to the Company, would individually or in the aggregate have a Material Adverse Effect.

(q) Except as disclosed in the General Disclosure Package, no labor dispute with the employees of the Company exists or, to the knowledge of the Company, is imminent that could reasonably be expected to have a Material Adverse Effect.

(r) Except as disclosed in the General Disclosure Package, the Company owns, possesses or can acquire on reasonable terms, adequate trademarks, trade names and other rights to inventions, know-how, patents, copyrights, confidential information and other intellectual property (collectively, “intellectual property rights”) necessary to conduct the business now operated by it or as proposed to be conducted as described in the General Disclosure Package by it, or presently employed by it, and, except as disclosed in the General Disclosure Package, has not received any notice of infringement of or conflict with asserted rights of others with respect to any intellectual property rights that, if determined adversely to the Company, would individually or in the aggregate have a Material Adverse Effect. Except as disclosed in the General Disclosure Package, there is no claim being made against the Company regarding intellectual property rights that would individually or in the aggregate have a Material Adverse Effect. Except as disclosed in the General Disclosure Package, the Company does not in the conduct of its business, as now conducted or proposed to be conducted as described in the General Disclosure Package, infringe or conflict with any enforceable intellectual property rights of a third party or, to its knowledge, any intellectual property rights which are the subject of any patent application filed by any third party, in each case which infringement or conflict could reasonably be expected to result in a Material Adverse Effect. The Company is not aware of any prior art that may render any patent application owned by the Company unpatentable which has not been or will not be disclosed to the United States Patent and Trademark Office and which could have Material Adverse Effect.

(s) Except as disclosed in the General Disclosure Package, the Company is not in violation in any material respect of any statute, any rule, regulation, decision or order of any governmental agency or body or any court, domestic or foreign, relating to the use, disposal or release of hazardous or toxic substances or relating to the protection or restoration of the environment or human exposure to hazardous or toxic substances (collectively, “environmental laws”), does not own or operate any real property contaminated with any substance that is subject to any environmental laws, is not liable for any off-site disposal or contamination pursuant to any environmental laws, or is not subject to any claim relating to any environmental laws, which violation, contamination, liability or claim would individually or in the aggregate have a Material Adverse Effect; and the Company is not aware of any pending investigation which might lead to such a claim.

(t) Except as disclosed in the General Disclosure Package, there are no pending actions, suits or proceedings against the Company or any of its properties that, if determined adversely to the Company would individually or in the aggregate have a Material Adverse Effect, or would materially and adversely affect the ability of the Company to perform its obligations under this Agreement, or which are otherwise material in the context of the sale of the Offered Securities; and to the Company’s knowledge, no such actions, suits or proceedings are threatened or contemplated.

(u) The financial statements included in each Registration Statement and the General Disclosure Package present fairly the financial position of the Company as of the dates shown and its results of operations and cash flows for the periods shown, and such financial statements have been prepared in conformity with the generally accepted accounting principles in the United States applied on a consistent basis.

(v) Except as disclosed in the General Disclosure Package, since the date of the latest audited financial statements included in the General Disclosure Package there has been no material adverse change, nor any development or event involving a prospective material adverse change, in the condition (financial or other), business, properties or results of operations of the Company and, except as disclosed in or contemplated by the General Disclosure Package, there has been no dividend or distribution of any kind declared, paid or made by the Company on any class of its capital stock.

(w) The Company is subject to the reporting requirements of either Section 13 or Section 15(d) of the Securities Exchange Act of 1934 and files reports with the Commission on the Electronic Data Gathering, Analysis, and Retrieval (EDGAR) system.

(x) The Company is not and, after giving effect to the offering and sale of the Offered Securities and the application of the proceeds thereof as described in the General Disclosure Package, will not be an “investment company” as defined in the Investment Company Act of 1940, as amended.

(y) Each Investigational New Drug (“IND”) application to the FDA or similar application to foreign regulatory bodies made by or on behalf of the Company, and related documents and information, has been filed, approved and maintained in compliance in all material respects with applicable statutes, rules, regulations, standards, guides or order administered or promulgated by the FDA or other regulatory body, and all pre-clinical and clinical studies undertaken by or on behalf of the Company to support approval of products for commercialization have been conducted in compliance with all applicable current Good Laboratory Practices and Good Clinical Practices in all material respects. No filing or submission to the FDA or any other regulatory body, that is intended to be the basis for any approval, contains any material omission or material false information. Except to the extent disclosed in the Registration Statements and the General Disclosure Package, the Company has operated and currently is in compliance in all material respects with all applicable rules, regulations and policies of the FDA and comparable drug regulatory agencies outside of the United States.

(z) The clinical trials conducted by or on behalf of the Company that are described in a Registration Statement and General Disclosure Package, or the results of which are referred to in a Registration Statement and General Disclosure Package, if any, are the only clinical trials currently being conducted by or on behalf of the Company, and, to the best of the Company’s knowledge, such studies and tests were and, if still pending, are being, conducted in accordance with experimental protocols, procedures and controls pursuant to accepted professional scientific standards; and the descriptions of the results of the studies, tests and trials contained in a Registration Statement and General Disclosure Package are accurate and complete in all material respects and fairly present the data derived from such studies and tests. Except as described in a Registration Statement or the General Disclosure Package, the Company has no knowledge of any other studies or test, the results of which are inconsistent with or otherwise call into question the results of the clinical trials described in a Registration Statement and General Disclosure Package. The Company has not received any notices or correspondence from the FDA or any other governmental agency requiring the termination, suspension or modification of any clinical trials conducted by, or on behalf of, the Company or in which the Company has participated that are described in a Registration Statement and the General Disclosure Package.

(aa) The Company has established and administers a compliance program (including a written compliance policy) applicable to the Company to assist the Company and its directors, officers and employees in complying with applicable regulatory guidelines (including, without limitation, those administered by the FDA and any other foreign, federal, state or local governmental or regulatory authority performing functions similar to those performed by the FDA).

(bb) The Company maintains a system of internal accounting controls sufficient to provide reasonable assurances that: (i) transactions are executed in accordance with management’s general or specific authorization; (ii) transactions are recorded as necessary to permit preparation of financial statements in conformity with GAAP and to maintain accountability for assets; (iii) access to assets is permitted only in accordance with management’s general or specific authorization; and (iv) the recorded accountability for assets is compared with existing assets at reasonable intervals and appropriate action is taken with respect to any differences.

(cc) As of the date of this Agreement and the date of the Registration Statement, (i) the Company satisfies the eligibility requirements for registrants for registration statement Form S-3 pursuant to the standards for such form prior to October 21, 1992 and (ii) the aggregate market value of voting stock held by non-affiliates of the Company is $100 million or more and the Company has an annual trading volume for its Securities of 3 million shares or more, as determined pursuant to the instructions set forth in the registration statement on Form S-3 prior to October 21, 1992.

3. Purchase, Sale and Delivery of Offered Securities. On the basis of the representations, warranties and agreements herein contained, but subject to the terms and conditions herein set forth, the Company agrees to sell to the Underwriters, and the Underwriters agree, severally and not jointly, to purchase from the Company, at a purchase price of $4.75 per share, the respective numbers of shares of Firm Securities set forth opposite the names of the Underwriters in Schedule A hereto.

The Company will deliver the Firm Securities to the Representatives for the accounts of the Underwriters, at the office of Cooley Godward LLP, 3175 Hanover Street, Palo Alto, California, against payment of the purchase price in Federal (same day) funds by official bank check or checks or wire transfer to an account at a bank acceptable to Credit Suisse drawn to the order of the Company at the office of Cooley Godward LLP , at 10:00 A.M., New York time, on April 4, 2005, or at such other time not later than seven full business days thereafter as Credit Suisse and the Company determine, such time being herein referred to

as the “First Closing Date”. For purposes of Rule 15c6-1 under the Securities Exchange Act of 1934, the First Closing Date (if later than the otherwise applicable settlement date) shall be the settlement date for payment of funds and delivery of securities for all the Offered Securities sold pursuant to the offering. The certificates for the Firm Securities so to be delivered will be in definitive form, in such denominations and registered in such names as Credit Suisse requests and will be made available for checking and packaging at the above office of Cooley Godward LLP at least 24 hours prior to the First Closing Date.

In addition, upon written notice from Credit Suisse given to the Company from time to time not more than 30 days subsequent to the date of the Prospectus, the Underwriters may purchase all or less than all of the Optional Securities at the purchase price per Security to be paid for the Firm Securities. The Company agrees to sell to the Underwriters the number of shares of Optional Securities specified in such notice and the Underwriters agree, severally and not jointly, to purchase such Optional Securities. Such Optional Securities shall be purchased for the account of each Underwriter in the same proportion as the number of shares of Firm Securities set forth opposite such Underwriter’s name bears to the total number of shares of Firm Securities (subject to adjustment by Credit Suisse to eliminate fractions) and may be purchased by the Underwriters only for the purpose of covering over-allotments made in connection with the sale of the Firm Securities. No Optional Securities shall be sold or delivered unless the Firm Securities previously have been, or simultaneously are, sold and delivered. The right to purchase the Optional Securities or any portion thereof may be exercised from time to time and to the extent not previously exercised may be surrendered and terminated at any time upon notice by Credit Suisse to the Company.

Each time for the delivery of and payment for the Optional Securities, being herein referred to as an “Optional Closing Date”, which may be the First Closing Date (the First Closing Date and each Optional Closing Date, if any, being sometimes referred to as a “Closing Date”), shall be determined by Credit Suisse but shall be not later than five full business days after written notice of election to purchase Optional Securities is given. The Company will deliver the Optional Securities being purchased on each Optional Closing Date to the Representatives for the accounts of the several Underwriters, at the office of Cooley Godward LLP, 3175 Hanover Street, Palo Alto, California, against payment of the purchase price therefor in Federal (same day) funds by official bank check or checks or wire transfer to an account at a bank acceptable to Credit Suisse drawn to the order of the Company, at the above office of Cooley Godward LLP. The certificates for the Optional Securities being purchased on each Optional Closing Date will be in definitive form, in such denominations and registered in such names as Credit Suisse requests upon reasonable notice prior to such Optional Closing Date and will be made available for checking and packaging at the above office of Cooley Godward LLP at a reasonable time in advance of such Optional Closing Date.

4. Offering by Underwriters. It is understood that the several Underwriters propose to offer the Offered Securities for sale to the public as set forth in the Prospectus.

5. Certain Agreements of the Company. The Company agrees with the several Underwriters that it will furnish to counsel for the Underwriters, one copy of the registration statement relating to the Offered Securities, including all exhibits, in the form it became effective and of all amendments thereto and that, in connection with the offering of Offered Securities:

(a) The Company will file the supplement to the Prospectus that reflects the terms of the offering of the Offered Securities with the Commission pursuant to and in accordance with subparagraph (2) of Rule 424(b) (or, if applicable and if consented to by Credit Suisse, subparagraph (5)) not later than the second business day following the execution and delivery of this Agreement. The Company has complied and will comply with Rule 433.

(b) The Company will advise Credit Suisse promptly of any proposal to amend or supplement a Registration Statement or any Statutory Prospectus and will not effect such amendment or supplementation without Credit Suisse’s consent; and the Company will also advise Credit Suisse

promptly of the filing of any such amendment or supplement and of the institution by the Commission of any stop order proceedings in respect of a Registration Statement or any part thereof and will use its best efforts to prevent the issuance of any such stop order and to obtain as soon as possible its lifting, if issued.

(c) If, at any time when a prospectus relating to the Offered Securities is (or but for the exemption in Rule 172 would be) required to be delivered under the Act in connection with sales by any Underwriter or dealer, any event occurs as a result of which the Prospectus as then amended or supplemented would include an untrue statement of a material fact or omit to state any material fact necessary to make the statements therein, in the light of the circumstances under which they were made, not misleading, or if it is necessary at any time to amend the Prospectus to comply with the Act, the Company will promptly notify Credit Suisse of such event and will promptly prepare and file with the Commission, at its own expense, an amendment or supplement which will correct such statement or omission or an amendment which will effect such compliance. Neither Credit Suisse’s consent to, nor the Underwriters’ delivery of, any such amendment or supplement shall constitute a waiver of any of the conditions set forth in Section 7.

(d) As soon as practicable, but not later than the Availability Date (as defined below), the Company will make generally available to its securityholders an earnings statement covering a period of at least 12 months beginning after the later of (i) the effective date of the Initial Registration Statement (or if later, the Additional Registration Statement), (ii) the effective date of the most recent post-effective amendment to a Registration Statement to become effective prior to the date of this Agreement and (iii) the date of the Company’s most recent Annual Report on Form 10-K filed with the Commission prior to the date of this Agreement, which will satisfy the provisions of Section 11(a) of the Act. “Availability Date” means the 40^th day after the end of the fourth fiscal quarter following the fiscal quarter that includes such effective date, except that, if such fourth fiscal quarter is the last quarter of the Company’s fiscal year, “Availability Date” means the 60^th day after the end of such fourth fiscal quarter.

(e) The Company will furnish to the Representatives copies of each Registration Statement, including all exhibits, any related preliminary prospectus, any related preliminary prospectus supplement, the Prospectus and all amendments and supplements to such documents, in each case as soon as available and in such quantities as Credit Suisse reasonably requests. The Company will pay the expenses of printing and distributing to the Underwriters all such documents.

(f) The Company will arrange for the qualification of the Offered Securities for sale under the laws of such jurisdictions as Credit Suisse designates and will continue such qualifications in effect so long as required for the distribution.

(g) The Company will pay all expenses incident to the performance of its obligations under this Agreement, for any filing fees and other expenses (including fees and disbursements of counsel) incurred in connection with qualification of the Offered Securities for sale under the laws of such jurisdictions as Credit Suisse designates and the printing of memoranda relating thereto, for any travel expenses of the Company’s officers and employees and any other expenses of the Company in connection with attending or hosting meetings with prospective purchasers of the Offered Securities, for expenses incurred in distributing preliminary prospectuses and the Prospectus (including any amendments and supplements thereto) to the Underwriters and for expenses incurred for preparing, printing and distributing any Issuer Free Writing Prospectuses to investors or prospective investors.

(h) For the period specified below (the “Lock-Up Period”), the Company will not offer, sell, contract to sell, pledge or otherwise dispose of, directly or indirectly, or file with the Commission a registration statement under the Act relating to, any additional shares of its Securities or securities

convertible into or exchangeable or exercisable for any shares of its Securities, or publicly disclose the intention to make any such offer, sale, pledge, disposition or filing, without the prior written consent of Credit Suisse, except issuances of Securities pursuant to the conversion or exchange of convertible or exchangeable securities or the exercise of warrants or options, in each case outstanding on the date hereof, grants of employee stock options pursuant to the terms of a plan as in effect on the date hereof, issuances of Securities pursuant to the exercise of such options or issuances of Securities pursuant to the Company’s employee stock purchase plan in effect on the date hereof. The initial Lock-Up Period will commence on the date hereof and will continue and include the date 90 days after the date hereof or such earlier date that Credit Suisse consents to in writing.

6. Free Writing Prospectuses. The Company represents and agrees that, unless it obtains the prior consent of Credit Suisse, and each Underwriter represents and agrees that, unless it obtains the prior consent of the Company and Credit Suisse, it has not made and will not make any offer relating to the Offered Securities that would constitute an Issuer Free Writing Prospectus, or that would otherwise constitute a “free writing prospectus,” as defined in Rule 405, required to be filed with the Commission. Any such free writing prospectus consented to by the Company and Credit Suisse is hereinafter referred to as a “Permitted Free Writing Prospectus.” The Company represents that it has treated and agrees that it will treat each Permitted Free Writing Prospectus as an “issuer free writing prospectus,” as defined in Rule 433, and has complied and will comply with the requirements of Rules 164 and 433 applicable to any Permitted Free Writing Prospectus, including timely Commission filing where required, legending and record keeping.

7. Conditions of the Obligations of the Underwriters. The obligations of the several Underwriters to purchase and pay for the Firm Securities on the First Closing Date and the Optional Securities to be purchased on each Optional Closing Date will be subject to the accuracy of the representations and warranties on the part of the Company herein, to the accuracy of the statements of Company officers made pursuant to the provisions hereof, to the performance by the Company of its obligations hereunder and to the following additional conditions precedent:

(a) On or prior to the date of this Agreement, the Representatives shall have received a letter, dated the date of delivery thereof, of Ernst & Young LLP confirming that they are independent public accountants within the meaning of the Act and the applicable published Rules and Regulations thereunder and stating to the effect that:

(i) in their opinion the financial statements audited by them and included or incorporated by reference in the Registration Statement and the General Disclosure Package comply as to form in all material respects with the applicable accounting requirements of the Act and the related published Rules and Regulations;

(ii) on the basis of a reading of the latest available interim financial statements of the Company, inquiries of officials of the Company who have responsibility for financial and accounting matters and other specified procedures, nothing came to their attention that caused them to believe that:

(A) at the date of the latest available balance sheet read by such accountants, or at a subsequent specified date not more than three business days prior to the date of this Agreement, there was any change in the capital stock or any increase in short-term debt or long-term debt of the Company or, at the date of the latest available balance sheet read by such accountants, there was any decrease in net current assets, as compared with amounts shown on the latest balance sheet included in the General Disclosure Package; or

(B) for the period from the closing date of the latest income statement included in the General Disclosure Package to the closing date of the latest available income statement read by such accountants there were any decreases, as compared with the corresponding period of the previous year and with the period of corresponding length ended the date of the latest income statement included in the General Disclosure Package, in net revenues, or in the total or per share amounts of net loss;

except in all cases set forth in clauses (A) and (B) above for changes, increases or decreases which the General Disclosure Package discloses have occurred or may occur or which are described in such letter; and

(iii) they have compared specified dollar amounts (or percentages derived from such dollar amounts) and other financial information contained in the Registration Statement, each Issuer Free Writing Prospectus (other than any Issuer Free Writing Prospectus that is an “electronic road show,” as defined in Rule 433(h)) and the General Disclosure Package (in each case to the extent that such dollar amounts, percentages and other financial information are derived from the general accounting records of the Company subject to the internal controls of the Company’s accounting system or are derived directly from such records by analysis or computation) with the results obtained from inquiries, a reading of such general accounting records and other procedures specified in such letter and have found such dollar amounts, percentages and other financial information to be in agreement with such results, except as otherwise specified in such letter.

All financial statements and schedules included in material incorporated by reference into the Prospectus or the General Disclosure Package shall be deemed included in the Prospectus or the General Disclosure Package for purposes of this subsection.

(b) The Prospectus shall have been filed with the Commission in accordance with the Rules and Regulations and Section 5(a) of this Agreement. Prior to such Closing Date, no stop order suspending the effectiveness of a Registration Statement shall have been issued and no proceedings for that purpose shall have been instituted or, to the knowledge of the Company or the Representatives, shall be contemplated by the Commission.

(c) Subsequent to the execution and delivery of this Agreement, there shall not have occurred (i) any change, or any development or event involving a prospective change, in the condition (financial or other), business, properties or results of operations of the Company which, in the judgment of a majority in interest of the Underwriters including the Representatives, is material and adverse and makes it impractical or inadvisable to proceed with completion of the public offering or the sale of and payment for the Offered Securities; (ii) any downgrading in the rating of any debt securities of the Company by any “nationally recognized statistical rating organization” (as defined for purposes of Rule 436(g) under the Act), or any public announcement that any such organization has under surveillance or review its rating of any debt securities of the Company (other than an announcement with positive implications of a possible upgrading, and no implication of a possible downgrading, of such rating) or any announcement that the Company has been placed on negative outlook; (iii) any change in U.S. or international financial, political or economic conditions or currency exchange rates or exchange controls as would, in the judgment of a majority in interest of the Underwriters including the Representatives, be likely to prejudice materially the success of the proposed issue, sale or distribution of the Offered Securities, whether in the primary market or in respect of dealings in the secondary market;

(iv) any material suspension or material limitation of trading in securities generally on the New York Stock Exchange, or any setting of minimum prices for trading on such exchange; (v) or any suspension of trading of any securities of the Company on any exchange or in the over-the-counter market; (vi) any banking moratorium declared by U.S. Federal or New York authorities; (vii) any major disruption of settlements of securities or clearance services in the United States or (viii) any attack on, outbreak or escalation of hostilities or act of terrorism involving the United States, any declaration of war by Congress or any other national or international calamity or emergency if, in the judgment of a majority in interest of the Underwriters including the Representatives, the effect of any such attack, outbreak, escalation, act, declaration, calamity or emergency makes it impractical or inadvisable to proceed with completion of the public offering or the sale of and payment for the Offered Securities.

(d) The Representatives shall have received an opinion, dated such Closing Date, of Cooley Godward LLP, counsel for the Company, to the effect that:

(i) The Company has been duly incorporated and is validly existing as a corporation in good standing under the laws of the State of Delaware, with requisite corporate power to own its properties and conduct its business as described in the General Disclosure Package and the Prospectus;

(ii) The Company is duly qualified to do business as a foreign corporation and is in good standing under the laws of the State of California;

(iii) The Offered Securities to be purchased by the Underwriters from the Company on such Closing Date have been duly authorized and, when issued by the Company and paid for by the Underwriters pursuant to this Agreement, will be validly issued, fully paid and nonassessable; the Offered Securities are consistent with the information in the General Disclosure Package and conform to the description thereof contained in the Prospectus under the heading “Description of Capital Stock”;

(iv) The statements in the General Disclosure Package and the Prospectus under the heading “Description of Capital Stock,” insofar as such statements purport to summarize legal matters, agreements or documents discussed therein, fairly present, to the extent required by the Act and the rules thereunder, in all material respects, such legal matters, agreements or documents;

(v) The statements in the General Disclosure Package and the Prospectus under the heading “U.S. Federal Income Tax Consequences to Non-United States Holders,” to the extent that they constitute matters of U.S. federal income tax laws and legal conclusions with respect thereto, are accurate in all material respects;

(vi) The holders of outstanding shares of capital stock of the Company are not entitled to preemptive or, to such counsel’s knowledge, rights of first refusal or other similar rights to subscribe for the Offered Securities;

(vii) To such counsel’s knowledge, except as set forth in the General Disclosure Package and the Prospectus, no holders of securities of the Company have rights to require the registration under the Act of resales of such securities;

(viii) The Company is not, and, after giving effect to the offering and sale of the Offered Securities and the application of the proceeds thereof as described in the General Disclosure Package and the Prospectus, will not be, an “investment company” as defined in the Investment Company Act of 1940, as amended;

(ix) No consent, approval, authorization or filing with, or order of, any court or governmental agency or body in the United States having jurisdiction over the Company is

required for the consummation by the Company of the transactions contemplated by this Agreement, except such as have been obtained under the Act and except such as may be required under the blue sky laws of any jurisdiction in connection with the purchase and distribution of the Securities by the Underwriters in the manner contemplated by this Agreement and in the General Disclosure Package and the Prospectus, or under the bylaws, rules and regulations of the NASD;

(x) The issue and sale of the Securities pursuant to the Agreement will not result in a breach or violation of (A) the charter or bylaws of the Company, (B) the terms of any contract identified on Exhibit A to such opinion (a “Material Contract”), or (C) any statute, law, rule, or regulation which, in such counsel’s experience, is typically applicable to transactions of the nature contemplated by the Agreement and is applicable to the Company, or any order, writ, judgment, injunction, decree or award that has been entered into against the Company and of which such counsel is aware;

(xi) The Registration Statement has become effective under the Act, and, to the best of the knowledge of such counsel, no stop order suspending the effectiveness of the Registration Statement has been issued and no proceedings for that purpose have been instituted or are pending or overtly threatened;

(xii) The Registration Statement and the Prospectus (other than the financial statements and notes thereto or other financial or statistical data derived therefrom, as to which such counsel expresses no opinion) comply as to form in all material respects with the applicable requirements of the Act and the rules thereunder;

(xiii) To such counsel’s knowledge, there is (A) no action, suit or proceeding by or before any court or other governmental agency, authority or body or any arbitrator pending or overtly threatened against the Company or its properties by a third party of a character required to be disclosed in the General Disclosure Package or the Prospectus that is not disclosed in the General Disclosure Package or the Prospectus as required by the Act and the rules thereunder and (B) no indenture, contract, lease, mortgage, deed of trust, note agreement, loan or other agreement or instrument of a character required to be filed as an exhibit to the Registration Statement, which is not filed as required by the Act and the rules thereunder; and

(xiv) This Agreement has been duly authorized by all necessary corporate action on the part of the Company and has been duly executed and delivered by the Company.

In addition, such counsel shall state that, in connection with the preparation of the Registration Statements, the General Disclosure Package and the Prospectus, such counsel has participated in conferences with officers and other representatives of the Company and with its independent registered public accountants, as well as with representatives of the Underwriters and their counsel. At such conferences, the contents of the Registration Statements, the General Disclosure Package, the Prospectus and related matters were discussed. Such counsel has not independently verified, and accordingly are not confirming and assume no responsibility for, the accuracy, completeness or fairness of the statements contained in the Registration Statements, the General Disclosure Package or the Prospectus (except to the extent of the opinions set forth in paragraphs (iv) and (v)). On the basis of the foregoing, no facts have come to such counsel’s attention that have caused such counsel to believe that (i) the Registration Statements (except as to the financial statements and schedules, related notes and other financial data and statistical data derived therefrom, as to which such counsel expresses no comment), at the date and time that the Registration Statements became effective, contained any untrue statement of a material fact or omitted to

state a material fact required to be stated therein or necessary to make the statements therein not misleading, (ii) the Prospectus (except as to the financial statements and schedules, related notes and other financial data and statistical data derived therefrom, as to which such counsel expresses no comment) as of its date or the date hereof contained or contains any untrue statement of a material fact or omitted or omits to state a material fact necessary in order to make the statements therein, in light of the circumstances under which they were made, not misleading, or (iii) the General Disclosure Package (except as to the financial statements and schedules, related notes and other financial data and statistical data derived therefrom, as to which such counsel expresses no comment) as of the Applicable Time contained any untrue statement of a material fact or omitted to state a material fact necessary in order to make the statements therein, in light of the circumstances under which they were made, not misleading.

(e) The Representatives shall have received an opinion, dated such Closing Date, of Morrison & Foerster, counsel for the Company, to the effect that:

(i) Such counsel has considered the statements contained in the Patent Information, although such counsel has not independently verified the accuracy, completeness or fairness of such statements. Based upon and subject to the foregoing, nothing has come to such counsel’s attention that leads such counsel to believe that the Patent Information as of the Applicable Time contained, or as of the date of the Prospectus or the date of such opinion, contained or contains, an untrue statement of a material fact or omitted or omits to state a material fact required to be stated therein or necessary to make the statements therein not misleading in light of the circumstances under which they were made. As of the Applicable Time and the date of such opinion, such counsel has no reason to believe that the Patent Information is not in all material respects a fair and accurate summary of the legal matters, documents and proceedings relating thereto.

“Patent Information” means the portions of the Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2005 under “Item 1. Business – Product Pipeline” and the portions of the preliminary prospectus and the Prospectus entitled “Risk Factors” under the headings “Any inability to protect our proprietary technologies could significantly harm our business and ability to successfully commercialize product candidates” on pages S-10 – S-11; “Patents related to one of our product candidates…” on page S-11-S-12; and “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” on pages S-12 – S-13.

(ii) Such counsel has also reviewed the two paragraphs headed under “Intellectual Property” on page 8 of the Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2005. To such counsel’s knowledge, there are no inaccurate statements in those paragraphs.

(iii) To such counsel’s knowledge, other than as disclosed in the preliminary prospectus and the Prospectus, there are no pending or threatened actions, suits, or proceedings against or affecting any patents, patent licenses, or intellectual property rights owned by or licensed to the Company in connection with the epothilone D program or the HSP 90 inhibitor program.

(iv) To such counsel’s knowledge, the Company has not received any demand letter or other written notice from any individual or entity alleging that the Company is infringing intellectual property rights with respect to the epothilone D program or the HSP 90 inhibitor program and, except as described in preliminary prospectus and the Prospectus, the Company is unaware of any United States patent that the Company currently infringes or would infringe if it makes or sells products developed under the epothilone D program or the HSP 90 inhibitor program.

In addition, such opinion shall also include a brief summary of the intellectual property position of the Company’s epothilone D program and HSP 90 inhibitor program as understood by such counsel at the time such opinion is rendered and in the form agreed to by the Representatives on or prior to the date hereof.

(f) The Representatives shall have received from Wilson Sonsini Goodrich & Rosati, P.C., counsel for the Underwriters, such opinion or opinions, dated the Closing Date, with respect to the incorporation of the Company, the validity of the Offered Securities, the Registration Statements, the Prospectus and other related matters as the Representatives may require, and the Company shall have furnished to such counsel such documents as they request for the purpose of enabling them to pass upon such matters.

(g) The Representatives shall have received a certificate, dated such Closing Date, of the President or any Vice President and a principal financial or accounting officer of the Company in which such officers, to the best of their knowledge after reasonable investigation, shall state that: the representations and warranties of the Company in this Agreement are true and correct; the Company has complied with all agreements and satisfied all conditions on its part to be performed or satisfied hereunder at or prior to such Closing Date; no stop order suspending the effectiveness of any Registration Statement has been issued and no proceedings for that purpose have been instituted or are contemplated by the Commission; the Additional Registration Statement (if any) satisfying the requirements of subparagraphs (1) and (3) of Rule 462(b) was filed pursuant to Rule 462(b), including payment of the applicable filing fee in accordance with Rule 111(a) or (b) under the Act, prior to the Applicable Time; and, subsequent to the date of the most recent financial statements in the General Disclosure Package, there has been no material adverse change, nor any development or event involving a prospective material adverse change, in the condition (financial or other), business, properties or results of operations of the Company except as set forth in the General Disclosure Package or as described in such certificate.

(h) The Representatives shall have received a letter, dated such Closing Date, of Ernst & Young LLP which meets the requirements of subsection (a) of this Section, except that the specified date referred to in such subsection will be a date not more than three days prior to such Closing Date for the purposes of this subsection.

(i) On or prior to the date of this Agreement, the Representatives shall have received lockup letters from (i) each of the executive officers and directors and of the Company and (ii) Daniel V. Santi.

The Company will furnish the Representatives with such conformed copies of such opinions, certificates, letters and documents as the Representatives reasonably request. Credit Suisse may in its sole discretion waive on behalf of the Underwriters compliance with any conditions to the obligations of the Underwriters hereunder, whether in respect of an Optional Closing Date or otherwise.

8. Indemnification and Contribution. (a) The Company will indemnify and hold harmless each Underwriter, its partners, members, directors, officers, affiliates and each person, if any, who controls such Underwriter within the meaning of Section 15 of the Act, against any losses, claims, damages or liabilities, joint or several, to which such Underwriter may become subject, under the Act or otherwise, insofar as such losses, claims, damages or liabilities (or actions in respect thereof) arise out of or are based upon any untrue statement or alleged untrue statement of any material fact contained in any part of any Registration Statement at any time, any Statutory Prospectus as of any time, the Prospectus, any Issuer Free Writing Prospectus, or arise out of or are based upon the omission or alleged omission to state therein a material fact

required to be stated therein or necessary to make the statements therein not misleading, and will reimburse each Underwriter for any legal or other expenses reasonably incurred by such Underwriter in connection with investigating or defending any such loss, claim, damage, liability or action as such expenses are incurred; provided, however, that the Company will not be liable in any such case to the extent that any such loss, claim, damage or liability arises out of or is based upon an untrue statement or alleged untrue statement in or omission or alleged omission from any of such documents in reliance upon and in conformity with written information furnished to the Company by any Underwriter through the Representatives specifically for use therein, it being understood and agreed that the only such information furnished by any Underwriter consists of the information described as such in subsection (b) below; and provided, further, that with respect to any untrue statement or alleged untrue statement in or omission or alleged omission from any preliminary prospectus the indemnity agreement contained in this subsection (a) shall not inure to the benefit of any Underwriter from whom the person asserting any such losses, claims, damages or liabilities purchased the Offered Securities concerned, to the extent that a prospectus relating to such Offered Securities was required to be delivered by such Underwriter under the Act in connection with such purchase and any such loss, claim, damage or liability of such Underwriter results from the fact that there was not sent or given to such person, at or prior to the written confirmation of the sale of such Offered Securities to such person, a copy of the Prospectus (exclusive of material incorporated by reference) if the Company had previously furnished copies thereof to such Underwriter.

(b) Each Underwriter will severally and not jointly indemnify and hold harmless the Company, its directors and officers and each person, if any who controls the Company within the meaning of Section 15 of the Act, against any losses, claims, damages or liabilities to which the Company may become subject, under the Act or otherwise, insofar as such losses, claims, damages or liabilities (or actions in respect thereof) arise out of or are based upon any untrue statement or alleged untrue statement of any material fact contained in any part of any Registration Statement at any time, any Statutory Prospectus as of any time, the Prospectus, any Issuer Free Writing Prospectus, or arise out of or are based upon the omission or the alleged omission to state therein a material fact required to be stated therein or necessary to make the statements therein not misleading, in each case to the extent, but only to the extent, that such untrue statement or alleged untrue statement or omission or alleged omission was made in reliance upon and in conformity with written information furnished to the Company by such Underwriter through the Representatives specifically for use therein, and will reimburse any legal or other expenses reasonably incurred by the Company, its directors and officers and each person, if any, who controls the Company within the meaning of Section 15 of the Act in connection with investigating or defending any such loss, claim, damage, liability or action as such expenses are incurred, it being understood and agreed that the only such information furnished by any Underwriter consists of the following information in the Prospectus furnished on behalf of each Underwriter: the concession and reallowance figures appearing in the fourth paragraph under the caption “Underwriting” and the information contained in the eleventh paragraph under the caption “Underwriting”.

(c) Promptly after receipt by an indemnified party under this Section of notice of the commencement of any action, such indemnified party will, if a claim in respect thereof is to be made against the indemnifying party under subsection (a) or (b) above, notify the indemnifying party of the commencement thereof; but the failure to notify the indemnifying party shall not relieve it from any liability that it may have under subsection (a) or (b) above except to the extent that it has been materially prejudiced (through the forfeiture of substantive rights or defenses) by such failure; and provided further that the failure to notify the indemnifying party shall not relieve it from any liability that it may have to an indemnified party otherwise than under subsection (a) or (b) above. In case any such action is brought against any indemnified party and it notifies the indemnifying party of the commencement thereof, the indemnifying party will be entitled to participate therein and, to the extent that it may wish, jointly with any other indemnifying party similarly notified, to assume the defense thereof, with counsel satisfactory to such indemnified party (who shall not, except with the consent of the indemnified party, be counsel to the indemnifying party), and after notice from the indemnifying party to such indemnified party of its election so to assume the defense thereof, the indemnifying party will not be liable to such indemnified party under this Section for any legal or other expenses subsequently incurred by such indemnified party in connection with the defense thereof other than

reasonable costs of investigation. No indemnifying party shall, without the prior written consent of the indemnified party, effect any settlement of any pending or threatened action in respect of which any indemnified party is or could have been a party and indemnity could have been sought hereunder by such indemnified party unless such settlement (i) includes an unconditional release of such indemnified party from all liability on any claims that are the subject matter of such action and (ii) does not include a statement as to, or an admission of, fault, culpability or a failure to act by or on behalf of an indemnified party.

(d) If the indemnification provided for in this Section is unavailable or insufficient to hold harmless an indemnified party under subsection (a) or (b) above, then each indemnifying party shall contribute to the amount paid or payable by such indemnified party as a result of the losses, claims, damages or liabilities referred to in subsection (a) or (b) above (i) in such proportion as is appropriate to reflect the relative benefits received by the Company on the one hand and the Underwriters on the other from the offering of the Securities or (ii) if the allocation provided by clause (i) above is not permitted by applicable law, in such proportion as is appropriate to reflect not only the relative benefits referred to in clause (i) above but also the relative fault of the Company on the one hand and the Underwriters on the other in connection with the statements or omissions which resulted in such losses, claims, damages or liabilities as well as any other relevant equitable considerations. The relative benefits received by the Company on the one hand and the Underwriters on the other shall be deemed to be in the same proportion as the total net proceeds from the offering (before deducting expenses) received by the Company bear to the total underwriting discounts and commissions received by the Underwriters. The relative fault shall be determined by reference to, among other things, whether the untrue or alleged untrue statement of a material fact or the omission or alleged omission to state a material fact relates to information supplied by the Company or the Underwriters and the parties’ relative intent, knowledge, access to information and opportunity to correct or prevent such untrue statement or omission. The amount paid by an indemnified party as a result of the losses, claims, damages or liabilities referred to in the first sentence of this subsection (d) shall be deemed to include any legal or other expenses reasonably incurred by such indemnified party in connection with investigating or defending any action or claim which is the subject of this subsection (d). Notwithstanding the provisions of this subsection (d), no Underwriter shall be required to contribute any amount in excess of the amount by which the total price at which the Securities underwritten by it and distributed to the public were offered to the public exceeds the amount of any damages which such Underwriter has otherwise been required to pay by reason of such untrue or alleged untrue statement or omission or alleged omission. No person guilty of fraudulent misrepresentation (within the meaning of Section 11(f) of the Act) shall be entitled to contribution from any person who was not guilty of such fraudulent misrepresentation. The Underwriters’ obligations in this subsection (d) to contribute are several in proportion to their respective underwriting obligations and not joint.

(e) The obligations of the Company under this Section shall be in addition to any liability which the Company may otherwise have and shall extend, upon the same terms and conditions, to each person, if any, who controls any Underwriter within the meaning of the Act; and the obligations of the Underwriters under this Section shall be in addition to any liability which the respective Underwriters may otherwise have and shall extend, upon the same terms and conditions, to each director of the Company, to each officer of the Company who has signed a Registration Statement and to each person, if any, who controls the Company within the meaning of the Act.

9. Default of Underwriters. If any Underwriter or Underwriters default in their obligations to purchase Offered Securities hereunder on either the First Closing Date or any Optional Closing Date and the aggregate number of shares of Offered Securities that such defaulting Underwriter or Underwriters agreed but failed to purchase does not exceed 10% of the total number of shares of Offered Securities that the Underwriters are obligated to purchase on such Closing Date, Credit Suisse may make arrangements satisfactory to the Company for the purchase of such Offered Securities by other persons, including any of the Underwriters, but if no such arrangements are made by such Closing Date, the non-defaulting Underwriters shall be obligated severally, in proportion to their respective commitments hereunder, to purchase the Offered Securities that such defaulting Underwriters agreed but failed to purchase on such

Closing Date. If any Underwriter or Underwriters so default and the aggregate number of shares of Offered Securities with respect to which such default or defaults occur exceeds 10% of the total number of shares of Offered Securities that the Underwriters are obligated to purchase on such Closing Date and arrangements satisfactory to Credit Suisse and the Company for the purchase of such Offered Securities by other persons are not made within 36 hours after such default, this Agreement will terminate without liability on the part of any non-defaulting Underwriter or the Company, except as provided in Section 11 (provided that if such default occurs with respect to Optional Securities after the First Closing Date, this Agreement will not terminate as to the Firm Securities or any Optional Securities purchased prior to such termination). As used in this Agreement, the term “Underwriter” includes any person substituted for an Underwriter under this Section. Nothing herein will relieve a defaulting Underwriter from liability for its default.

10. Absence of Fiduciary Relationship. The Company acknowledges and agrees that:

(a) the Representatives have been retained solely to act as underwriters in connection with the sale of the Offered Securities and that no fiduciary, advisory or agency relationship between the Company and the Representatives have been created in respect of any of the transactions contemplated by this Agreement or the Prospectus, irrespective of whether the Representatives have advised or are advising the Company on other matters;

(b) the price of the Offered Securities set forth in this Agreement was established by the Company following discussions and arms-length negotiations with the Representatives and the Company is capable of evaluating and understanding and understands and accepts the terms, risks and conditions of the transactions contemplated by this Agreement;

(c) the Company has been advised that the Representatives and their affiliates are engaged in a broad range of transactions which may involve interests that differ from those of the Company and that the Representatives have no obligation to disclose such interests and transactions to the Company by virtue of any fiduciary, advisory or agency relationship; and

(d) the Company waives, to the fullest extent permitted by law, any claims it may have against the Representatives for breach of fiduciary duty or alleged breach of fiduciary duty and agrees that the Representatives shall have no liability (whether direct or indirect) to the Company in respect of such a fiduciary duty claim or to any person asserting a fiduciary duty claim on behalf of or in right of the Company, including stockholders, employees or creditors of the Company.

11. Survival of Certain Representations and Obligations. The respective indemnities, agreements, representations, warranties and other statements of the Company or its officers and of the several Underwriters set forth in or made pursuant to this Agreement will remain in full force and effect, regardless of any investigation, or statement as to the results thereof, made by or on behalf of any Underwriter, the Company or any of their respective representatives, officers or directors or any controlling person, and will survive delivery of and payment for the Offered Securities. If this Agreement is terminated pursuant to Section 9 or if for any reason the purchase of the Offered Securities by the Underwriters is not consummated, the Company shall remain responsible for the expenses to be paid or reimbursed by it pursuant to Section 5 and the respective obligations of the Company and the Underwriters pursuant to Section 8 shall remain in effect, and if any Offered Securities have been purchased hereunder the representations and warranties in Section 2 and all obligations under Section 5 shall also remain in effect. If the purchase of the Offered Securities by the Underwriters is not consummated for any reason other than solely because of the termination of this Agreement pursuant to Section 9 or the occurrence of any event specified in clause (iii), (iv), (vi), (vii) or (viii) of Section 7(c), the Company will reimburse the Underwriters for all out-of-pocket expenses (including fees and disbursements of counsel) reasonably incurred by them in connection with the offering of the Offered Securities.

12. Notices. All communications hereunder will be in writing and, if sent to the Underwriters, will be mailed, delivered or faxed and confirmed to the Representatives c/o Credit Suisse Securities (USA) LLC, Eleven Madison Avenue, New York, N.Y. 10010-3629, Attention: IBD Legal (Fax No.: (212) 325-4296), or, if sent to the Company, will be mailed, delivered or faxed and confirmed to it at 3832 Bay Center Place, Hayward, California 94545, Attention: Chief Financial Officer (Fax No.: (510) 731-5118); provided, however, that any notice to an Underwriter pursuant to Section 8 will be mailed, delivered or faxed and confirmed to such Underwriter.

13. Successors. This Agreement will inure to the benefit of and be binding upon the parties hereto and their respective successors and the officers and directors and controlling persons referred to in Section 8, and no other person will have any right or obligation hereunder.

14. Representation of Underwriters. The Representatives will act for the several Underwriters in connection with the transactions contemplated by this Agreement, and any action under this Agreement taken by the Representatives will be binding upon all the Underwriters.

15. Counterparts. This Agreement may be executed in any number of counterparts, each of which shall be deemed to be an original, but all such counterparts shall together constitute one and the same Agreement.

16. Applicable Law. This Agreement shall be governed by, and construed in accordance with, the laws of the State of New York, without regard to principles of conflicts of laws.

The Company hereby submits to the non-exclusive jurisdiction of the Federal and state courts in the Borough of Manhattan in The City of New York in any suit or proceeding arising out of or relating to this Agreement or the transactions contemplated hereby.

If the foregoing is in accordance with the Representatives’ understanding of our agreement, kindly sign and return to the Company one of the counterparts hereof, whereupon it will become a binding agreement between the Company and the several Underwriters in accordance with its terms.


Very truly yours,

KOSAN BIOSCIENCES INCORPORATED

By:		/s/ MARGARET A. HORN
Name:		Margaret A. Horn
Title:		Senior Vice President, Legal and Corporate Development, General Counsel and Secretary


The foregoing Underwriting Agreement is hereby confirmed and accepted as of the date first above written.

CREDIT SUISSE SECURITIES (USA) LLC COWEN & CO., LLC

		Acting on behalf of themselves and as the Representatives of the several Underwriters

By CREDIT SUISSE SECURITIES (USA) LLC

By:		/s/ JENNIFER JARRETT
Name:		Jennifer Jarrett
Title:		Director


Underwriter		Number of Firm Securities
Credit Suisse Securities (USA) LLC		3,500,000
Cowen & Co., LLC		1,500,000

Total		5,000,000