Exhibit 99.1

EXECUTION VERSION

IDENIX PHARMACEUTICALS, INC.
(a Delaware corporation)
6,460,672 Shares of Common Stock
UNDERWRITING AGREEMENT

Dated: April 29, 2010

IDENIX PHARMACEUTICALS, INC.
(a Delaware corporation)
6,460,672 Shares of Common Stock
(Par Value $0.001 per Share)

UNDERWRITING AGREEMENT

April 29, 2010

Thomas Weisel Partners LLC
as representative of the Underwriters
named in Schedule A hereto,
c/o Thomas Weisel Partners LLC
One Montgomery Street, Suite 3700
San Francisco, CA 94104

Ladies and Gentlemen:

     Idenix Pharmaceuticals, Inc., a Delaware corporation (the “Company”) confirms its agreement with Thomas Weisel Partners LLC (“Thomas Weisel”) and each of the other Underwriters named in Schedule A hereto (collectively, the “Underwriters,” which term shall also include any underwriter substituted as hereinafter provided in Section 10 hereof), for whom Thomas Weisel is acting as representative (in such capacity, the “Representative”), with respect to the issue and sale by the Company and the purchase by the Underwriters, acting severally and not jointly, of the respective numbers of shares of Common Stock, par value $0.001 per share, of the Company (“Common Stock”) set forth in said Schedule A. The aforesaid 6,460,672 shares of Common Stock to be purchased by the Underwriters are hereinafter called the “Securities.” If the only firm named in Schedule A hereto is Thomas Weisel, then the term “Underwriters” shall be deemed to refer solely to Thomas Weisel.

     The Company understands that the Underwriters propose to make a public offering of the Securities as soon as the Representative deems advisable after this Agreement has been executed and delivered.

     The Company has prepared and filed with the Securities and Exchange Commission (the “Commission”) a registration statement on Form S-3 (No. 333-153471), including the related preliminary prospectus or prospectuses, covering the registration of the Securities under the Securities Act of 1933, as amended (the “1933 Act”). Promptly after execution and delivery of this Agreement, the Company will prepare and file a prospectus in accordance with the provisions of Rule 430B (“Rule 430B”) of the rules and regulations of the Commission under the 1933 Act (the “1933 Act Regulations”) and paragraph (b) of Rule 424 (“Rule 424(b)”) of the 1933 Act Regulations. The information included in such prospectus that was omitted from such registration statement at the time it became effective but that is deemed to be part of and included in such registration statement pursuant to Rule 430B (or Rule 430A of the 1933 Act Regulations, as applicable) is referred to as “Rule 430B Information.” Each prospectus used in connection with the offering of the Securities that omitted the Rule 430B Information is herein called a “preliminary prospectus.” Such registration statement, at any given time, including all exhibits, financial schedules and the documents incorporated by reference therein pursuant to Item 12 of Form S-3 under the 1933 Act, as amended from time to time, and the documents otherwise deemed to be a part thereof or included therein by 1933 Act Regulations, is herein called the “Registration Statement.” Any registration statement filed pursuant to Rule 462(b) of the 1933 Act Regulations is herein referred to as the “Rule 462(b) Registration Statement,” and after such filing, if applicable, the term “Registration Statement” shall include the Rule 462(b) Registration Statement. The final prospectus in the form first furnished to the Underwriters for use in connection with the offering of the Securities, including the documents incorporated by reference therein pursuant to Item 12 of Form S-3 under the 1933 Act at the time of the execution of this Agreement and any preliminary prospectuses that form a part thereof, is herein called the “Prospectus.” For purposes of this Agreement, all references to the Registration Statement, any preliminary prospectus, the Prospectus or any amendment or supplement to any of the foregoing shall be deemed to include the copy filed with the Commission pursuant to its Electronic Data Gathering, Analysis and Retrieval system (“EDGAR”).

     All references in this Agreement to financial statements and schedules and other information which is “contained,” “included” or “stated” in the Registration Statement, any preliminary prospectus or the Prospectus (or other references of like import) shall be deemed to mean and include all such financial statements and schedules and other information which is incorporated by reference in or otherwise deemed by 1933 Act Regulations to be a part of or included in the Registration Statement, any preliminary prospectus or the Prospectus, as the case may be; and all references in this Agreement to amendments or supplements to the Registration Statement, any preliminary prospectus or the Prospectus shall be deemed to mean and include the filing of any document under the Securities Exchange Act of 1934, as amended (the “1934 Act”), which is incorporated by reference in or otherwise deemed by 1933 Act Regulations to be a part of or included in the Registration Statement, such preliminary prospectus or the Prospectus, as the case may be.

     SECTION 1. Representations and Warranties.

     (a) Representations and Warranties by the Company. The Company represents and warrants to each Underwriter as of the date hereof, as of the Applicable Time referred to in Section 1(a)(i) hereof, and as of the Closing Time referred to in Section 2(b) hereof and agrees with each Underwriter, as follows:

          (i) Compliance with Registration Requirements. At the time of filing it met and for the purposes of this offering, the Company meets the requirements for use of Form S-3 under the 1933 Act. Each of the Registration Statement, any Rule 462(b) Registration Statement and any post-effective amendment thereto has become effective under the 1933 Act and no stop order suspending the effectiveness of the Registration Statement, any Rule 462(b) Registration Statement or any post-effective amendment thereto has been issued under the 1933 Act and no proceedings for that purpose have been instituted or are pending or, to the knowledge of the Company, are contemplated by the Commission, and any request on the part of the Commission for additional information has been complied with.

     At the respective times the Registration Statement, any Rule 462(b) Registration Statement and any post-effective amendments thereto became effective, the Registration Statement, the Rule 462(b) Registration Statement and any amendments and supplements thereto complied and, as amended, as applicable, will comply in all material respects with the requirements of the 1933 Act and the 1933 Act Regulations and did not and, as amended, as applicable, will not contain an untrue statement of a material fact or omit to state a material fact required to be stated therein or necessary to make the statements therein not misleading. Neither the Prospectus nor any amendments or supplements thereto, at the time the Prospectus or any such amendment or supplement was issued and at the Closing Time, included or, as amended or supplemented, as applicable, will include an untrue statement of a material fact or omitted or, as amended or supplemented, as applicable, will omit to state a material fact necessary in order to make the statements therein, in the light of the circumstances under which they were made, not misleading.

     As of the Applicable Time (as defined below), neither (x) the Issuer General Use Free Writing Prospectus(es) (as defined below) issued at or prior to the Applicable Time, the Statutory Prospectus (as defined below) and the Prospectus (collectively, the “General Disclosure Package”), nor (y) any individual Issuer Limited Use 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.

     As used in this subsection and elsewhere in this Agreement:

     “Applicable Time” means 7:45 AM (Eastern time) on April 29, 2010 or such other time as agreed upon in writing by the Company and Thomas Weisel.

     “Issuer Free Writing Prospectus” means any “issuer free writing prospectus,” as defined in Rule 433 of the 1933 Act Regulations (“Rule 433”), relating to the Securities that (i) is required to be filed with the Commission by the Company, (ii) is a “road show for an offering that is a written communication” within the meaning of Rule 433(d)(8)(i) whether or not required to be filed with the Commission or (iii) is exempt from filing pursuant to Rule 433(d)(5)(i) because it contains a description of the Securities or of the offering that does not reflect the final terms, in each case in the form filed or required to be filed with the Commission or, if not required to be filed, in the form required to be retained in the Company’s records pursuant to Rule 433(g).

     “Issuer General Use Free Writing Prospectus” means any Issuer Free Writing Prospectus that is intended for general distribution to prospective investors (other than a Bona Fide Electronic Road Show (as defined in Rule 433), as evidenced by its being specified in Schedule D hereto.

     “Issuer Limited Use Free Writing Prospectus” means any Issuer Free Writing Prospectus that is not an Issuer General Use Free Writing Prospectus.

     “Statutory Prospectus” as of any time means the prospectus relating to the Securities that is included in the Registration Statement immediately prior to that time, including any document incorporated by reference therein and any preliminary or other prospectus deemed to be a part thereof.

     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 Securities or until any earlier date that the issuer notified or notifies Thomas Weisel as described in Section 3(e), did not, does not and will not include any information that conflicted, conflicts or will conflict with the information contained in the Registration Statement or the Prospectus, including any document incorporated by reference therein, and any preliminary or other prospectus deemed to be a part thereof that has not been superseded or modified.

     The representations and warranties in this subsection shall not apply to statements in or omissions from the Registration Statement, the Prospectus or any Issuer Free Writing Prospectus, or any amendment or supplement to the foregoing, made in reliance upon and in conformity with written information furnished to the Company by any Underwriter through Thomas Weisel expressly for use therein.

     Each preliminary prospectus (including the prospectus filed as part of the Registration Statement as originally filed or as part of any amendment thereto) complied when so filed in all material respects with the 1933 Act Regulations and each preliminary prospectus and the Prospectus delivered to the Underwriters for use in connection with this offering was identical to the electronically transmitted copies thereof filed with the Commission pursuant to EDGAR, except to the extent permitted by Regulation S-T.

     At the time of filing the Registration Statement, any Rule 462(b) Registration Statement and any post-effective amendments thereto and at the date hereof, the Company was not and is not an “ineligible issuer,” as defined in Rule 405 of the 1933 Act Regulations.

          (ii) Incorporated Documents. The documents incorporated or deemed to be incorporated by reference in the Registration Statement and the Prospectus, when they became effective or at the time they were or hereafter are filed with the Commission, complied and will comply in all material respect with the requirements of the 1934 Act, and the rules and regulations of the Commission under the 1934 Act (the “1934 Regulations”).

          (iii) Independent Accountants. The accountants who certified the financial statements and supporting schedules included or incorporated by reference in the Registration Statement are independent public accountants as required by the 1933 Act and the 1933 Act Regulations.

          (iv) Financial Statements. The financial statements included in the Registration Statement, the General Disclosure Package and the Prospectus, together with the related schedules and notes, present fairly the financial position of the Company at the dates indicated and the statement of operations, stockholders’ equity and cash flows of the Company for the periods specified; said financial statements, together with the related schedules and notes, have been prepared in conformity with generally accepted accounting principles (“GAAP”) applied on a consistent basis throughout the periods involved, except as stated therein. The supporting schedules to such financial statements included or incorporated by reference in the Registration Statement, if any, present fairly in accordance with GAAP the information required to be stated therein. The selected financial data included in the Prospectus present fairly the information shown therein and have been compiled on a basis consistent with that of the audited financial statements included in the Registration Statement.

          (v) No Material Adverse Change in Business. Since the respective dates as of which information is given in the Registration Statement, the General Disclosure Package or the Prospectus, except as otherwise stated

therein, (A) there has been no material adverse change in the condition, financial or otherwise, or in the earnings, business affairs or business prospects of the Company and its subsidiaries, taken as a whole, whether or not arising in the ordinary course of business (a “Material Adverse Effect”), (B) there have been no transactions entered into by the Company or its subsidiaries other than those in the ordinary course of business, which are material with respect to the Company and its subsidiaries, taken as a whole, and (C) there has been no dividend or distribution of any kind declared, paid or made by the Company on any class of its capital stock.

          (vi) Good Standing of the Company. Each of the Company and its significant subsidiaries (as defined in Rule 1-02 of Regulation S-X) has been duly organized and is validly existing as a corporation or other valid legal entity in good standing under the laws of the jurisdiction in which it is chartered and organized and has corporate power and authority to own, lease and operate its properties and to conduct its business as described in the Prospectus and to enter into and perform its obligations under this Agreement; and the Company and each of its significant subsidiaries is duly qualified as a foreign corporation to transact business and is in good standing in each other jurisdiction in which such qualification is required, whether by reason of the ownership or leasing of property or the conduct of business, except where the failure so to qualify or to be in good standing would not reasonably be expected to result in a Material Adverse Effect.

          (vii) Subsidiaries. The subsidiaries set forth in Exhibit 21.1 of the Company’s Annual Report on Form 10-K for the year ended December 31, 2009, as filed with the Commission on March 9, 2010 (the “10-K”), which is incorporated by reference in the Prospectus, are the only subsidiaries of the Company, and all of the issued shares of capital stock of such subsidiaries have been duly authorized and validly issued and are fully paid and non-assessable and (except for directors’ qualifying shares) are owned by the Company, either directly or indirectly, free and clear of any liens, encumbrances, equities or claims.

          (viii) Capitalization. The authorized, issued and outstanding capital stock of the Company is as set forth in the Prospectus, except for subsequent issuances, if any, pursuant to this Agreement, or the employee, director and/or consultant compensation, benefit or stock plans described in the Prospectus or pursuant to the exercise, conversion or exchange of convertible or exchangeable securities or options or rights referred to in the Prospectus. All of the shares of issued and outstanding capital stock of the Company have been duly authorized and validly issued and are fully paid and non-assessable; none of the outstanding shares of capital stock was issued in violation of the preemptive or other similar rights of any securityholder of the Company.

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

          (x) Authorization and Description of Securities. The Securities to be purchased by the Underwriters from the Company have been duly authorized for issuance and sale to the Underwriters pursuant to this Agreement and, when issued and delivered by the Company pursuant to this Agreement against payment of the consideration set forth herein, will be validly issued and fully paid and non-assessable; the Common Stock conforms as to legal matters in all material respects to the description thereof contained in the Prospectus and such description conforms to the rights set forth in the instruments defining the same; no holder of the Securities will be subject to personal liability by reason of being such a holder; and the issuance of the Securities is not subject to the preemptive or other similar rights of any securityholder of the Company that have not been validly waived.

          (xi) Absence of Defaults and Conflicts. Neither the Company nor any of its significant subsidiaries is (i) in violation of its charter or by-laws or (ii) in default in the performance or observance of any obligation, agreement, covenant or condition contained in any contract, indenture, mortgage, deed of trust, loan or credit agreement, note, lease or other agreement or instrument to which the Company or any of its subsidiaries is a party or by which it may be bound, or to which any of the property or assets of the Company or any of its subsidiaries is subject (collectively, “Agreements and Instruments”), except for such defaults that would not reasonably be expected to result in a Material Adverse Effect; and the execution, delivery and performance of this Agreement and the consummation of the transactions contemplated herein (including the issuance and sale of the Securities) and compliance by the Company with its obligations hereunder have been duly authorized by all necessary corporate action of the Company and do not and will not, whether with or without the giving of notice or passage of time or both, conflict with or constitute a breach of, or default or Repayment Event (as defined below) under, or result in the creation or imposition of any lien, charge or encumbrance upon any property or assets of the Company pursuant to,

the Agreements and Instruments (except for such conflicts, breaches, defaults or Repayment Events or liens, charges or encumbrances that would not reasonably be expected to result in a Material Adverse Effect), nor will such action result in any violation of the provisions of the charter or by-laws of the Company or, except as would not reasonably be expected to result in a Material Adverse Effect, any applicable law, statute, rule, regulation, judgment, order, writ or decree of any government, government instrumentality or court, domestic or foreign, having jurisdiction over the Company or any of its assets, properties or operations. As used herein, a “Repayment Event” means any event or condition which gives the holder of any note, debenture or other evidence of indebtedness (or any person acting on such holder’s behalf) the right to require the repurchase, redemption or repayment of all or a portion of such indebtedness by the Company.

          (xii) Related Parties. No relationship, direct or indirect, exists between or among the Company, on the one hand, and the directors, officers, stockholders, customers or suppliers of the Company, on the other hand, which is required to be described in the Registration Statement and the Prospectus and which is not so described. There are no outstanding loans, advance or guarantees of indebtedness by the Company to or for the benefit of any of the executive offers or directors of the Company, except as disclosed in the Registration Statement and the Prospectus.

          (xiii) Fair Summaries. The statements set forth (i) in the Prospectus under the captions “Description of Common Stock” and “Underwriting,” (ii) in the Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2009, as filed with the Commission on March 9, 2010, which is incorporated by reference in the Prospectus, under the captions “Business — Collaborations” through and including “Business — Cooperative Laboratory Agreements — University of Cagliari”, “Business — Patents and Licenses,” “Risk Factors — Factors Related to Our Relationship with Novartis”, “Risk Factors — Factors Related to Patents and Licenses” and “Risk Factors — Factors Related to Our Dependence on Third Parties — Our license agreement with GSK is important to our business. The royalties and other payments we receive under our licensing arrangement with GSK could be delayed, reduced or terminated if GSK terminates or fails to perform its obligations under its agreement with us or if GSK is unsuccessful in its sales efforts”, and (iii) in the Company’s Proxy Statement, as filed with the Commission on April 28, 2009, portions of which are incorporated by reference in the Prospectus, under the captions “Compensation Discussion and Analysis” and “Certain Relationships, Related Transactions and Director Independence”, insofar as they purport to describe the documents referred to therein, are accurate, complete and fair in all material respects.

          (xiv) Absence of Labor Dispute. No labor dispute with the employees of the Company or any of its subsidiaries exists or, to the knowledge of the Company, is imminent, and the Company is not aware of any existing or imminent labor disturbance by the employees of any of its or its subsidiaries’ principal suppliers, manufacturers, customers or contractors, which, in either case, would reasonably be expected to result in a Material Adverse Effect.

          (xv) Absence of Proceedings. There is no action, suit, proceeding, inquiry or investigation before or brought by any court or governmental agency or body, domestic or foreign, now pending, or, to the knowledge of the Company, threatened, against or affecting the Company or any of its subsidiaries, which is required to be disclosed in the Registration Statement and is not disclosed therein, or, other than as disclosed therein, which would reasonably be expected to result in a Material Adverse Effect, or which would materially and adversely affect the properties or assets of the Company and its subsidiaries, taken as a whole, or the consummation of the transactions contemplated in this Agreement or the performance by the Company of its obligations hereunder; the aggregate of all pending legal or governmental proceedings to which the Company and its subsidiaries is a party or of which any of its property or assets is the subject which are not described in the Registration Statement, the General Disclosure Package or the Prospectus, including ordinary routine litigation incidental to the business, could not reasonably be expected to result in a Material Adverse Effect.

          (xvi) Accuracy of Exhibits. There are no contracts or documents which are required to be described in the Registration Statement or the Prospectus or to be filed as exhibits thereto which have not been so described and filed as required.

          (xvii) Intellectual Property.

(A)

The Company and its subsidiaries own, possess, license or have other rights under the patents and patent applications, copyrights, trademarks, service marks, trade names, technology and know-

how (including trade secrets and other unpatented and/or unpatentable proprietary rights) necessary, or used, in any material respect, for the conduct of their business in the manner in which it is being conducted and in the manner in which it is intended to be conducted as set forth in the Prospectus and reasonably necessary in connection with the commercialization of product candidates (the “Product Candidates”) described in the Prospectus as being under development (collectively, the “Company Intellectual Property”).

	(B)		To the Company’s and its subsidiaries’ knowledge, (I) none of the claims of patents owned or licensed by the Company or any of its subsidiaries and constituting Company Intellectual Property are unenforceable or invalid, and (II) none of the claims of patent applications owned or licensed by Company or any of its subsidiaries and constituting Company Intellectual Property specifically covering Product Candidates or their use would be unenforceable or invalid if issued as patents, in each case except as enforceability may be limited by bankruptcy and other similar laws affecting the rights of creditors generally and general principles of equity.
	(C)		The Company and its subsidiaries own or possess valid licenses or other rights under the patents and patent applications set forth in the patent schedule provided by the Company to you on the date hereof (the “Patent Schedule”), which Patent Schedule lists all such patents and patent applications necessary for, or used in, any material respect to conduct the business of the Company and its subsidiaries in the manner in which it is being conducted and in the manner intended to be conducted as described in the Prospectus and necessary in connection with the development and commercialization of Product Candidates described in the Prospectus as being under development by the Company.
	(D)		Neither the Company nor any of its subsidiaries is obligated to pay a material royalty, grant a license, or provide other material consideration to any third party in connection with the Company Intellectual Property other than as disclosed in the Prospectus.
	(E)		Neither the Company nor any of its subsidiaries has received any written notice of infringement with respect to the Company Intellectual Property other than as disclosed in the Prospectus or as would not reasonably be expected to have a Material Adverse Effect; except as disclosed in the Prospectus, there are no pending or, to the knowledge of the Company, threatened actions, suits, proceedings or claims by others against the Company or any of its subsidiaries claiming that the Company or any of its subsidiaries is infringing any patent, trade secret, trade mark, service mark, copyright or other proprietary right, information or materials.
	(F)		The discoveries, inventions, products or processes of the Company and its subsidiaries referred to in the Prospectus do not, to the knowledge of the Company and any of its subsidiaries, infringe or interfere with any right or patent of any third party, or any discovery, invention, product or process that is the subject of a patent application filed by any third party, which could reasonably be expected to have a Material Adverse Effect.
	(G)		Except as described in the Prospectus, neither the Company nor any of its subsidiaries has received any notice of any inventorship challenges with respect to the patents and patent applications within Company Intellectual Property nor has any interference, conflict, opposition, declaratory action or nullity proceeding been declared or provoked with respect to such patents and patent applications.
	(H)		Except as disclosed in the Prospectus with respect to Novartis and GSK with respect to the IDX899 Product Candidate, no third party, including any academic or governmental organization, possesses rights to the Company Intellectual Property which, if exercised, could enable such party to develop products competitive to the Product Candidates or could reasonably be expected to have a Material Adverse Effect.
	(I)		The Company and its subsidiaries and, to the Company’s and its subsidiaries’ knowledge, their respective licensors are not in breach of, and have complied with all terms of, any license or other

			agreement relating to Company Intellectual Property, except for such breaches or non-compliance that could not reasonably be expected to have a Material Adverse Effect.
	(J)		There are no contracts or other documents material to the Company Intellectual Property other than those described in the Prospectus. Neither the Company nor any of its subsidiaries is aware that any of its employees is obligated under any contract (including licenses, covenants or commitments of any nature) or other agreement, or subject to any judgment, decree or order of any court or administrative agency, that would conflict with the Company’s or its subsidiaries’ business.
	(K)		None of the execution or delivery of this Agreement, or the carrying on of the Company’s and its subsidiaries’ business by the employees of the Company and its subsidiaries, will, to the Company’s and its subsidiaries’ knowledge, conflict with or result in a breach of terms, conditions, or provisions of, or constitute a default under, any contract, covenant or instrument under which any such employee is now obligated.
	(L)		To the Company’s and its subsidiaries’ knowledge, the U.S. government does not have any right in any Company Intellectual Property.
	(M)		To the Company’s and its subsidiaries’ knowledge, it is not and will not be necessary to use any inventions, trade secrets or proprietary information or materials of any of its consultants, or its employees (or persons it currently intends to hire) made prior to their employment or engagement by the Company or its subsidiaries except for those inventions and proprietary information licensed to the Company or its subsidiaries.
	(N)		The Company and its subsidiaries have complied with the required duty of candor and good faith in dealing with the United States Patent and Trademark Office (the “PTO”), including the duty to disclose to the PTO all information believed to be material to the patentability of the Company’s patents and pending U.S. patent applications within the Company Intellectual Property.
	(O)		The Company, CNRS, L’Universite Montpellier II (“UMII”) and Universita Degli Studi di Cagliari (“UDSC”) are identified or will be identified, as applicable, in the records of the PTO as the holders of record to the U.S. patents and patent applications as set forth in the Patent Schedule.
	(P)		To the knowledge of the Company, except as described in the Prospectus, no other entity or individual (i.e., other than the Company, CNRS, UMII and UDSC) has any rights, title or interest in the patents or patent applications set forth in the Patent Schedule or any trademarks, copyrights or other intellectual property rights of the Company.
	(Q)		The Company, CNRS, UMII and UDSC are similarly listed or will be listed in the records of corresponding foreign agencies with respect to the foreign counterparts of the foregoing set forth in the Patent Schedule.
	(R)		There are no legal or governmental proceedings pending relating to patents, trade secrets, trade marks, service marks, copyrights or other proprietary information or materials constituting Company Intellectual Property, other than PTO or foreign patent office review of pending applications for patents and trademarks, and no such proceedings are contemplated or, to the knowledge of the Company, threatened by governmental authorities or others.
	(S)		The Company and its subsidiaries have diligently prosecuted, and are diligently prosecuting, claims in the patent applications within the Company Intellectual Property, which contain claims covering products of the Company and Product Candidates or their method of use.

          (xviii) Contracts. Each of (i) the Cooperative Antiviral Research Activity Agreement, dated January 4, 1999, between Idenix France and UDSC, as amended by the Letter Agreement, dated April 14, 2002, by and

between Idenix France and UDSC, and by the Letter Agreement, dated May 8, 2003, by and among the Company, Idenix France, Novartis and UDSC, and by the Agreement, dated June 30, 2004, by and between the Company and UDSC (the “UDSC Cooperative Agreement”), (ii) the License Agreement, dated December 14, 2000, between the Company and UDSC, as amended by the Letter Agreement, dated April 14, 2002, by and between Idenix France and UDSC, by the Letter Agreement, dated May 8, 2003, by and among the Company, Idenix France, Novartis and UDSC, and by the Agreement, dated June 30, 2004, by and between the Company and UDSC (the “UDSC License Agreement”), (iii) the Development, License and Commercialization Agreement, dated as of May 8, 2003, by and among the Company, Idenix Cayman and Novartis as amended on April 30, 2004, December 21, 2004, February 27, 2006, September 28, 2007 and January 28, 2009 and (iv) the License Agreement, dated as of February 4, 2009, by and among the Company and SmithKline Beecham Corporation, doing business as GlaxoSmithKline (“GSK”), whereby the Company granted GSK an exclusive license to develop, manufacture and commercialize certain non-nucleoside reverse transcriptase inhibitor compounds claimed in certain patents and patent applications owned or controlled by the Company, including the compound developed by the Company known as IDX899, for the treatment and prophylaxis of human diseases and conditions on a worldwide basis, is a valid and binding obligation of the parties thereto and enforceable against such parties in accordance with its terms, except as enforceability may be limited by bankruptcy and other similar laws affecting the rights of creditors generally and general principles of equity.

          (xix) University of Cagliari. Under the Research Agreement and License Agreement with the University of Cagliari, as amended in 2002, 2003, 2004, 2005 and 2006, the Company has the right to take any and all actions which are necessary to enforce or defend Company Intellectual Property rights that are co-owned with and/or licensed from the University of Cagliari under such agreements, and the Company has the right to join the University of Cagliari as a party plaintiff in any such action. The University of Cagliari has agreed to cooperate, assist and participate, including without limitation as a party plaintiff, in enforcing or defending the Company Intellectual Property rights that are co-owned with and/or licensed from the University of Cagliari under such agreements.

          (xx) Absence of Further Requirements. No filing with, or authorization, approval, consent, license, order, registration, qualification or decree of, any court or governmental authority or agency is necessary or required for the performance by the Company of its obligations hereunder, in connection with the offering, issuance or sale of the Securities hereunder or the consummation of the transactions contemplated by this Agreement, except such as have been already obtained or as may be required under the 1933 Act, the 1933 Act Regulations, state securities laws or the rules and regulations of the Financial Industry Regulatory Authority (“FINRA”) or The Nasdaq Stock Market.

          (xxi) Absence of Manipulation. Neither the Company nor, to the knowledge of the Company, any affiliate of the Company has taken, nor will the Company or any affiliate take, directly or indirectly, any action which is designed to or which has constituted or which would be expected to cause or result in stabilization or manipulation of the price of any security of the Company to facilitate the sale or resale of the Securities.

          (xxii) Possession of Licenses and Permits. The Company and its subsidiaries possesses such permits, licenses, approvals, consents and other authorizations (collectively, “Governmental Licenses”) issued by the appropriate federal, state, local or foreign regulatory agencies or bodies necessary to conduct the business of the Company and its subsidiaries as described in the Prospectus, including without limitation, all such registrations, approvals, certificates, authorizations and permits required by the United States Food and Drug Administration (the “FDA”), the United States Drug Enforcement Administration, and/or other federal, state, local or foreign agencies or bodies engaged in the regulation of clinical trials, pharmaceuticals, biologics or biohazardous substances or materials, except where the failure so to possess would not, singly or in the aggregate, reasonably be expected to result in a Material Adverse Effect; the Company and its subsidiaries are in compliance with the terms and conditions of all such Governmental Licenses, except where the failure so to comply would not, singly or in the aggregate, reasonably be expected to result in a Material Adverse Effect; all of the Governmental Licenses are valid and in full force and effect, except when the invalidity of such Governmental Licenses or the failure of such Governmental Licenses to be in full force and effect would not, singly or in the aggregate, reasonably be expected to result in a Material Adverse Effect; and neither the Company nor any of its subsidiaries has received any notice of proceedings (nor, to the Company’s and its subsidiaries’ knowledge, are any proceedings threatened against the Company or any of its subsidiaries) relating to the revocation or modification of any such Governmental Licenses

which, singly or in the aggregate, if the subject of an unfavorable decision, ruling or finding, would reasonably be expected to result in a Material Adverse Effect. Where required by applicable laws and regulations of the FDA, the Company and its subsidiaries have submitted to the FDA an Investigational New Drug Application or amendment or supplement thereto for each clinical trial it has conducted or sponsored or is conducting or sponsoring, except where such failure would not, singly or in the aggregate, reasonably be expected to have a Material Adverse Effect; all such submissions were in material compliance with applicable laws and rules and regulations when submitted and no material deficiencies have been asserted by the FDA with respect to any such submissions, except any deficiencies which could not, singly or in the aggregate, reasonably be expected to have a Material Adverse Effect.

          (xxiii) Tests and Preclinical and Clinical Studies. The Company and its subsidiaries have operated and currently are in compliance with the United States Federal Food, Drug, and Cosmetic Act, all applicable rules and regulations of the FDA and other federal, state, local and foreign governmental bodies exercising comparable authority, except where the failure to so operate or be in compliance would not reasonably be expected to have a Material Adverse Effect. The clinical studies conducted by or, to the Company’s knowledge, on behalf of the Company or its subsidiaries that are described in the Registration Statement and the Prospectus were, and, if still pending, are being, conducted in all material respects in accordance with the protocols submitted to the FDA and all applicable laws and regulations; the preclinical studies conducted by or, to the Company’s knowledge, on behalf of the Company or its subsidiaries that are described in the Registration Statement and the Prospectus were, and, if still pending, are being, conducted in all material respects in accordance with experimental protocols, procedures and controls pursuant to, where applicable, accepted professional and scientific standards for products or product candidates comparable to those being developed by the Company and its subsidiaries; the descriptions of the tests and preclinical and clinical studies, and results thereof, conducted by or, to the Company’s knowledge, on behalf of the Company or its subsidiaries contained in the Registration Statement and the Prospectus are accurate and complete in all material respects; the Company is not aware of any other trials or studies, the results of which reasonably call into question the results described or referred to in the Registration Statement and the Prospectus; neither the Company nor any of its subsidiaries are in receipt of any notices or correspondence from the FDA or any foreign, state or local governmental body exercising comparable authority that reasonably call into question the results of the trials or studies described or referred to in the Registration Statement and the Prospectus; and neither the Company nor any of its subsidiaries have received any notice or correspondence from the FDA or any foreign, state or local governmental body exercising comparable authority requiring the termination, suspension, or clinical hold of any tests or preclinical or clinical studies with respect to product candidates currently under development, or such notice or correspondence from any Institutional Review Board or comparable authority requiring the termination or suspension of a clinical study, conducted by or on behalf of the Company, which termination, suspension, or clinical hold would reasonably be expected to have a Material Adverse Effect.

          (xxiv) Title to Property. Each of the Company and its subsidiaries has good and marketable title to all real property owned by it and good title to all other properties owned by it that are material to the Company and its subsidiaries, taken as a whole, in each case, free and clear of all mortgages, pledges, liens, security interests, claims, restrictions or encumbrances of any kind except such as (a) are described in the Prospectus or (b) do not, singly or in the aggregate, materially affect the value of such property and do not materially interfere with the use made and proposed to be made of such property by the Company or its subsidiaries; and all of the leases and subleases material to the business of the Company and its subsidiaries, taken as a whole, and under which the Company or its subsidiaries holds properties described in the Prospectus, are in full force and effect, and, except as described in the Prospectus or as would not, singly or in the aggregate, reasonably be expected to result in a Material Adverse Effect, neither the Company nor its subsidiaries has notice of any material claim of any sort that has been asserted by anyone adverse to the rights of the Company or its subsidiaries under any of the leases or subleases mentioned above, or affecting or questioning the rights of the Company or its subsidiaries to the continued possession of the leased or subleased premises under any such lease or sublease.

          (xxv) Investment Company Act. The Company is not required, and upon the issuance and sale of the Securities as herein contemplated and the application of the net proceeds therefrom as described in the Prospectus will not be required, to register as an “investment company” under the Investment Company Act of 1940, as amended (the “1940 Act”).

          (xxvi) Environmental Laws. Except as described in the Prospectus and except as would not, singly or in the aggregate, reasonably be expected to result in a Material Adverse Effect, (A) neither the Company not its

subsidiaries is in violation of any federal, state, local or foreign statute, law, rule, regulation, ordinance, code, policy or rule of common law or any judicial or administrative interpretation thereof, including any judicial or administrative order, consent, decree or judgment, relating to pollution or protection of human health, the environment (including, without limitation, ambient air, surface water, groundwater, land surface or subsurface-strata) or wildlife, including, without limitation, laws and regulations relating to the release or threatened release of chemicals, pollutants, contaminants, wastes, toxic substances, hazardous substances, petroleum or petroleum products, asbestos containing materials or mold (collectively, “Hazardous Materials”) or to the manufacture, processing, distribution, use, treatment, storage, disposal, transport or handling of Hazardous Materials (collectively, “Environmental Laws”), (B) the Company and its subsidiaries have all permits, authorizations and approvals required under any applicable Environmental Laws to conduct their business as described in the Prospectus and are in compliance with their requirements, (C) there are no pending or, to the knowledge of the Company, threatened administrative, regulatory or judicial actions, suits, demands, demand letters, claims, liens, notices of noncompliance or violation, investigation or proceedings relating to any Environmental Law against the Company or any of its subsidiaries and (D) to the knowledge of the Company, there are no events or circumstances that would reasonably be expected to form the basis of an order for clean-up or remediation, or an action, suit or proceeding by any private party or governmental body or agency, against or affecting the Company or any of its subsidiaries relating to Hazardous Materials or any Environmental Laws.

          (xxvii) Registration Rights. There are no persons with rights to have any securities registered pursuant to the Registration Statement, which rights have not been waived or complied with, nor, except as specifically set forth in the Prospectus, are there any persons with rights to have any securities otherwise registered by the Company under the 1933 Act.

          (xxviii) Accounting Controls and Disclosure Controls.

	(A)		The Company maintains a system of internal accounting controls sufficient to provide reasonable assurances that (A) transactions are executed in accordance with management’s general or specific authorization; (B) transactions are recorded as necessary to permit preparation of financial statements in conformity with GAAP and to maintain accountability for assets; (C) access to assets is permitted only in accordance with management’s general or specific authorization; and (D) the recorded accountability for assets is compared with the existing assets at reasonable intervals and appropriate action is taken with respect to any differences. Except as described in the Prospectus, since the end of the Company’s most recent audited fiscal year, there has been (1) no material weakness in the Company’s internal control over financial reporting (whether or not remediated) and (2) no change in the Company’s internal control over financial reporting that has materially affected, or is reasonably likely to materially affect, the Company’s internal control over financial reporting.
	(B)		The Company employs disclosure controls and procedures that are designed to ensure that information required to be disclosed by the Company in the reports that it files or submits under the 1934 Act is recorded, processed, summarized and reported, within the time periods specified in the Commission’s rules and forms, and is accumulated and communicated to the Company’s management, including its principal executive officer or officers and the principal financial officer or officers, as appropriate, to allow timely decisions regarding disclosure.

          (xxix) Compliance with the Sarbanes-Oxley Act. The Company is in compliance in all material respects with all provisions of the Sarbanes-Oxley Act of 2002 and all rules and regulations promulgated thereunder or implementing the provisions thereof (the “Sarbanes-Oxley Act”), including Section 402 related to loans and Sections 302 and 906 related to certifications.

          (xxx) FINRA Matters. Neither the Company nor, to the Company’s knowledge, the Company’s officers, directors or any of its affiliates (within the meaning of FINRA Conduct Rule 2720(b)(1)(a)), directly or indirectly controls, is controlled by, or is under common control with, or is an associated person (within the meaning of Article I, Section 1(rr) of the By-laws of FINRA) of, any member firm of FINRA.

          (xxxi) Payment of Taxes. All United States federal income tax returns of the Company and its subsidiaries required by law to be filed have been filed and all taxes shown by such returns or otherwise assessed, which are due and payable, have been paid, except assessments against which appeals have been or will be promptly taken and as to which adequate reserves have been provided. The United States federal income tax returns of the Company through the fiscal year ended December 31, 2005 have been settled and no assessment in connection therewith has been made against the Company. The Company has filed all other tax returns that are required to have been filed by it pursuant to applicable foreign, state, local or other law except insofar as the failure to file such returns would not reasonably be expected to result in a Material Adverse Effect, and has paid all taxes due pursuant to such returns or pursuant to any assessment received by the Company, except for such taxes, if any, as are being contested in good faith and as to which adequate reserves have been provided. The charges, accruals and reserves on the books of the Company in respect of any income and corporation tax liability for any years not finally determined are adequate to meet any assessments or re-assessments for additional income tax for any years not finally determined, except to the extent of any inadequacy that would not reasonably be expected to result in a Material Adverse Effect.

          (xxxii) Insurance. The Company and each of its significant subsidiaries carries or is entitled to the benefits of insurance, with financially sound and reputable insurers, in such amounts and covering such risks as is generally maintained by companies of established repute engaged in the same or similar business and at the same or a similar stage of development, and all such insurance is in full force and effect. The Company has no reason to believe that it and its significant subsidiaries will not be able (A) to renew its existing insurance coverage as and when such policies expire or (B) to obtain comparable coverage from similar institutions as may be necessary or appropriate to conduct its business as now conducted and at a cost that would not result in a Material Adverse Effect. Neither the Company nor any of its significant subsidiaries has been denied any insurance coverage which it has sought or for which it has applied.

          (xxxiii) Statistical and Market-Related Data. Any statistical and market-related data included in the Registration Statement and the Prospectus are based on or derived from sources that the Company believes to be reliable and accurate, and, to the extent required by such sources, the Company has obtained the written consent to the use of such data from such sources.

          (xxxiv) Foreign Corrupt Practices Act. Neither the Company nor, to the knowledge of the Company, any director, officer, agent, employee, affiliate or other person acting on behalf of the Company is aware of or has taken any action, directly or indirectly, that would result in a violation by such persons of the Foreign Corrupt Practices Act of 1977, as amended, and the rules and regulations thereunder (the “FCPA”), including, without limitation, making use of the mails or any means or instrumentality of interstate commerce corruptly in furtherance of an offer, payment, promise to pay or authorization of the payment of any money, or other property, gift, promise to give, or authorization of the giving of anything of value to any “foreign official” (as such term is defined in the FCPA) or any foreign political party or official thereof or any candidate for foreign political office, in contravention of the FCPA and the Company, except in each case as would not, singly or in the aggregate, reasonably be expected to result in a Material Adverse Effect, and, to the knowledge of the Company, its affiliates have conducted their businesses in compliance with the FCPA, except as would not reasonably be expected to have a Material Adverse Effect, and have instituted and maintain policies and procedures designed to ensure, and which are reasonably expected to continue to ensure, continued compliance therewith.

          (xxxv) No Brokerage Fees. Except as contemplated by this Agreement and as disclosed in the Registration Statement and the Prospectus, no person is entitled to receive from the Company a brokerage commission, finder’s fee or other like payment in connection with the transactions contemplated herein.

          (xxxvi) Money Laundering Laws. The operations of the Company and its subsidiaries are and have been conducted at all times in compliance with applicable financial recordkeeping and reporting requirements of the Currency and Foreign Transactions Reporting Act of 1970, as amended, the money laundering statutes of all applicable jurisdictions, the rules and regulations thereunder and any related or similar rules, regulations or guidelines, issued, administered or enforced by any governmental agency (collectively, the “Money Laundering Laws”), except in each case as would not, singly or in the aggregate, reasonably be expected to have a Material Adverse Effect, and no action, suit or proceeding by or before any court or governmental agency, authority or body

or any arbitrator involving the Company or its subsidiaries with respect to the Money Laundering Laws is pending or, to the knowledge of the Company, threatened.

          (xxxvii) OFAC. Neither the Company nor, to the knowledge of the Company, any director, officer, agent, employee, affiliate or person acting on behalf of the Company is currently subject to any U.S. sanctions administered by the Office of Foreign Assets Control of the U.S. Treasury Department (“OFAC”); and the Company will not directly or indirectly use the proceeds of the offering, or lend, contribute or otherwise make available such proceeds to any subsidiary, joint venture partner or other person or entity, for the purpose of financing the activities of any person currently subject to any U.S. sanctions administered by OFAC.

     (b) Officer’s Certificates. Any certificate signed by any officer of the Company delivered to the Representative or to counsel for the Underwriters shall be deemed a representation and warranty by the Company to each Underwriter as to the matters covered thereby.

     SECTION 2. Sale and Delivery to Underwriters; Closing.

     (a) Securities. On the basis of the representations and warranties herein contained and subject to the terms and conditions herein set forth, the Company agrees to sell to each Underwriter, severally and not jointly, and each Underwriter, severally and not jointly, agrees to purchase from the Company, at the price per share set forth in Schedule B, the number of Securities set forth in Schedule A opposite the name of such Underwriter, plus any additional number of Securities which such Underwriter may become obligated to purchase pursuant to the provisions of Section 10 hereof.

     (b) Payment. Payment of the purchase price for, and delivery of certificates for, the Securities shall be made at the offices of Wilmer Cutler Pickering Hale and Dorr LLP at 60 State Street, Boston, MA 02109, or at such other place as shall be agreed upon by the Representative and the Company, at 10:00 A.M. (Eastern time) on the fourth (third, if the pricing occurs before 4:30 P.M. (Eastern time) on any given day) business day after the date hereof (unless postponed in accordance with the provisions of Section 10), or such other time not later than ten business days after such date as shall be agreed upon by the Representative and the Company (such time and date of payment and delivery being herein called “Closing Time”).

     Payment shall be made to the Company by wire transfer of immediately available funds to a bank account designated by the Company against delivery to the Representative for the respective accounts of the Underwriters of certificates for the Securities to be purchased by them. It is understood that each Underwriter has authorized the Representative, for its account, to accept delivery of, receipt for, and make payment of the purchase price for, the Securities that it has agreed to purchase. Subject to Section 10, Thomas Weisel, individually and not as representative of the Underwriters, may (but shall not be obligated to) make payment of the purchase price for the Securities to be purchased by any Underwriter whose funds have not been received by the Closing Time, but such payment shall not relieve such Underwriter from its obligations hereunder.

     (c) Denominations; Registration. The Securities to be purchased by each Underwriter hereunder will be represented by one or more definitive global Securities in book-entry form which will be deposited by or on behalf of the Company with the Depository Trust Company (“DTC”) or its designated custodian. On the date and at the time specified in paragraph (b) above, the Company will deliver the Securities to Thomas Weisel Partners LLC, for the account of each Underwriter, against payment by or on behalf of each such Underwriter of the purchase price therefor by wire transfer of Federal (same-day) funds to the account specified by the Company to Thomas Weisel Partners LLC by causing DTC to credit the Securities to the account of Thomas Weisel Partners LLC at DTC.

     SECTION 3. Covenants of the Company. The Company covenants with each Underwriter as follows:

     (a) Compliance with Securities Regulations and Commission Requests. The Company, subject to Section 3(b), will comply with the requirements of Rule 430B, and will notify the Representative immediately, and confirm the notice in writing, (i) when any post-effective amendment to the Registration Statement shall become effective, or any supplement to the Prospectus or any amended Prospectus shall have been filed, (ii) of the receipt of any comments from the Commission, (iii) of any request by the Commission for any amendment to the Registration

Statement or any amendment or supplement to the Prospectus or any document incorporated by reference therein or otherwise deemed to be a part thereof or for additional information, (iv) of the issuance by the Commission of any stop order suspending the effectiveness of the Registration Statement or of any order preventing or suspending the use of any preliminary prospectus, or of the suspension of the qualification of the Securities for offering or sale in any jurisdiction, or of the initiation or threatening of any proceedings for any of such purposes or of any examination pursuant to Section 8(e) of the 1933 Act concerning the Registration Statement and (v) if the Company becomes the subject of a proceeding under Section 8A of the 1933 Act in connection with the offering of the Securities. The Company will effect the filings required under Rule 424(b), in the manner and within the time period required by Rule 424(b) (without reliance on Rule 424(b)(8)), and will take such steps as it deems necessary to ascertain promptly whether the form of prospectus transmitted for filing under Rule 424(b) was received for filing by the Commission and, in the event that it was not, it will promptly file such prospectus. The Company will make every reasonable effort to prevent the issuance of any stop order and, if any stop order is issued, to obtain the lifting thereof at the earliest possible moment.

     (b) Filing of Amendments and Exchange Act Documents. The Company will give the Representative notice of its intention to file or prepare any amendment to the Registration Statement (including any filing under Rule 462(b)) or any amendment, supplement or revision to either any preliminary prospectus (including any prospectus included in the Registration Statement at the time it became effective) or to the Prospectus, and will furnish the Representative with copies of any such documents a reasonable amount of time prior to such proposed filing or use, as the case may be, and will not file or use any such document to which the Representative or counsel for the Underwriters shall object. The Company has given the Representative notice of any filings made pursuant to the 1934 Act or the 1934 Act Regulations within 48 hours prior to the Applicable Time; the Company will give the Representative notice of its intention to make any such filing from the Applicable Time to the Closing Time and will furnish the Representative with copies of any such documents a reasonable amount of time prior to such proposed filing, as the case may be, and will not file or use any such document to which the Representative or counsel for the Underwriters shall reasonably object.

     (c) Delivery of Registration Statements. The Company has furnished or will deliver to the Representative and counsel for the Underwriters, without charge, signed copies of the Registration Statement as originally filed and of each amendment thereto (including exhibits filed therewith ) and signed copies of all consents and certificates of experts, and will also deliver to the Representative, without charge, a conformed copy of the Registration Statement as originally filed and of each amendment thereto (without exhibits) for each of the Underwriters. The copies of the Registration Statement and each amendment thereto furnished to the Underwriters will be identical to the electronically transmitted copies thereof filed with the Commission pursuant to EDGAR, except to the extent permitted by Regulation S-T.

     (d) Delivery of Prospectuses. The Company has delivered to each Underwriter, without charge, as many copies of each preliminary prospectus as such Underwriter reasonably requested, and the Company hereby consents to the use of such copies for purposes permitted by the 1933 Act. The Company will furnish to each Underwriter, without charge, during the period when the Prospectus is required to be delivered under the 1933 Act, such number of copies of the Prospectus (as amended or supplemented) as such Underwriter may reasonably request. The Prospectus and any amendments or supplements thereto furnished to the Underwriters will be identical to the electronically transmitted copies thereof filed with the Commission pursuant to EDGAR, except to the extent permitted by Regulation S-T.

     (e) Continued Compliance with Securities Laws. The Company will comply with the 1933 Act and the 1933 Act Regulations and the 1934 Act and 1934 Act Regulations so as to permit the completion of the distribution of the Securities as contemplated in this Agreement and in the Prospectus. If at any time when a prospectus is required by the 1933 Act to be delivered in connection with sales of the Securities, any event shall occur or condition shall exist as a result of which it is necessary, in the opinion of counsel for the Underwriters or for the Company, to amend the Registration Statement or amend or supplement the Prospectus in order that the Prospectus will not include any untrue statements of a material fact or omit to state a material fact necessary in order to make the statements therein not misleading in the light of the circumstances existing at the time it is delivered to a purchaser, or if it shall be necessary, in the opinion of such counsel, at any such time to amend the Registration Statement or amend or supplement the Prospectus in order to comply with the requirements of the 1933 Act or the 1933 Act Regulations, the Company will promptly prepare and file with the Commission, subject to Section 3(b), such amendment or

supplement as may be necessary to correct such statement or omission or to make the Registration Statement or the Prospectus comply with such requirements, and the Company will furnish to the Underwriters such number of copies of such amendment or supplement as the Underwriters may reasonably request. 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 contained in the Registration Statement relating to the Securities 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, the Company will promptly notify Thomas Weisel and will promptly amend or supplement, at its own expense, such Issuer Free Writing Prospectus to eliminate or correct such conflict, untrue statement or omission.

     (f) Blue Sky Qualifications. The Company will use its best efforts, in cooperation with the Underwriters, to qualify the Securities for offering and sale under the applicable securities laws of such states and other jurisdictions (domestic or foreign) as the Representative may designate and to maintain such qualifications in effect for a period of not less than one year from the later of the effective date of the Registration Statement and any Rule 462(b) Registration Statement; provided, however, that the Company shall not be obligated to file any general consent to service of process or to qualify as a foreign corporation or as a dealer in securities in any jurisdiction in which it is not so qualified or to subject itself to taxation in respect of doing business in any jurisdiction in which it is not otherwise so subject.

     (g) Rule 158. The Company will timely file such reports pursuant to the 1934 Act as are necessary in order to make generally available to its securityholders as soon as practicable an earnings statement for the purposes of, and to provide to the Underwriters the benefits contemplated by, the last paragraph of Section 11(a) of the 1933 Act.

     (h) Use of Proceeds. The Company will use the net proceeds received by it from the sale of the Securities in the manner specified in the Prospectus under “Use of Proceeds.”

     (i) Listing. The Company will use its reasonable best efforts to effect and maintain the quotation of the Securities on the Nasdaq Global Market.

     (j) Restriction on Sale of Securities. During a period of 60 days from the date of the Prospectus (the “Lock Up Period”), the Company will not, without the prior written consent of Thomas Weisel, (i) directly or indirectly, offer, pledge, 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 any share of Common Stock or any securities convertible into or exercisable or exchangeable for Common Stock or file any registration statement under the 1933 Act with respect to any of the foregoing or (ii) enter into any swap or any other agreement or any transaction that transfers, in whole or in part, directly or indirectly, the economic consequence of ownership of the Common Stock, whether any such swap or transaction described in clause (i) or (ii) above is to be settled by delivery of Common Stock or such other securities, in cash or otherwise. The foregoing sentence shall not apply to (A) the Securities to be sold hereunder, (B) the issuance by the Company of any shares of Common Stock upon the exercise of an option or warrant or the conversion of a security outstanding on the date hereof and referred to in the Prospectus; (C) the issuance by the Company of any shares of Common Stock, or options to purchase Common Stock, to employees, directors and/or consultants of the Company pursuant to the employee benefit and stock plans described in the Prospectus, (D) the issuance by the Company pursuant to the Amended and Restated Stockholders’ Agreement, dated as of July 23, 2004, with Novartis Pharma AG (“Novartis”), except to the extent Novartis has waived its rights to such issuances, including pursuant to the S-3 Financing Waiver and Consent between the Company and Novartis, dated as of May 19, 2009 with regards to the issuance of the Securities or (E) the issuance of up to an aggregate of 3,000,000 shares of Common Stock in connection with any strategic transaction that includes a commercial relationship (including joint ventures, marketing or distribution arrangements, collaboration agreements or intellectual property license agreements) or any acquisition of assets or not less than a majority or controlling portion of the equity of another entity, provided that any such shares of Common Stock and securities issued pursuant to this clause (E) during the 60-day restricted period described above shall be subject to the restrictions described above for the remained of such restricted period; provided, however, that if (1) during the last 17 days of the initial Lock-Up Period, the Company releases earnings results or announces material news or a material event or (2) prior to the expiration of the initial Lock-Up Period, the Company announces that it will release earnings results during the 16-day period following the last day of the Lock-Up Period, then in each case the Lock-Up Period will be

automatically extended until the expiration of the 18-day period beginning on the date of release of the earnings results or the announcement of the material news or material event, as applicable, unless Thomas Weisel waives, in writing, such extension.

     (k) Reporting Requirements. The Company, during the period when the Prospectus is required to be delivered under the 1933 Act, will file all documents required to be filed with the Commission pursuant to the 1934 Act within the time periods required by the 1934 Act and the rules and regulations of the Commission thereunder.

     (l) Issuer Free Writing Prospectuses. The Company represents and agrees that, unless it obtains the prior consent of the Representative, and each Underwriter represents and agrees that, unless it obtains the prior consent of the Company and the Representative, it has not made and will not make any offer relating to the Securities that would constitute an “issuer free writing prospectus,” as defined in Rule 433, 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 Representative or by the Company and the Representative, as the case may be, 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 Rule 433 applicable to any Permitted Free Writing Prospectus, including timely filing with the Commission where required, legending and record keeping.

     (m) Trademarks. The Company will, upon request of any Underwriter, furnish, or cause to be furnished, to such Underwriter an electronic version of the Company’s trademarks, servicemarks and corporate logo for us on the website, if any, operated by such Underwriter for the purpose of facilitating the on-line offering of the Securities (the “License”); provided, however, that the License shall be used solely for the purpose described above, is granted without any fee and may not be assigned or transfered.

     SECTION 4. Payment of Expenses.

     (a) Expenses. The Company will pay or cause to be paid all expenses incident to the performance of its obligations under this Agreement, including (i) the preparation, printing and filing of the Registration Statement (including financial statements and exhibits) as originally filed and of each amendment thereto, (ii) the preparation, printing and delivery to the Underwriters of this Agreement, any Agreement among Underwriters and such other documents as may be required in connection with the offering, purchase, sale, issuance or delivery of the Securities, (iii) the preparation, issuance and delivery of the certificates for the Securities to the Underwriters, including any stock or other transfer taxes and any stamp or other duties payable upon the sale, issuance or delivery of the Securities to the Underwriters, (iv) the fees and disbursements of the Company’s counsel, accountants and other advisors, (v) the qualification of the Securities under securities laws in accordance with the provisions of Section 3(f) hereof, including filing fees and the reasonable fees and disbursements of counsel for the Underwriters in connection therewith and in connection with the preparation of a Blue Sky Survey and any supplement thereto, (vi) the printing and delivery to the Underwriters of copies of each preliminary prospectus, any Permitted Free Writing Prospectus and of the Prospectus and any amendments or supplements thereto and any costs associated with electronic delivery of any of the foregoing by the Underwriters to investors, (vii) the preparation, printing and delivery to the Underwriters of copies of the Blue Sky Survey and any supplement thereto, (viii) the fees and expenses of any transfer agent or registrar for the Securities, (ix) the costs and expenses of the Company relating to investor presentations on any “road show” undertaken in connection with the marketing of the Securities, including without limitation, expenses associated with the production of road show slides and graphics, fees and expenses of any consultants engaged in connection with the road show presentations, travel and lodging expenses of the representatives and officers of the Company and any such consultants, and the cost of aircraft and other transportation chartered in connection with the road show, (x) the filing fees incident to, and the reasonable fees and disbursements of counsel to the Underwriters in connection with, the review by FINRA of the terms of the sale of the Securities, (xi) the fees and expenses incurred in connection with the listing of the Securities in the Nasdaq Global Market, and (xii) the costs and expenses (including without limitation any damages or other amounts payable in connection with legal or contractual liability) associated with the reforming of any contracts for sale of the Securities made by the Underwriter caused by a breach of the representation contained in the third paragraph of Section 1(a)(i). It is understood that, subject to this section and Section 4(b) hereof, the Underwriters will pay all of their costs and expenses, including fees and disbursements of their counsel.

     (b) Termination of Agreement. If this Agreement is terminated by the Representative in accordance with the provisions of Section 5 or Section 9(a)(i) hereof, the Company shall reimburse the Underwriters for all of their accountable out-of-pocket expenses, including the reasonable fees and disbursements of counsel for the Underwriters.

     SECTION 5. Conditions of Underwriters’ Obligations. The obligations of the several Underwriters hereunder are subject to the accuracy of the representations and warranties of the Company contained in Section 1 hereof or in certificates of any officer of the Company delivered pursuant to the provisions hereof, to the performance by the Company of its covenants and other obligations hereunder that are required to be performed or satisfied by it at or prior to the Closing Time, and to the following further conditions:

     (a) Effectiveness of Registration Statement. The Registration Statement, including any Rule 462(b) Registration Statement, has become effective and at Closing Time no stop order suspending the effectiveness of the Registration Statement shall have been issued under the 1933 Act or proceedings therefor initiated or threatened by the Commission, and any request on the part of the Commission for additional information shall have been complied with to the reasonable satisfaction of counsel to the Underwriters. A prospectus containing the Rule 430B Information shall have been filed with the Commission in the manner and within the time frame required by Rule 424(b) without reliance on Rule 424(b)(8) or a post-effective amendment providing such information shall have been filed and declared effective in accordance with the requirements of Rule 430B.

     (b) Opinion of Counsel for the Company. At Closing Time, the Representative shall have received the favorable opinion, dated as of Closing Time, of Wilmer Cutler Pickering Hale and Dorr LLP, counsel for the Company, in form and substance reasonably satisfactory to counsel for the Underwriters, together with signed or reproduced copies of such letter for each of the other Underwriters, to the effect set forth in Exhibit A hereto.

     (c) Opinion of in-house Counsel. At Closing Time, the Representative shall have received the favorable opinion, dated as of Closing Time, of John Weidenbruch, Executive Vice President, General Counsel and Secretary of the Company, in form and substance reasonably satisfactory to counsel for the Underwriters, together with signed or reproduced copies of such letter for each of the other Underwriters, to the effect set forth in Exhibit A1 hereto.

     (d) Opinion of Intellectual Property Counsel. At Closing Time, the Representative shall have received the favorable opinion, dated as of Closing Time, of Jones Day, intellectual property counsel for the Company, in form and substance reasonably satisfactory to counsel for the Underwriters, together with signed or reproduced copies of such letter for each of the other Underwriters, to the effect set forth in Exhibit A2 hereto.

     (e) Opinion of Counsel for Underwriters. At Closing Time, the Representative shall have received the favorable opinion, dated as of Closing Time, of Ropes & Gray LLP, counsel for the Underwriters, together with signed or reproduced copies of such letter for each of the other Underwriters, with respect to such matters as the Representative may reasonably request.

     (f) Officers’ Certificate. At Closing Time, there shall not have been, since the date hereof or since the respective dates as of which information is given in the Prospectus or the General Disclosure Package, any Material Adverse Effect, and the Representative shall have received a certificate of the President or a Vice President of the Company and of the chief financial or chief accounting officer of the Company, dated as of Closing Time, to the effect that (i) there has been no such Material Adverse Effect, (ii) the representations and warranties in Section 1(a) hereof are true and correct with the same force and effect as though expressly made at and as of Closing Time, (iii) the Company has complied with all agreements and satisfied all conditions on its part to be performed or satisfied at or prior to Closing Time, and (iv) 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, to their knowledge, contemplated by the Commission.

     (g) Accountant’s Comfort Letter. At the time of the execution of this Agreement, the Representative shall have received from PricewaterhouseCoopers LLP a letter dated such date, in form and substance satisfactory to the Representative and PricewaterhouseCoopers LLP, together with signed or reproduced copies of such letter for each of the other Underwriters containing statements and information of the type ordinarily included in accountants’

“comfort letters” to underwriters with respect to the financial statements and certain financial information contained in the Registration Statement and the Prospectus.

     (h) Bring-down Comfort Letter. At Closing Time, the Representative shall have received from PricewaterhouseCoopers LLP a letter, dated as of Closing Time, to the effect that they reaffirm the statements made in the letter furnished pursuant to subsection (g) of this Section, except that the specified date referred to shall be a date not more than three business days prior to Closing Time.

     (i) Approval of Listing. At Closing Time, the Securities shall have been approved for inclusion in the Nasdaq Global Market, subject only to official notice of issuance.

     (j) Lock-up Agreements. At the date of this Agreement, the Representative shall have received a lock-up agreement substantially in the form of Exhibit B hereto signed by the persons listed on Schedule C hereto.

     (k) No Objection. FINRA shall not have raised any objection with respect to the fairness and reasonableness of the underwriting terms and arrangements.

     (l) Additional Documents. At Closing Time counsel for the Underwriters shall have been furnished with such documents and opinions as they may require for the purpose of enabling them to pass upon the issuance and sale of the Securities as herein contemplated, or in order to evidence the accuracy of any of the representations or warranties, or the fulfillment of any of the conditions, herein contained; and all proceedings taken by the Company in connection with the issuance and sale of the Securities as herein contemplated shall be satisfactory in form and substance to the Representative and counsel for the Underwriters.

     (m) Termination of Agreement. If any condition specified in this Section shall not have been fulfilled when and as required to be fulfilled, this Agreement may be terminated by the Representative by notice to the Company at any time at or prior to Closing Time and such termination shall be without liability of any party to any other party except as provided in Section 4 and except that Sections 1, 6, 7 and 8 shall survive any such termination and remain in full force and effect.

     SECTION 6. Indemnification.

     (a) Indemnification of Underwriters. The Company agrees to indemnify and hold harmless each Underwriter, its affiliates, as such term is defined in Rule 501(b) under the 1933 Act (each, an “Affiliate”), its selling agents and each person, if any, who controls any Underwriter within the meaning of Section 15 of the 1933 Act or Section 20 of the 1934 Act as follows:

          (i) against any and all loss, liability, claim, damage and expense whatsoever, as incurred, arising out of any untrue statement or alleged untrue statement of a material fact contained in the Registration Statement (or any amendment thereto), including the Rule 430B Information, or the omission or alleged omission therefrom of a material fact required to be stated therein or necessary to make the statements therein not misleading or arising out of any untrue statement or alleged untrue statement of a material fact included in any preliminary prospectus, any Issuer Free Writing Prospectus or the Prospectus (or any amendment or supplement thereto), or the omission or alleged omission therefrom of a material fact necessary in order to make the statements therein, in the light of the circumstances under which they were made, not misleading;

          (ii) against any and all loss, liability, claim, damage and expense whatsoever, as incurred, to the extent of the aggregate amount paid in settlement of any litigation, or any investigation or proceeding by any governmental agency or body, commenced or threatened, or of any claim whatsoever based upon any such untrue statement or omission, or any such alleged untrue statement or omission; provided that (subject to Section 6(d) below) any such settlement is effected with the written consent of the Company; and

          (iii) against any and all expense whatsoever, as incurred (including the fees and disbursements of counsel chosen by Thomas Weisel), reasonably incurred in investigating, preparing or defending against any litigation, or any investigation or proceeding by any governmental agency or body, commenced or threatened, or any

claim whatsoever based upon any such untrue statement or omission, or any such alleged untrue statement or omission, to the extent that any such expense is not paid under (i) or (ii) above;

provided, however, that this indemnity agreement shall not apply to any loss, liability, claim, damage or expense to the extent arising out of any untrue statement or omission or alleged untrue statement or omission made in reliance upon and in conformity with written information furnished to the Company by any Underwriter through Thomas Weisel expressly for use in the Registration Statement (or any amendment thereto), including the Rule 430B Information, or any preliminary prospectus, any Issuer Free Writing Prospectus or the Prospectus (or any amendment or supplement thereto).

     (b) Indemnification of Company, Directors and Officers. Each Underwriter severally agrees to indemnify and hold harmless the Company, its directors, each of its officers who signed the Registration Statement, and each person, if any, who controls the Company within the meaning of Section 15 of the 1933 Act or Section 20 of the 1934 Act against any and all loss, liability, claim, damage and expense described in the indemnity contained in Section 6(a) above, as incurred, but only with respect to untrue statements or omissions, or alleged untrue statements or omissions, of material fact, made in the Registration Statement (or any amendment thereto), including the Rule 430B Information, or any preliminary prospectus, any Issuer Free Writing Prospectus or the Prospectus (or any amendment or supplement thereto) in reliance upon and in conformity with written information furnished to the Company by such Underwriter through Thomas Weisel expressly for use therein.

     (c) Actions against Parties; Notification. Each indemnified party shall give notice as promptly as reasonably practicable to each indemnifying party of any action commenced against it in respect of which indemnity may be sought hereunder, but failure to so notify an indemnifying party shall not relieve such indemnifying party from any liability hereunder to the extent it is not materially prejudiced as a result thereof and in any event shall not relieve it from any liability which it may have otherwise than on account of this indemnity agreement. In the case of parties indemnified pursuant to Section 6(a) above, counsel to the indemnified parties shall be selected by Thomas Weisel and, in the case of parties indemnified pursuant to Section 6(b) above, counsel to the indemnified parties shall be selected by the Company. An indemnifying party may participate at its own expense in the defense of any such action and, to the extent it shall elect, jointly with any other indemnifying party, assume the defense thereof, with counsel reasonably satisfactory to such indemnified parties; provided, however, that counsel to the indemnifying party shall not (except with the consent of the indemnified party) also be counsel to the indemnified party. In no event shall the indemnifying parties be liable for fees and expenses of more than one counsel (in addition to any local counsel) separate from their own counsel for all indemnified parties in connection with any one action or separate but similar or related actions in the same jurisdiction arising out of the same general allegations or circumstances; provided, however, that after notice from the indemnifying party to an indemnified party that the indemnifying party has elected to assume the defense of any action, the indemnifying party shall not be liable to such indemnified party for any legal fees or expenses of other counsel or any other expenses, in each case 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 parties, settle or compromise or consent to the entry of any judgment with respect to any litigation, or any investigation or proceeding by any governmental agency or body, commenced or threatened, or any claim whatsoever in respect of which indemnification or contribution could be sought under this Section 6 or Section 7 hereof (whether or not the indemnified parties are actual or potential parties thereto), unless such settlement, compromise or consent (i) includes an unconditional release of each indemnified party from all liability arising out of such litigation, investigation, proceeding or claim 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 any indemnified party.

     (d) Settlement without Consent if Failure to Reimburse. If at any time an indemnified party shall have requested an indemnifying party to reimburse the indemnified party for fees and expenses of counsel, such indemnifying party agrees that it shall be liable for any settlement of the nature contemplated by Section 6(a)(ii) effected without its written consent if (i) such settlement is entered into more than 45 days after receipt by such indemnifying party of the aforesaid request, (ii) such indemnifying party shall have received notice of the terms of such settlement at least 30 days prior to such settlement being entered into and (iii) such indemnifying party shall not have reimbursed such indemnified party in accordance with such request prior to the date of such settlement.

     SECTION 7. Contribution. If the indemnification provided for in Section 6 hereof is for any reason unavailable to or insufficient to hold harmless an indemnified party in respect of any losses, liabilities, claims, damages or expenses referred to therein, then each indemnifying party shall contribute to the aggregate amount of such losses, liabilities, claims, damages and expenses incurred by such indemnified party, as incurred, (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 hand from the offering of the Securities pursuant to this Agreement or (ii) if the allocation provided by clause (i) 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 of the Underwriters on the other hand in connection with the statements or omissions which resulted in such losses, liabilities, claims, damages or expenses, 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 hand in connection with the offering of the Securities pursuant to this Agreement shall be deemed to be in the same respective proportions as the total net proceeds from the offering of the Securities pursuant to this Agreement (before deducting expenses) received by the Company and the total underwriting discount received by the Underwriters, in each case as set forth on the cover of the Prospectus bear to the aggregate initial public offering price of the Securities as set forth on the cover of the Prospectus.

     The relative fault of the Company on the one hand and the Underwriters on the other hand shall be determined by reference to, among other things, whether any such untrue or alleged untrue statement of a material fact or omission or alleged omission to state a material fact relates to information supplied by the Company or by the Underwriters and the parties’ relative intent, knowledge, access to information and opportunity to correct or prevent such statement or omission.

     The Company and the Underwriters agree that it would not be just and equitable if contribution pursuant to this Section 7 were determined by pro rata allocation (even if the Underwriters were treated as one entity for such purpose) or by any other method of allocation which does not take account of the equitable considerations referred to above in this Section 7. The aggregate amount of losses, liabilities, claims, damages and expenses incurred by an indemnified party and referred to above in this Section 7 shall be deemed to include any legal or other expenses reasonably incurred by such indemnified party in investigating, preparing or defending against any litigation, or any investigation or proceeding by any governmental agency or body, commenced or threatened, or any claim whatsoever based upon any such untrue or alleged untrue statement or omission or alleged omission.

     Notwithstanding the provisions of this Section 7, 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 any 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 1933 Act) shall be entitled to contribution from any person who was not guilty of such fraudulent misrepresentation.

     For purposes of this Section 7, each person, if any, who controls an Underwriter within the meaning of Section 15 of the 1933 Act or Section 20 of the 1934 Act and each Underwriter’s Affiliates and selling agents shall have the same rights to contribution as such Underwriter, and each director of the Company, each officer of the Company who signed the Registration Statement, and each person, if any, who controls the Company within the meaning of Section 15 of the 1933 Act or Section 20 of the 1934 Act shall have the same rights to contribution as the Company. The Underwriters’ respective obligations to contribute pursuant to this Section 7 are several in proportion to the number of Securities set forth opposite their respective names in Schedule A hereto and not joint.

     SECTION 8. Representations, Warranties and Agreements to Survive. All representations, warranties and agreements contained in this Agreement or in certificates of officers of the Company submitted pursuant hereto, shall remain operative and in full force and effect regardless of (i) any investigation made by or on behalf of any Underwriter or its Affiliates or selling agents, any person controlling any Underwriter, its officers or directors, or any person controlling the Company and (ii) delivery of and payment for the Securities.

     SECTION 9. Termination of Agreement.

     (a) Termination; General. The Representative may terminate this Agreement, by notice to the Company, at any time at or prior to Closing Time (i) if there has been, since the time of execution of this Agreement or since the respective dates as of which information is given in the Prospectus or General Disclosure Package, any Material Adverse Effect, or (ii) if there has occurred any material adverse change in the financial markets in the United States or the international financial markets, any outbreak of hostilities or escalation thereof or other calamity or crisis or any change or development involving a prospective change in national or international political, financial or economic conditions, in each case the effect of which is such as to make it, in the judgment of the Representative, impracticable or inadvisable to market the Securities or to enforce contracts for the sale of the Securities, or (iii) if trading in any securities of the Company has been suspended or materially limited by the Commission or the Nasdaq Global Market, or if trading generally on the American Stock Exchange or the New York Stock Exchange or in the Nasdaq Global Market has been suspended or materially limited, or minimum or maximum prices for trading have been fixed, or maximum ranges for prices have been required, by any of said exchanges or by such system or by order of the Commission, FINRA or any other governmental authority, or (iv) a material disruption has occurred in commercial banking or securities settlement or clearance services in the United States, or (v) if a banking moratorium has been declared by either Federal or New York authorities.

     (b) Liabilities. If this Agreement is terminated pursuant to this Section, such termination shall be without liability of any party to any other party except as provided in Section 4 hereof, and provided further that Sections 1, 6, 7 and 8 shall survive such termination and remain in full force and effect.

     (c) Termination of Lock-Up Agreements and Non-distribution Agreements. In the event this Agreement is terminated for any reason, the Representative shall contemporaneously terminate or waive in their entirety the agreements referred to in Section 5(j).

     SECTION 10. Default by One or More of the Underwriters. If one or more of the Underwriters shall fail at Closing Time to purchase the Securities which it or they are obligated to purchase under this Agreement (the “Defaulted Securities”), the Representative shall have the right, within 36 hours thereafter, to make arrangements for one or more of the non-defaulting Underwriters, or any other underwriters, to purchase all, but not less than all, of the Defaulted Securities in such amounts as may be agreed upon and upon the terms herein set forth; if, however, the Representative shall not have completed such arrangements within such 36-hour period, then:

          (i) if the number of Defaulted Securities does not exceed one-eleventh of the number of Securities to be purchased on such date, each of the non-defaulting Underwriters shall be obligated, severally and not jointly, to purchase the full amount thereof in the proportions that their respective underwriting obligations hereunder bear to the underwriting obligations of all non-defaulting Underwriters, or

          (ii) if the number of Defaulted Securities exceeds one-eleventh of the number of Securities to be purchased on such date, this Agreement shall terminate without liability on the part of any non-defaulting Underwriter.

     No action taken pursuant to this Section shall relieve any defaulting Underwriter from liability in respect of its default.

     In the event of any such default which does not result in a termination of this Agreement either the (i) Representative or (ii) the Company shall have the right to postpone Closing Time for a period not exceeding seven days in order to effect any required changes in the Registration Statement or Prospectus or in any other documents or arrangements. As used herein, the term “Underwriter” includes any person substituted for an Underwriter under this Section 10.

     SECTION 11. Tax Disclosure. Notwithstanding any other provision of this Agreement, immediately upon commencement of discussions with respect to the transactions contemplated hereby, the Company (and each employee, representative or other agent of the Company) may disclose to any and all persons, without limitation of any kind, the tax treatment and tax structure of the transactions contemplated by this Agreement and all materials of any kind (including opinions or other tax analyses) that are provided to the Company relating to such tax treatment and tax structure. For purposes of the foregoing, the term “tax treatment” is the purported or claimed federal income tax treatment of the transactions contemplated hereby, and the term “tax structure” includes any fact that may be

relevant to understanding the purported or claimed federal income tax treatment of the transactions contemplated hereby.

     SECTION 12. Notices. All notices and other communications hereunder shall be in writing and shall be deemed to have been duly given if mailed or transmitted by any standard form of telecommunication. Notices to the Underwriters shall be directed to the Representative at One Montgomery Street, Suite 3700, San Francisco, CA 94104, attention: General Counsel with a copy to Ropes & Gray LLP, One International Place, Boston, MA 02110, attention of Patrick O’Brien, Esq.; notices to the Company shall be directed to it at Idenix Pharmaceuticals, Inc., 60 Hampshire Street, Cambridge, MA 02139, attention of President and Chief Executive Officer, with a copy to Wilmer Cutler Pickering Hale and Dorr LLP at 60 State Street, Boston, MA 02109, attention of Susan Murley, Esq.

     SECTION 13. No Advisory or Fiduciary Relationship. The Company acknowledges and agrees that (a) the purchase and sale of the Securities pursuant to this Agreement, including the determination of the public offering price of the Securities and any related discounts and commissions, is an arm’s-length commercial transaction between the Company, on the one hand, and the several Underwriters, on the other hand, (b) in connection with the offering contemplated hereby and the process leading to such transaction each Underwriter is and has been acting solely as a principal and is not the agent or fiduciary of the Company or its respective stockholders, creditors, employees or any other party, (c) no Underwriter has assumed or will assume an advisory or fiduciary responsibility in favor of the Company with respect to the offering contemplated hereby or the process leading thereto (irrespective of whether such Underwriter has advised or is currently advising the Company on other matters) and no Underwriter has any obligation to the Company with respect to the offering contemplated hereby except the obligations expressly set forth in this Agreement, (d) the Underwriters and their respective affiliates may be engaged in a broad range of transactions that involve interests that differ from those of the Company, and (e) the Underwriters have not provided any legal, accounting, regulatory or tax advice with respect to the offering contemplated hereby and the Company has consulted its own respective legal, accounting, regulatory and tax advisors to the extent it deemed appropriate.

     SECTION 14. Parties. This Agreement shall inure to the benefit of and be binding upon the Underwriters, the Company and their respective successors. Nothing expressed or mentioned in this Agreement is intended or shall be construed to give any person, firm or corporation, other than the Underwriters, the Company and their respective successors and the controlling persons and officers and directors referred to in Sections 6 and 7 and their heirs and legal representatives, any legal or equitable right, remedy or claim under or in respect of this Agreement or any provision herein contained. This Agreement and all conditions and provisions hereof are intended to be for the sole and exclusive benefit of the Underwriters, the Company and their respective successors, and said controlling persons and officers and directors and their heirs and legal representatives, and for the benefit of no other person, firm or corporation. No purchaser of Securities from any Underwriter shall be deemed to be a successor by reason merely of such purchase.

     SECTION 15. GOVERNING LAW. THIS AGREEMENT SHALL BE GOVERNED BY AND CONSTRUED IN ACCORDANCE WITH THE LAWS OF THE STATE OF NEW YORK.

     SECTION 16. TIME. TIME SHALL BE OF THE ESSENCE OF THIS AGREEMENT. EXCEPT AS OTHERWISE SET FORTH HEREIN, SPECIFIED TIMES OF DAY REFER TO NEW YORK CITY TIME.

     SECTION 17. 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.

     SECTION 18. Effect of Headings. The Section headings herein are for convenience only and shall not affect the construction hereof.

     If the foregoing is in accordance with your understanding of our agreement, please sign and return to the Company a counterpart hereof, whereupon this instrument, along with all counterparts, will become a binding agreement among the Underwriters and the Company in accordance with its terms.

For itself and as Representative of the other Underwriters named in Schedule A hereto.

     If the foregoing is in accordance with your understanding of our agreement, please sign and return to the Company a counterpart hereof, whereupon this instrument, along with all counterparts, will become a binding agreement among the Underwriters and the Company in accordance with its terms.

For itself and as Representative of the other Underwriters named in Schedule A hereto.

SCHEDULE A

Name of Underwriter		Number of Securities
Thomas Weisel Partners LLC			2,584,270
Leerink Swann LLC			1,938,201
William Blair & Company, L.L.C.			1,938,201
Total.			6,460,672

SCHEDULE B
IDENIX PHARMACEUTICALS, INC.
6,460,672 Shares of Common Stock

(Par Value $0.001 Per Share)

1. The number of Securities offered to the public shall be 6,460,672 and the offering price per share to the public for the Securities shall be $4.35.

2. The purchase price per share for the Securities to be paid by the several Underwriters shall be $4.089, being an amount equal to the public offering price set forth above less $0.261 per share.

SCHEDULE C
LIST OF PERSONS SUBJECT TO LOCK-UP

Jean-Pierre Sommadossi
Ronald C. Renaud, Jr.
Douglas L. Mayers, M.D.
John F. Weidenbruch
Paul J. Fanning
David N. Standring, Ph.D.
Charles W. Cramb
Wayne T. Hockmeyer, Ph.D.
Thomas R. Hodgson
Robert Pelzer
Denise Pollard-Knight, Ph.D.
Steven Projan, Ph.D.
Anthony Rosenberg
Tamar D. Howson

SCHEDULE D

None

EXHIBIT A

FORM OF OPINION OF COMPANY COUNSEL

EXHIBIT A1

FORM OF OPINION OF IN-HOUSE COUNSEL

EXHIBIT A2

FORM OF OPINION OF INTELLECTUAL PROPERTY COUNSEL

EXHIBIT B

FORM OF LOCK-UP AGREEMENT

[                    ], 2010

Thomas Weisel Partners LLC
One Montgomery Street, Suite 3700
San Francisco, CA 94104

Ladies and Gentlemen:

          The undersigned understands that Thomas Weisel Partners LLC (“Thomas Weisel”) proposes to enter into an Agreement (the “Agreement”) with Idenix Pharmaceuticals, Inc., a Delaware corporation (the “Company”), providing for an offering of shares of the common stock, $0.001 par value per share, of the Company (the “Common Stock”) registered under the Securities Act of 1933, as amended (the “Offering”).

          To induce Thomas Weisel to continue its efforts in connection with the Offering, the undersigned hereby agrees that, without the prior written consent of Thomas Weisel, it will not, during the period commencing on the date hereof and ending 60 days after the date of the final prospectus supplement relating to the Offering (the “Prospectus”), (1) offer, pledge, 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, lend, or otherwise transfer or dispose of, directly or indirectly, any shares of Common Stock or any securities convertible into or exercisable or exchangeable for Common Stock or (2) enter into any swap or other arrangement that transfers to another, in whole or in part, any of the economic consequences of ownership of the Common Stock, 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. The foregoing sentence shall not apply to (a) transactions relating to shares of Common Stock or other securities acquired in open market transactions after the completion of the Offering, provided that no filing under Section 16(a) of the Securities Exchange Act of 1934, as amended (the “Exchange Act”), shall be required or shall be voluntarily made in connection with subsequent sales of Common Stock or other securities acquired in such open market transactions, (b) transfers of shares of Common Stock or any security convertible into or exercisable or exchangeable for Common Stock as a bona fide gift, (c) 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, (d) 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, (e) 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, (f) transactions pursuant to any trading plan established pursuant to Rule 10b5-1 of the Exchange Act that has been entered into by the undersigned prior to the date of this agreement, or (g) the entry into any trading plan established pursuant to Rule 10b5-1 of the Exchange Act, provided that no sales or other dispositions may occur under such plan until the expiration of the restricted period referred to in the foregoing sentence; provided further that in the case of any transfer or distribution pursuant to clause (b), (c) or (d), (i) each donee or distributee shall sign and deliver a lock-up letter substantially in the form of this letter and (ii) no filing under Section 16(a) of the Exchange Act, reporting a reduction in beneficial ownership of shares of Common Stock, shall be required or shall be voluntarily made during the restricted period referred to in the foregoing sentence. In addition, the undersigned agrees that, without the prior written consent of Thomas Weisel, it will not, during the period commencing on the date hereof and ending 60 days after the date of the Prospectus, 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. The undersigned also agrees and consents to the entry of stop transfer instructions with the Company’s transfer agent and registrar against the transfer of the undersigned’s shares of Common Stock except in compliance with the foregoing restrictions. For the purposes of this paragraph,

“immediate family” shall mean spouse, domestic partner, lineal descendant (including adopted children), father, mother, brother or sister of the transferor.

          If:

          (1) during the last 17 days of the 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 restricted period, the Company announces that it will release earnings results during the 16-day period beginning on the last day of the restricted period,

          the restrictions imposed by this 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.

          The undersigned shall not engage in any transaction that may be restricted by this agreement during the 34-day period beginning on the last day of the initial restricted period unless the undersigned requests and receives prior written confirmation from the Company or Thomas Weisel that the restrictions imposed by this agreement have expired.

          The undersigned understands that the Company and Thomas Weisel are relying upon this agreement in proceeding toward consummation of the Offering. The undersigned further understands that this agreement is irrevocable and shall be binding upon the undersigned’s heirs, legal representatives, successors and assigns.

          Whether or not the Offering actually occurs depends on a number of factors, including market conditions. Any Offering will only be made pursuant to an Agreement, the terms of which are subject to negotiation between the Company and Thomas Weisel.

          If (1) either the Company, on the one hand, or Thomas Weisel, on the other hand, notifies the other in writing that it does not intend to proceed with the Offering, (2) the registration statement or Prospectus filed with the Securities and Exchange Commission with respect to the Offering is withdrawn, (3) for any reason the Agreement shall be terminated prior to the closing of the Offering, or (4) if the closing of the Offering does not occur on or prior to June 15, 2010, this agreement shall be terminated and the undersigned shall be released from any obligations hereunder.

Very truly yours,

(Name)