SC 13D/A

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13D/A

Under the Securities Exchange Act of 1934
(Amendment No. 5)*

Idenix Pharmaceuticals, Inc.

(Name of Issuer)

Common Stock

(Title of Class of Securities)

45166R 20 4

(CUSIP Number)

Matt Owens, Esq.	Adam H. Golden, Esq.
Novartis Pharma AG	Kaye Scholer LLP
Lichtstrasse 35	425 Park Avenue
CH-4056 Basel, Switzerland	New York, New York 10022
+41 61 324 8323	(212) 259-8000

(Name, Address and Telephone Number of Person
Authorized to Receive Notices and Communications)

April 6, 2011

(Date of Event Which Requires Filing of this Statement)

If the filing person has previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because of Rule 13d-1(e), 13d-1(f) or 13d-1(g), check the following box. o

Note: Schedules filed in paper format shall include a signed original and five copies of the schedule, including all exhibits. See Rule 13d-7 for other parties to whom copies are to be sent.

The information required on this cover page shall not be deemed to be "filed" for the purpose of Section 18 of the Securities Exchange Act of 1934 ("Exchange Act") or otherwise subject to the liabilities of that section of the Exchange Act but shall be subject to all other provisions of the Exchange Act (however, see the Notes).

	1	Name of Reporting Person I.R.S. Identification Nos. of Above Person (Entities Only)* NOVARTIS AG
	2	Check the Appropriate Box if a Member of a Group*
		(a)			o
		(b)			o
	3	SEC Use Only
	4	Source of Funds* AF
	5	Check Box if Disclosure of Legal Proceedings Is Required Pursuant to Items 2(d) or 2(e)  o
	6	Citizenship or Place of Organization SWITZERLAND
Number of Shares Beneficially Owned by Each Reporting Person With			7	Sole Voting Power 0
			8	Shared Voting Power 33,185,191
			9	Sole Dispositive Power 0
			10	Shared Dispositive Power 33,185,191
	11	Aggregate Amount Beneficially Owned by Each Reporting Person 33,185,191
	12	Check Box if the Aggregate Amount in Row (11) Excludes Certain Shares*
	13	Percent of Class Represented by Amount in Row (11) 36.0%
	14	Type of Reporting Person* CO

*SEE INSTRUCTIONS

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	1	Name of Reporting Person I.R.S. Identification Nos. of Above Person (Entities Only)* NOVARTIS PHARMA AG
	2	Check the Appropriate Box if a Member of a Group*
		(a)			o
		(b)			o
	3	SEC Use Only
	4	Source of Funds* WC
	5	Check Box if Disclosure of Legal Proceedings Is Required Pursuant to Items 2(d) or 2(e)  o
	6	Citizenship or Place of Organization SWITZERLAND
Number of Shares Beneficially Owned by Each Reporting Person With			7	Sole Voting Power 0
			8	Shared Voting Power 33,185,191
			9	Sole Dispositive Power 0
			10	Shared Dispositive Power 33,185,191
	11	Aggregate Amount Beneficially Owned by Each Reporting Person 33,185,191
	12	Check Box if the Aggregate Amount in Row (11) Excludes Certain Shares*  o
	13	Percent of Class Represented by Amount in Row (11) 36.0%
	14	Type of Reporting Person* CO

*SEE INSTRUCTIONS

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Item 1.	Security and Issuer
This Amendment No. 5 to Schedule 13D (this “Amendment”) amends the initial statement on Schedule 13D filed with the Securities and Exchange Commission on July 27, 2004, as previously amended on September 7, 2005, November 2, 2005, May 20, 2009 and June 18, 2010 (the “Original Schedule 13D”) relating to Common Stock (the “Common Stock”), par value $0.001 per share, of Idenix Pharmaceuticals, Inc., a Delaware corporation (the “Company”).  The address of the Company’s principal executive offices is 60 Hampshire Street, Cambridge, Massachusetts 02139. Except as otherwise described herein, the information contained in the Original Schedule 13D remains in effect.  Capitalized terms used but not defined in this Amendment shall have the respective meanings set forth with respect thereto in the Original Schedule 13D.
Item 2.	Identity and Background
No change except as described below. (a) - (c) and (f) The name, business address, present principal occupation or employment and citizenship of the executive officers and members of the Board of Directors of each of the Reporting Persons is set forth on Schedule I hereto and is incorporated herein by reference. (d) and (e) Neither the Reporting Persons nor, to the best knowledge of each of them, any of the persons listed on Schedule I hereto with respect to each such Reporting Person during the last five years, (i) has been convicted in a criminal proceeding (excluding traffic violations or similar misdemeanors) or (ii) was a party to a civil proceeding of a judicial or administrative body of competent jurisdiction and as a result of such proceeding was or is subject to a judgment, decree or final order enjoining future violations of, or prohibiting or mandating activities subject to, federal or state securities laws or finding any violation with respect to such laws.
Item 3.	Source and Amount of Funds or Other Consideration
No change except as described below. Novartis Pharma acquired the following shares of Common Stock pursuant to the exercise of stock subscription rights under the Stockholders Agreement: On August 13, 2010, Novartis Pharma acquired 36,048 shares of Common Stock for $2.41 per share or an aggregate of $86,875.68. On August 13, 2010, Novartis Pharma acquired 262 shares of Common Stock for $2.96 per share or an aggregate of $775.52. On August 13, 2010, Novartis Pharma acquired 91 shares of Common Stock for $3.04 per share or an aggregate of $276.64. On August 13, 2010, Novartis Pharma acquired 813 shares of Common Stock for $3.79 per share or an aggregate of $3,081.27. On November 26, 2010, Novartis Pharma acquired 14,234 shares of Common Stock for $2.41 per share or an aggregate of $34,303.94.

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On November 26, 2010, Novartis Pharma acquired 822 shares of Common Stock for $2.64 per share or an aggregate of $2,170.08. On November 26, 2010, Novartis Pharma acquired 44 shares of Common Stock for $2.96 per share or an aggregate of $130.24. On November 26, 2010, Novartis Pharma acquired 165 shares of Common Stock for $3.79 per share or an aggregate of $625.35. On November 26, 2010, Novartis Pharma acquired 1,332 shares of Common Stock for $5.11 per share or an aggregate of $6,806.52. On November 26, 2010, Novartis Pharma acquired 486 shares of Common Stock for $5.46 per share or an aggregate of $2,653.56. On February 25, 2011, Novartis Pharma acquired 17,270 shares of Common Stock for $2.41 per share or an aggregate of $41,620.70. On February 25, 2011, Novartis Pharma acquired 897 shares of Common Stock for $2.96 per share or an aggregate of $2,655.12. On February 25, 2011, Novartis Pharma acquired 860 shares of Common Stock for $3.79 per share or an aggregate of $3,259.40. On February 25, 2011, Novartis Pharma acquired 1,760 shares of Common Stock for $3.88 per share or an aggregate of $6,828.80. On February 25, 2011, Novartis Pharma acquired 206 shares of Common Stock for $5.11 per share or an aggregate of $1,052.66. On April 13, 2011, Novartis Pharma acquired 1,785,714 shares of Common Stock for $2.80 per share or an aggregate of $5,000,000, pursuant to a Concurrent Private Placement Stock Purchase Agreement (the “Stock Purchase Agreement”), by and between Novartis Pharma and the Company, dated as of April 8, 2011. The source of funds for these acquisitions was Novartis Pharma’s working capital.
Item 4.	Purpose of Transaction
No change except as described below. On April 8, 2011, the Company commenced a public offering of up to 21,056,500 shares of Common Stock, including shares that may be issued pursuant to an underwriter’s over-allotment option (the “Over-Allotment Option”) at a price per share of $2.80 (the “Public Offering”). Novartis Pharma has the right under the Stockholders Agreement to approve new issuances of Common Stock by the Company (subject to certain exceptions) and to purchase a pro rata portion of such new Common Stock.

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On April 6, 2011, the Company and Novartis Pharma entered into a General Waiver and Consent (the “Waiver and Consent”), pursuant to which Novartis Pharma approved the Public Offering.  The Waiver and Consent is conditioned on Novartis Pharma and its affiliates’ continuing to own thirty percent (30%) or more of the Voting Stock at all times prior to and upon consummation of the Public Offering.  In addition, Novartis Pharma exercised its purchase rights to acquire 1,785,714 shares of Common Stock for $2.80 per share or an aggregate of $5,000,000, pursuant to the Stock Purchase Agreement.  The acquisition of such shares was consummated concurrently with the consummation of the Public Offering on April 13, 2011. Under Section 3.1(e) of the Stockholders’ Agreement, the Company is obligated to use its reasonable best efforts to nominate for election as a director of the Company at least one Novartis designee so long as Novartis and its affiliates own 19.4% or more of the Voting Stock and at least two Novartis designees, so long as Novartis Pharma and its affiliates own thirty-five percent (35%) or more of the Voting Stock.  On April 6, 2011, pursuant to the Waiver and Consent, the Company, Novartis Pharma and the stockholders listed on Exhibit A thereto entered into an Amendment No. 1 to Stockholders’ Agreement, pursuant to which the percentage of the Voting Stock which Novartis Pharma and its affiliates must own in order to be entitled to nominate two designees to the Board of Directors of the Company was reduced from thirty five percent (35%) to thirty percent (30%). Novartis Pharma and the Company are parties to a Letter Agreement, dated March 21, 2003, as amended on January 28, 2009 (the “Letter”), pursuant to which, so long as Novartis Pharma and its affiliates own at least forty percent (40%) of the Common Stock, Novartis Pharma’s consent is required for the selection and appointment of the Company’s Chief Financial Officer. On April 6, 2011, pursuant to the Waiver and Consent, the Company and Novartis Pharma entered into an amendment to the Letter (the “Letter of Amendment”), pursuant to which the percentage of the Voting Stock which Novartis Pharma and its affiliates must own in order for Novartis Pharma to maintain its rights with respect to the selection and appointment of the Company’s Chief Financial Officer was reduced from forty percent (40%) to thirty percent (30%).  In addition, pursuant to the Letter and the Letter of Amendment, if Novartis Pharma and its affiliates own at least thirty percent (30%) of the Voting Stock, the Company is required to terminate the Chief Financial Officer, if in Novartis Pharma’s reasonable judgment, the Chief Financial Officer is not satisfactorily performing his or her duties. The descriptions of each of the Waiver and Consent, the Stock Purchase Agreement, Amendment No. 1 to the Stockholders Agreement and the Letter of Amendment are qualified by reference to the full text thereof, which is included as an exhibit hereto and incorporated herein by reference.
Item 5.	Interest in Securities of the Issuer
No change except as described below. (a) and (b) Novartis is the beneficial owner of 33,185,191 shares of Common Stock representing 36.0% of the outstanding shares of Common Stock (35.0% if the Over-Allotment Option is exercised), all of which shares are owned directly by Novartis Pharma. Novartis has shared power to vote or direct the vote and shared power to dispose or to direct the disposition of all shares of Common Stock beneficially owned by it.

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Novartis Pharma is the record and beneficial owner of 33,185,191 shares of Common Stock, representing 36.0% of the outstanding shares of Common Stock (35.0% if the Over-Allotment Option is exercised).  Novartis Pharma has shared power to vote or direct the vote and shared power to dispose or to direct the disposition of all shares of Common Stock beneficially owned by it. To the best knowledge of each of the Reporting Persons, none of the persons listed on Schedule I hereto with respect to such Reporting Person is the beneficial owner of any shares of Common Stock. (c) Except for the transactions described in Item 3 neither the Reporting Persons nor, to the best knowledge of each of the Reporting Persons, any of the persons listed on Schedule I has engaged in any transaction in the Common Stock in the past 60 days.
Item 6.	Contracts, Arrangements, Understandings or Relationships with Respect to Securities of the Issuer
Novartis Pharma has agreed in a lock-up agreement (the “Lock-Up Agreement”) entered into on April 6, 2011 with J.P. Morgan Securities LLC (“J.P. Morgan”), as underwriter of the Public Offering, not to (i) sell or otherwise dispose of or transfer, directly or indirectly, any shares of Common Stock or any securities convertible into, exercisable or exchangeable for Common Stock, (ii) enter into any hedging agreements with respect to the Common Stock or such other securities or (iii) demand registration of the Common Stock with the Securities and Exchange Commission, during a 60-day period after the date of the prospectus relating to the Public Offering.  Novartis Pharma is permitted to sell, transfer or hedge sooner, subject to certain conditions, in the following circumstances: ·with written consent of J.P. Morgan; ·as a bona fide gift, provided the donee agrees to be bound by the lock-up restriction; ·pursuant to the exercise of any option to purchase shares of Common Stock, provided that the underlying shares of Common Stock continues to be subject to the lock-up restriction; ·with respect to shares of Common Stock acquired in open market transactions after the completion of the Public Offering; and ·transfers to Affiliates (as defined in Rule 12b-2 of the Exchange Act) or transfers by or on behalf of any employee benefit plan maintained by Novartis Pharma or its Affiliates, provided the transferee agrees to be bound by the lock-up restriction. The description of the Lock-Up Agreement is qualified by reference to the full text thereof, which is included as an exhibit hereto and incorporated herein by reference. No other changes except as described in Item 4 of this Amendment.
Item 7.	Material to be Filed as Exhibits
No change except for the additional exhibits described below.

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Exhibit 1	General Waiver and Consent, dated as of April 6, 2011, by and between the Company and Novartis Pharma.
Exhibit 2	Amendment No. 1 to Stockholders’ Agreement, dated as of April 6, 2011, by and among the Company, Novartis Pharma and the stockholders listed in Exhibit A thereto, incorporated by reference to Exhibit 10.2 to the Current Report on Form 8-K filed by the Company on April 7, 2011.
Exhibit 3	Letter of Amendment, dated as of April 6, 2011, by and between the Company and Novartis Pharma, incorporated by reference to Exhibit 10.3 to the Current Report on Form 8-K filed by the Company on April 7, 2011.
Exhibit 4	Amendment No. 6 to Development, License and Commercialization Agreement, dated as of April 6, 2011, by and among the Company, Idenix (Cayman) Limited and Novartis Pharma, incorporated by reference to Exhibit 10.1 to the Current Report on Form 8-K filed by the Company on April 7, 2011.
Exhibit 5	Concurrent Private Placement Stock Purchase Agreement, dated as of April 8, 2011, by and between the Company and Novartis Pharma, incorporated by reference to Exhibit 1.1 to the Current Report on Form 8-K filed by the Company on April 11, 2011.
Exhibit 6	Lock-Up Agreement, dated as of April 6, 2011, by and between J.P. Morgan Securities LLC and Novartis Pharma.

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Signatures

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

Date:	April 20, 2011
		NOVARTIS AG
			By:	/s/ Jonathan Symonds
			Name:		Jonathan Symonds
			Title:		Chief Financial Officer
			By:	/s/ Thomas Werlen
			Name:		Thomas Werlen
			Title:		Group General Counsel
		NOVARTIS PHARMA AG
			By:	/s/ Tony Rosenberg
			Name:		Tony Rosenberg
			Title:		Head Partnering & Emerging Businesses
			By:	/s/ Matt Owens
			Name:		Matt Owens
			Title:		Senior Legal Counsel

SCHEDULE I - to Form 13-D/A

DIRECTORS AND EXECUTIVE OFFICERS OF

NOVARTIS AND NOVARTIS PHARMA

DIRECTORS AND EXECUTIVE OFFICERS OF NOVARTIS

The name, address, citizenship and present principal occupation or employment of each of the directors and executive officers of Novartis are set forth below. Unless otherwise indicated below, each occupation set forth opposite an individual’s name refers to employment with Novartis.

Name	Relationship to Novartis	Principal Occupation	Citizenship
Daniel L. Vasella, M.D	Chairman of the Board of Directors	Chairman of the Board of Directors	Swiss
Ulrich Lehner, Ph.D.	Vice Chairman of the Board of Directors	Member of Shareholder’s Committee of Henkel AG & Co. KGaA, manufacturer and vendor of consumer products; Henkelstrasse 67, 40191 Düsseldorf, Germany	German
William R. Brody, M.D., Ph.D.	Director	President of the Salk Institute for Biological Studies, 10010 North Torrey Pines Road, La Jolla, CA 92037, USA	American
Srikant Datar, Ph.D.	Director	Arthur Lowes Dickinson Professor at the Graduate School of Business Administration at Harvard University Harvard Business School, Soldiers Field, Boston, MA 02163, USA	American
Ann Fudge	Director	Member of the Board of General Electric Company, diversified technology, media, and financial services corporation; 3135 Easton Turnpike, Fairfield, CT 06828, USA	American

Pierre Landolt	Director	Chairman of the Sandoz Family Foundation; 85 Avenue Général-Guisan, CH-1009 Pully, Switzerland	Swiss
Enrico Vanni, Ph.D.	Director	Novartis International AGNovartis Campus, 4002 Basel Switzerland	Swiss
Andreas von Planta, Ph.D.	Director	Partner at the law firm of Lenz & Staehelin; Route de Chêne 30, CH- 1211 Geneva 17, Switzerland	Swiss
Dr. Ing. Wendelin Wiedeking	Director	Novartis International AGNovartis Campus, 4002 Basel Switzerland	German
Marjorie M. Yang	Director	Chairman of the Esquel Group, textile and apparel manufacturer; The Esquel Group, 12/F, Harbour Centre, 25 Harbour Road, Wanchai, Hong Kong, People’s Republic of China	Chinese
Rolf M. Zinkernagel, M.D.	Director	Institute of Experimental Immunology, University of Zurich, Schmelzbergstrasse 12, CH-8091 Zurich, Switzerland	Swiss
Jürgen Brokatzky-Geiger, Ph.D.	Member of the Executive Committee, Head of Human Resources	Member of the Executive Committee, Head of Human Resources	German
David Epstein	Member of the Executive Committee, Head of Novartis Pharmaceuticals Division	Member of the Executive Committee, Head of Novartis Pharmaceuticals Division	American
Mark C. Fishman, M.D.	Member of the Executive Committee, President of the Novartis Institutes for BioMedical Research	Member of the Executive Committee, President of the Novartis Institutes for BioMedical Research	American
Jeff George	Member of the Executive Committee; Head of Sandoz Division	Member of the Executive Committee; Head of Sandoz Division	American

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George Gunn, MRCVS	Member of the Executive Committee; Head of Novartis Animal Health Division	Member of the Executive Committee; Head of Novartis Animal Health Division	British
Joseph Jimenez	Member of the Executive Committee, Chief Executive Officer	Member of the Executive Committee, Chief Executive Officer	American
Andrin Oswald, M.D.	Member of the Executive Committee, Head of Novartis Vaccine and Diagnostics Division	Member of the Executive Committee, Head of Novartis Vaccine and Diagnostics Division	Swiss
Jonathan Symonds	Member of the Executive Committee, Chief Financial Officer	Member of the Executive Committee, Chief Financial Officer	British
Thomas Werlen, Ph.D.	Member of the Executive Committee, Group General Counsel	Member of the Executive Committee, Group General Counsel	Swiss

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DIRECTORS AND EXECUTIVE OFFICERS OF NOVARTIS PHARMA

The name, address, citizenship and present principal occupation or employment of each of the directors and executive officers of Novartis Pharma are set forth below.  Unless otherwise indicated below, each occupation set forth opposite an individual’s name refers to employment with Novartis Pharma.

Name	Relationship to Novartis Pharma	Principal Occupation	Citizenship
Daniel L. Vasella, M.D. Director	Chairman of the Board of Directors	Chairman of the Board of Directors	Swiss
Joseph Jimenez Director	Member of the Board of Directors	Chief Executive Officer	American
Jonathan Symonds Director	Member of the Board of Directors	Chief Financial Officer	British
Thomas Werlen, Ph.D. Director	Member of the Board of Directors	Group General Counsel	Swiss

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JOINT FILING AGREEMENT

In accordance with Rule 13d-1(k) under the Securities Exchange Act of 1934, as amended, each of the persons named below agrees to the joint filing of a Statement on Schedule 13D (including amendments thereto) with respect to the common stock, par value $0.001 per share, of Idenix Pharmaceuticals, Inc., a Delaware corporation.

Date: April 20, 2011
	NOVARTIS AG
		By:	/s/ Jonathan Symonds
		Name:		Jonathan Symonds
		Title:		Chief Financial Officer
		By:	/s/ Thomas Werlen
		Name:		Thomas Werlen
		Title:		Group General Counsel
	NOVARTIS PHARMA AG
		By:	/s/ Tony Rosenberg
		Name:		Tony Rosenberg
		Title:		Head Partnering & Emerging Businesses
		By:	/s/ Matt Owens
		Name:		Matt Owens
		Title:		Senior Legal Counsel

EX-99.1

Exhibit 1

GENERAL WAIVER AND CONSENT

This General Waiver and Consent is made and effective as of the 6th of April, 2011 (“Effective Date”) between Idenix Pharmaceuticals, Inc., with offices at 60 Hampshire Street, Cambridge, Massachusetts 02139, USA (“Idenix US”) and Novartis Pharma AG, with offices at Forum 1, Novartis Campus, 4056 Basel, Switzerland (“Novartis”).

INTRODUCTION

A.	Idenix US, Idenix (Cayman) Limited and Novartis are parties to a Development, License and Commercialization Agreement dated as of 8 May 2003 (as amended) (“Novartis License Agreement”).
B.	Idenix US and Novartis are parties to an Amended and Restated Stockholders’ Agreement dated as of 27 July 2004 (“Stockholders’ Agreement”).
C.	Under Section 3.3 of the Stockholders’ Agreement, Novartis has the right in certain circumstances to approve the issuance of shares of capital stock of Idenix US.
D.	Under Section 4.1 of the Stockholders’ Agreement, Novartis has the right to buy its pro rata portion of the New Shares (as defined below).
E.	Idenix US and Novartis are parties to a letter from Idenix US to Novartis dated March 21, 2003 relating to Novartis’ right regarding the appointment and removal of Idenix US’ Chief Financial Officer and other matters (as amended) (the “Letter”).
F.	Idenix has filed a registration statement on Form S-3 (File No. 33- 153471) for the issuance and sale by Idenix of up to $100,000,000 of shares of Common Stock, which registration statement was declared effective on October 17, 2008 (together with any amendment thereto or any additional registration statement filed pursuant to Rule 462(b) promulgated under the Securities Act of 1933, as amended, the “ Shelf Registration Statement”).
G.	Idenix US has proposed to issue and sell in one or more public offerings pursuant to the Registration Statement up to US$59,000,000 in shares (“New Shares”) of common stock, US $0.001 par value per share, of Idenix US (“Common Stock”) at a price per share as yet to be determined (the “Financing”).
H.	The Board of Directors of Idenix US has approved, and will recommend that at its 2011 Annual Meeting of Stockholders the stockholders approve, a proposal to increase from 125,000,000 to 200,000,000 the number of shares of Common

Stock authorized for issuance (the “Share Increase”).

THE PARTIES AGREE

1.Provided that Novartis and its Affiliates (as defined in the Novartis License Agreement) at all times during, and immediately upon expiration of, the Waiver Term (as hereinafter defined) collectively own at least thirty percent (30%) of the Voting Stock (as defined in the Stockholders’ Agreement) of Idenix US, then:

(a)	Novartis approves the Financing under Section 3.3 of the Stockholders’ Agreement;
(b)	Novartis agrees that this General Waiver and Consent fulfills the notice requirements of Sections 2.2 and 3.3 of the Stockholders’ Agreement with respect to the Financing; and
(c)	Pursuant to Sections 3.3(a) and 3.3(c) of the Stockholders’ Agreement, Novartis approves the Share Increase and any and all amendments to the Restated Certificate of Incorporation, solely to the extent necessary to effect the Share Increase.

This General Waiver and Consent applies solely with respect to the Financing and the Share Increase and does not constitute a waiver of any rights of Novartis with respect to other current or future events.  For the sake of clarity, in the event any action(s) or proposal(s) associated with the Financing would drop the Voting Stock ownership of Novartis and its Affiliates below thirty percent (30%), then Section 1 above shall not apply.  The intent is for the Financing to not drop the Voting Stock ownership of Novartis and its Affiliates below thirty percent (30%).  Furthermore, nothing contained in this General Waiver and Consent shall constitute a waiver of Novartis’ rights or Idenix’s obligations under Section 4.1 of the Stockholders’ Agreement.

2.Idenix US and Novartis agree to amend, and Idenix US agrees to cause Idenix (Cayman) Limited, to the extent it is a party, to amend, simultaneously with execution of this General Waiver and Consent, the following agreements:

(a)	the Novartis License Agreement in accordance with Amendment No. 6 to that agreement, as set out in Exhibit A;
(b)	the Stockholders’ Agreement in accordance with Amendment No. 1 to that agreement, as set out in Exhibit B hereto; and
(c)	the Letter in accordance with Letter of Amendment, as set out in Exhibit C hereto.

This General Waiver and Consent will be effective as of the Effective Date. If the Financing is not consummated on or before 30 September 2011, Section 1 of this General Waiver and Consent will terminate and be of no further force or effect.  The period of time commencing with the Effective Date and ending on the earlier to occur of (i) consummation of the Financing or (ii) 30 September 2011 is referred to herein as the “Waiver Term”.

This General Waiver and Consent is governed by the laws of the State of New York and may be executed in one or more counterparts.

EXECUTION

Idenix US and Novartis have caused this General Waiver and Consent to be duly executed by their authorized representatives, as of the date first written above.

IDENIX PHARMACEUTICALS, INC.
By:	/s/ Maria Stahl
	Name:	Maria Stahl
	Title:	SVP and General Counsel
NOVARTIS PHARMA AG
By:	/s/ Tony Rosenberg
	Name:	Tony Rosenberg
	Title:	Head Partnering & Emerging Businesses
By:	/s/ Matt Owens
	Name:	Matt Owens
	Title:	Senior Legal Counsel

EX-99.6

Exhibit 6

LOCK-UP AGREEMENT

April 6, 2011

J.P. MORGAN SECURITIES LLC

383 Madison Avenue

New York, NY 10179

Re:          Idenix Pharmaceuticals, Inc. — Public Offering

Ladies and Gentlemen:

The undersigned understands that you propose to enter into an Underwriting Agreement (the “Underwriting Agreement”) with Idenix Pharmaceuticals, Inc., a Delaware corporation (the “Company”), providing for the public offering (the “Public Offering”) by J.P. Morgan Securities LLC (the “Underwriter”), of common stock, $0.001 par value, of the Company (the “Securities”).

In consideration of the Underwriter’s agreement to purchase and make the Public Offering of the Securities, and for other good and valuable consideration receipt of which is hereby acknowledged, the undersigned hereby agrees that, without the prior written consent of the Underwriter, the undersigned will not, during the period ending 60 days after the date of the prospectus relating to the Public Offering (the “Prospectus”), (1) offer, pledge, announce the intention to sell, sell, contract to sell, sell any option or contract to purchase, purchase any option or contract to sell, grant any option, right or warrant to purchase, or otherwise transfer or dispose of, directly or indirectly, any shares of common stock, $0.001 per share par value, of the Company (the “Common Stock”) or any securities convertible into or exercisable or exchangeable for Common Stock (including without limitation, Common Stock or such other securities which may be deemed to be beneficially owned by the undersigned in accordance with the rules and regulations of the Securities and Exchange Commission and securities which may be issued upon exercise of a stock option or warrant) or (2) enter into any swap or other agreement that transfers,in whole or in part, any of the economic consequences of ownership of the Common Stock or such other securities, whether any such transaction described in clause (1) or (2) above is to be settled by delivery of Common Stock or such other securities, in cash or otherwise or (3) make any demand for or exercise any right with respect to the registration of any shares of Common Stock or any security convertible into or exercisable or exchangeable for Common Stock (other than the exercise of “piggyback” registration rights to the extent the Company initiates a registration), in each case other than (A) the Securities to be sold by the undersigned pursuant to the Underwriting Agreement, (B) transfers of shares of Common Stock as a bona fide gift or gifts, (C) the exercise of any option to purchase shares of Common Stock, provided that the underlying Common Stock continues to be subject to the restrictions set forth above, (D) transactions relating to shares of Common Stock or other securities acquired in open market transactions after the completion of the Public Offering, (E) transfers of shares of Common Stock or any security convertible into or exercisable or exchangeable for Common Stock to the immediate family of the undersigned, to a trust the beneficiaries of which are exclusively the undersigned and/or a member or members of the immediate family of the undersigned or to any corporation, partnership, limited liability company or other entity all of the beneficial ownership interests of which are held exclusively by the undersigned and/or a member or members of the immediate family of the undersigned, (F) transfers of shares of Common Stock or any security convertible into or exercisable or exchangeable for Common Stock upon death by will or intestate succession (G) the entry into any trading plan established pursuant to Rule

10b5-1 of the Securities Exchange Act of 1934, as amended (“Exchange Act”), provided that no sales or other dispositions may occur under such plan until the expiration of the 60-day restricted period and that no filing or other public announcement, whether under the Exchange Act or otherwise, shall be required or shall be made by the undersigned or the Company in connection with the trading plan during such restricted period and, before the trading plan is established, the Company shall have provided to the Underwriter written confirmation that no such filing or public announcement shall be required or shall be made by the Company in connection with the trading plan during such period and (H) if the undersigned is an entity, transfers of shares of Common Stock or other securities to an Affiliate (as defined in Rule 12b-2 under the Exchange Act) and/or transfers by or on behalf of any employee benefit plan maintained by the undersigned or an Affiliate; provided, further, that in the case of any transfer or distribution pursuant to clause (B), (E), (F) or (H), each donee, distributee or transferee shall execute and deliver to the Underwriter a lock-up letter in the form of this paragraph; and provided, further, that in the case of any transfer or distribution pursuant to clause (B), (D), (E), (F) or (H), no filing by any party (donor, donee, transferor or transferee) under the Exchange Act or other public announcement shall be required or shall be made voluntarily in connection with such transfer or distribution (other than a filing on a Form 5 made after the expiration of the 60-day period referred to above). Notwithstanding the foregoing, if (1) during the last 17 days of the 60-day restricted period, the Company issues an earnings release or material news or a material event relating to the Company occurs; or (2) prior to the expiration of the 60-day restricted period, the Company announces that it will release earnings results during the 16-day period beginning on the last day of the 60-day period, the restrictions imposed by this Letter Agreement shall continue to apply until the expiration of the 18-day period beginning on the issuance of the earnings release or the occurrence of the material news or material event.

In furtherance of the foregoing, the Company, and any duly appointed transfer agent for the registration or transfer of the securities described herein, are hereby authorized to decline to make any transfer of securities if such transfer would constitute a violation or breach of this Letter Agreement.

The undersigned hereby represents and warrants that the undersigned has full power and authority to enter into this Letter Agreement. All authority herein conferred or agreed to be conferred and any obligations of the undersigned shall be binding upon the successors, assigns, heirs or personal representatives of the undersigned.

The undersigned understands that, if the Underwriting Agreement does not become effective prior to June 30, 2011, or if the Underwriting Agreement (other than the provisions thereof which survive termination) shall terminate or be terminated prior to payment for and delivery of the Common Stock to be sold thereunder, the undersigned shall be released from, all obligations under this Letter Agreement. The undersigned understands that the Underwriter is entering into the Underwriting Agreement and proceeding with the Public Offering in reliance upon this Letter Agreement.

This Letter Agreement and the undersigned’s obligations hereunder are conditioned upon and shall have no force or effect unless all executive officers and directors of the Company on the date hereof have executed a letter agreement in the same form.

This Letter Agreement shall be governed by and construed in accordance with the laws of the State of New York, without regard to the conflict of laws principles thereof.

Very truly yours,
NOVARTIS PHARMA AG
By:	/s/ Tony Rosenberg
Name:	Tony Rosenberg
Title:	Head Partnering & Emerging Businesses
By:	/s/ Matt Owens
Name:	Matt Owens
Title:	Senior Legal Counsel