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                                                                   EXHIBIT 10.8

                              EMPLOYMENT AGREEMENT

                  THIS EMPLOYMENT AGREEMENT (the "Agreement") is made this 8th
day of May, 2003 between Idenix Pharmaceuticals, Inc., a corporation
domesticated under the laws of the State of Delaware (together with its
successors and assigns, the "Company") and Nathaniel A. Brown, M.D. (the
"Employee").

                  WHEREAS, the Employee is currently employed as the Senior Vice
President, Hepatitis Clinical Research of the Company;

                  WHEREAS, the Employee is a holder of options to purchase
shares of the Company's capital stock;

                  WHEREAS, Novartis Pharma AG, a corporation organized under the
laws of Switzerland, has entered into a Stock Purchase Agreement with the
Company, dated as of March 21, 2003 (the "Purchase Agreement") pursuant to which
Novartis Pharma AG will acquire a majority of the fully diluted outstanding
shares of the Company from the shareholders and option holders of the Company,
including the Employee; and

                  WHEREAS, the consummation of the transactions contemplated by
the Purchase Agreement is conditioned upon the Employee entering into an
employment agreement upon the terms and conditions set forth herein.

                  NOW, THEREFORE, in consideration of the mutual covenants and
representations contained herein and in the Purchase Agreement, the Company and
the Employee (individually, a "Party" and, together, the "Parties") hereto agree
as follows:

1.       EMPLOYMENT PERIOD

                  Commencing upon the closing date under the Purchase Agreement
(the "Effective Date"), the Company shall employ the Employee, and the Employee
shall serve the Company, under the terms of this Agreement for a term of five
(5) years from the Effective Date (the "Term"), provided, however, that the Term
shall automatically renew for successive one-year terms, unless either Party
gives written notice to the other party not less than 90 calendar days prior to
the expiration of any such term that such Party is electing not to so extend the
Term. Notwithstanding the foregoing, the Employee's employment hereunder may be
terminated at any time, subject to Section 4 and Section 5 hereof, following the
Effective Date. The period of time during which the Employee is employed
hereunder shall be referred to herein as the "Employment Period."

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2.       DUTIES AND STATUS; PLACE OF EMPLOYMENT

                  During the Employment Period, the Company hereby engages the
Employee as the Senior Vice President, Hepatitis Clinical Research of the
Company on the terms and conditions set forth in this Agreement. During the
Employment Period, the Employee shall report to the Chief Executive Officer of
the Company, and shall exercise such authority and perform such duties and
functions and discharge such responsibilities as are customarily associated with
the Employee's position and shall not be assigned duties that are materially
inconsistent with his ability to discharge the foregoing duties and
responsibilities. During the Employment Period, the Employee agrees to devote
substantially all of the Employee's business time, efforts and skills to the
performance of the Employee's duties and responsibilities under this Agreement,
provided, however, that the Employee shall be permitted to manage his personal
investments and affairs, to serve with the prior consent of the Company's Chief
Executive Officer on such scientific advisory boards or other similar
organizations as agreed to promote the interests of the Company and to engage in
charitable activities and community affairs provided such activities do not
materially interfere with the proper performance of the Employee's duties and
responsibilities hereunder. During the Employment Period, the Employee's
principal place of employment shall be in the Boston, Massachusetts metropolitan
area.

3.       COMPENSATION AND BENEFITS

         A. Salary. During the Employment Period, the Company shall pay to the
Employee, as compensation for the performance of the Employee's duties and
obligations under this Agreement, an annual base salary of $252,000 ("Base
Salary"), payable in accordance with the normal payroll practices of the
Company. The Base Salary shall be reviewed annually for additional increases, if
any, which in the sole discretion of the Board of Directors of the Company (the
"Board") are merited or necessary to adjust for cost of living increases or to
retain a competitive Base Salary for the Employee. After any such increase, the
term "Base Salary" as utilized in this Agreement shall thereafter refer to the
increased amount. Base Salary shall not be reduced at any time without the
express prior written consent of the Employee.

         B. Annual Bonus. During the Employment Period, the Employee shall be
eligible for an annual target bonus equal to no less than 30% of the Employee's
Base Salary (the "Target Bonus"), based upon criteria established by the Board
in consultation with senior management of the Company and payable in such amount
if the relevant criteria are met. If such performance goals are exceeded, the
Employee shall receive a larger amount of up to 200% of the annual target bonus
opportunity. Any annual bonus shall be payable in accordance with the terms of
the bonus program established by the Board, provided that any bonus earned shall
be paid no later than 90 calendar days after the end of the applicable
performance period. The Employee's target

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bonus opportunity as a percentage of Base Salary may be reviewed periodically
for any increase in the discretion of the Board. After any such increase, the
term "Target Bonus" as utilized in this Agreement shall thereafter refer to the
increased amount. The Target Bonus shall not be reduced at any time without the
express prior written consent of the Employee.

         C.       Equity.

                  (i)      Waiver. The Employee hereby waives the right under
         any employment agreement, equity compensation plan, award agreement or
         any other plan or agreement with respect to the Company to any
         acceleration of vesting (or lapsing of restrictions) of any options to
         purchase shares of the Company's capital stock, any restricted shares
         of the Company's capital stock or any other equity-based awards of the
         Company held by the Employee on the date hereof (together, the "Equity
         Awards") that would otherwise be triggered, either directly or
         indirectly, by the Purchase Agreement or the transactions contemplated
         by the Purchase Agreement.

                  (ii)     Continued Vesting. Subject to paragraph (i) above,
         any unvested or restricted Equity Awards will continue to vest (or have
         restrictions lapse) in accordance with the terms of this Agreement or
         the applicable plan and award agreement, including any provision
         providing for acceleration of vesting (or lapsing of restrictions) upon
         any "change in control" of the Company (as defined in the applicable
         plan or award agreement) that may occur following the consummation of
         the transactions contemplated by the Purchase Agreement.

                  (iii)    Equity Holding Requirement. The Employee agrees not
         to sell, transfer, pledge or otherwise dispose of any interest in more
         than 50% of the "Holding Period Shares" until the fifth anniversary of
         the Effective Date. For purposes of this Agreement, "Holding Period
         Shares" shall mean the sum of (i) the unrestricted shares of the
         Company's capital stock owned by the Employee on the date hereof and
         (ii) any shares acquired upon the exercise or vesting of the Equity
         Awards (other than unrestricted shares included in clause (i)). The
         foregoing restriction shall not apply in the case of (i) Disability (as
         defined below) of the Employee, (ii) the death of the Employee, (iii)
         dispositions to a family member, family trust or family-controlled
         entity for estate planning purposes of the Employee, (iv) termination
         by the Company for any reason other than Cause, (v) termination by the
         Employee for Good Reason or (vi) dispositions otherwise specifically
         provided for in the Stockholders' Agreement to be entered into by and
         among Idenix Pharmaceuticals, Inc., the Preference Holders, and Other
         Holders named therein, dated May 8th, 2003 (the "Stockholders'
         Agreement"), in each such case, whether before or following a Change in
         Control (as defined in Section 6.C hereof).

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                  (iv)     New Equity Awards. The Employee shall be eligible to
         participate in any long-term incentive award program of the Company,
         including, but not limited to, the Company's 1998 Equity Incentive
         Plan, as amended from time to time, and any successor thereto. In
         addition, the Employee shall have an annual performance target equity
         opportunity to be awarded an option to purchase 30,000 shares of the
         Company's stock (with such award vesting ratably over the 48 months
         following the date of grant). The actual number of shares to be awarded
         under such target equity opportunity shall be subject to annual
         approval by the Company's Board of Directors and conditioned upon the
         achievement of annual performance targets established by the Board.

         D. Employee Benefits. During the Employment Period, the Employee shall
be entitled to participate in all of the employee and fringe benefit plans of
the Company in effect during the Employment Period on the same basis as provided
generally to other senior-level executives of the Company, subject to, and on a
basis consistent with, the participation requirements and other terms and
conditions of such plans. The Employee shall be entitled to four (4) weeks
vacation per calendar year, subject to such carry-over policy as is approved by
the Board.

         E. Business Expenses. During the Employment Period, the Company shall
promptly reimburse the Employee for all appropriately documented, reasonable
business expenses incurred by the Employee in the performance of the Employee's
duties under this Agreement, in accordance with the Company's policies.

4.       TERMINATION OF EMPLOYMENT

         A. Termination by the Company. The Company shall have the right to
terminate the Employment Period and the Employee's employment hereunder for any
reason at any time, including for any reason that does not constitute Cause,
subject to the consequences of such termination as provided in this Agreement.
For purposes of this Agreement, and subject to the Employee's opportunity to
cure as provided below, the Company shall have "Cause" to terminate the
Employee's employment hereunder if such termination shall be the result of: (i)
willful fraud or willful material dishonesty in connection with the Employee's
employment by the Company; (ii) intentional failure by the Employee to
substantially perform the Employee's duties hereunder or gross neglect in the
performance of such duties; (iii) gross misconduct by the Employee that is
materially detrimental to the Company's reputation, goodwill or business
operations; (iv) a breach of any of the Employee's covenants as provided in
Section 7 hereof; or (v) the conviction of, or plea of nolo contendere to, a
charge of commission of a felony.

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         B. Termination by the Employee. The Employee shall have the right to
terminate the Employment Period for any reason at any time, including for any
reason that does not constitute Good Reason, subject to the consequences of such
termination and the Employee's covenants as provided in this Agreement and such
termination shall not be deemed to be a breach of this Agreement. For purposes
of this Agreement, and subject to the Company's opportunity to cure as provided
below, the Employee shall have "Good Reason" to terminate the Employee's
employment hereunder if such termination shall be the result of the following,
without the Employee's prior written consent: (i) any adverse change in the
Employee's title or a material diminution in the Employee's authority or
responsibilities as set forth in Section 2 hereof, (ii) a reduction in the
Employee's Base Salary, the Target Bonus or target equity amount, (iii) a
relocation of the Employee's primary place of employment to a location more than
40 miles from the Employee's current primary place of employment or (iv) any
other material breach by the Company of this Agreement, which breach adversely
affects the Employee and is not promptly cured after the Employee's delivery of
a written notice of such breach to the Company.

         C. Notice and Opportunity to Cure. Notwithstanding the foregoing, it
shall be a condition precedent to the Company's right to terminate the
Employee's employment for Cause and the Employee's right to terminate the
Employee's employment for Good Reason that (1) the party seeking the termination
shall first have given the other party written notice stating with reasonable
specificity the reason for the termination ("breach"), (2) if such breach is
susceptible of cure or remedy, a period of 30 calendar days from and after the
giving of such notice shall have elapsed without the breaching party having
effectively cured or remedied such breach during such 30-day period and (3) in
the case of a termination for Cause, if the Employee fails to cure such neglect
or conduct within such 30-day period, the Employee has an opportunity to be
heard before the Board or, if the Board shall so designate, before a committee
comprised of the Chief Executive Officer of the Company and two other senior
executives of the Company, in each such case subject to a majority vote to
terminate the Employee's employment for Cause.

         D. Termination Upon Death or Disability. The Employment Period and the
Employee's employment hereunder shall be terminated by the death of the
Employee. The Employment Period and the Employee's employment hereunder may be
terminated by the Company or the Employee if the Employee shall be rendered
incapable of performing the Employee's duties to the Company by reason of any
medically determined physical or mental impairment that can reasonably be
expected to result in death or has lasted for a period of six (6) or more
consecutive months from the first date of the Employee's absence due to the
disability (a "Disability").

         E. Expiration of Term. The Employment Period shall terminate upon the
expiration of the Term. Any continuation of the Employee's employment beyond the

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expiration of the original Term or any extended Term shall not be subject to the
terms and conditions of this Agreement, except that the "Inventions Agreement"
set forth as Exhibit A shall survive the Term and the expiration of the
Employment Period, and Sections 7 and 9 hereof shall survive in accordance with
their terms for the periods specified therein.

5.       CONSEQUENCES OF TERMINATION

         A. Without Cause; For Good Reason. In the event of a termination of the
Employee's employment during the Employment Period by the Company other than for
Cause or by the Employee for Good Reason (a "Covered Termination"), the Company
shall provide the Employee with the following:

                  (i)      payment of a cash lump-sum amount equal to the sum of
         (x) the Employee's Base Salary and (y) the greater of (1) Target Bonus
         or (2) the bonus earned by the Employee for the year preceding the year
         in which termination of employment occurs, payable as soon as
         practicable following the termination of employment and the expiration
         of the revocation period in the Employee Release (as defined in Section
         8 hereof);

                  (ii)     immediate vesting and exercisability of all
         outstanding equity awards, including any Equity Awards, upon a Covered
         Termination, with any equity awards that are stock options remaining
         exercisable for 24 months following the date of a Covered Termination
         (or until the expiration of the term of the option, if earlier);

                  (iii)    continued coverage for the Employee and the
         Employee's eligible dependents under all group medical, dental and life
         insurance coverages that are provided to employees of the Company
         generally for a period of 12 months following a Covered Termination,
         with such coverage to be at the Company's cost (subject to standard
         employee contribution requirements). Any such coverage shall be
         discontinued in the event that the Employee obtains substitute coverage
         from subsequent employment or service during such 12-month period; and

                  (iv)     payment of (x) any earned but unpaid amounts as of
         the date of termination, including, but not limited to, Base Salary
         through the date of termination, reimbursement of business expenses and
         any incentive awards earned for performance periods that have ended,
         (y) any compensation previously deferred by the Employee together with
         any vested Company matching contributions and (z) any accrued but
         unpaid vacation days under Company policy through the date of
         termination ("Accrued Obligations"), payable as soon as practicable
         following such termination.

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         B. Death or Disability. In the event the Employee's employment with the
Company is terminated on account of death or Disability during the Employment
Period, the Company shall provide the Employee (or his estate or legal
representative, as the case may be) with such death or disability benefits as
are provided under the death and disability plans that are available to
employees of the Company generally on the date of termination. In addition, the
Employee (or his estate or legal representative, as the case may be) shall be
entitled to:

                  (i)      in the case of Disability, payment of 60% of Base
         Salary per year until the Employee reaches age 65 (with such payment
         obligation offset by any amounts the Employee receives from any
         insurance plan or policy of the Company);

                  (ii)     immediate vesting and exercisability of all
         outstanding equity awards, including any Equity Awards, upon such
         termination of employment, with any equity awards that are stock
         options remaining exercisable for 24 months following the date of
         termination (or until the expiration of the term of the option, if
         earlier);

                  (iii)    continued coverage for the Employee (in the case of
         Disability) and the Employee's eligible dependents under all group
         medical, dental and life insurance coverages that are provided to
         employees of the Company generally for a period of 12 months following
         such termination, with such coverage to be at the Company's cost
         (subject to standard employee contribution requirements). Any such
         coverage shall be discontinued in the event that the Employee obtains
         substitute coverage from subsequent employment or service during such
         12-month period; and

                  (iv)     payment of Accrued Obligations as soon as practicable
         following the termination of employment.

         C. Other Termination. In the event that the Employee's employment with
the Company is terminated during the Employment Period (i) by the Company for
Cause or (ii) by the Employee other than for Good Reason or Disability, the
Employee shall not be entitled to any further payments, compensation or other
benefits under this Agreement, except the Employee shall be entitled to (i)
payment of Accrued Obligations as soon as practicable following such termination
and (ii) treatment of any outstanding equity awards in accordance with the
applicable plan or award agreement.

         D. Other Benefits. The benefits payable to the Employee under this
Agreement are not in lieu of any benefits payable or entitlements due under any
applicable plan, program, policy or arrangement of, or other agreement with, the
Company or any affiliate, and upon termination the Employee shall receive such
benefits, payments or

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entitlements, if any, as he may be entitled to receive pursuant to the terms of
such plans, programs, policies, arrangements or other agreements. Except for the
obligations of the Company provided by the foregoing and this Section 5, the
Company shall have no further severance obligations to the Employee or the
Employee's beneficiaries upon the Employee's termination of employment.

         E. No Mitigation or Offset. The Employee shall have no obligation to
mitigate the damages provided by this Section 5 by seeking substitute employment
or otherwise and, except as provided in Section 5.A(iii) and Section 5.B(iii)
hereof, there shall be no offset of the payments or benefits provided in this
Section 5. In addition, except as provided in Section 7.E hereof, the Company's
obligation to make any payment pursuant to, or otherwise perform its obligations
under, this Agreement shall not be affected by any claim or other right the
Company or any affiliate may have against the Employee for any reason.

6.       CHANGE IN CONTROL PROTECTIONS

         A. Termination Within One Year After a Change in Control. If the
Employee's employment is terminated by the Company without Cause or the Employee
terminates his employment for Good Reason, in each case within one (1) year
following a Change in Control and during the Term, in addition to the payments,
benefits and entitlements provided pursuant to Section 5.A hereof, the Employee
shall also be entitled to an additional lump-sum amount equal to the sum of (x)
the Employee's Base Salary and (y) the greater of (1) Target Bonus or (2) the
bonus earned by the Employee for the year preceding the year in which
termination of employment occurs, payable as soon as practicable following the
termination of employment.

         B. Excise Tax Provision. Anything herein to the contrary
notwithstanding, to the extent that any payment, entitlement or benefit provided
under this Agreement or any other agreement, plan, policy, program or
arrangement of the Company (the "Payments") would be subject to the imposition
of the excise tax imposed under Section 4999 of the Internal Revenue Code of
1986, as amended, or any similar Federal or state law (an "Excise Tax"), the
Payments shall be reduced (but not below zero) to the maximum amount as will
result in no portion of the Payments being subject to such Excise Tax (the "Safe
Harbor Cap"), but only if the net after-tax amount that would be received by the
Employee, taking into account all applicable Federal, state and local income
taxes and the imposition of the Excise Tax, is greater than the net after-tax
amount that would be received by the Employee if Payments are not reduced to the
Safe Harbor Cap. Unless the Employee has given prior written notice specifying a
different order to the Company to effectuate the reductions described in the
preceding sentence, the Company shall reduce or eliminate the Payments to the
Safe Harbor Cap, by first reducing or eliminating those payments or benefits
which are not payable in cash and then by reducing or eliminating cash payments.
Any notice given by the Employee

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pursuant to the preceding sentence shall take precedence over the provisions of
any other plan, arrangement or agreement governing the Employee's rights and
entitlements to any benefit, entitlement or compensation.

         C. Definition of Change in Control. For purposes of Section 3 hereof
and this Section 6, the term "Change in Control" shall mean:

                           (i)      any "person," as such term is used in
                  Sections 3(a)(9) and 13(d) of the Securities Exchange Act of
                  1934, becomes a "beneficial owner," as such term is used in
                  Rule 13d-3 promulgated under that act, of fifty percent (50%)
                  or more of the Voting Stock of the Company, or Novartis Pharma
                  AG disposes of its entire interest in the Company's Voting
                  Stock (except in connection with an initial public offering of
                  the Voting Stock);

                           (ii)     the Company adopts any plan of liquidation
                  providing for the distribution of all or substantially all of
                  its assets;

                           (iii)    all or substantially all of the assets or
                  business of the Company is disposed of pursuant to a merger,
                  consolidation or other transaction (unless the shareholders of
                  the Company immediately prior to such merger, consolidation or
                  other transaction beneficially own, directly or indirectly, at
                  least fifty percent (50%) of the Voting Stock or other
                  ownership interests of the entity or entities, if any, that
                  succeed to the business of the Company); or

                           (iv)     the Company combines with another company
                  and is the surviving corporation but, immediately after the
                  combination, the shareholders of the Company immediately prior
                  to the combination hold, directly or indirectly, fifty percent
                  (50%) or less of the Voting Stock of the combined company
                  (there being excluded from the number of shares held by such
                  shareholders, but not from the Voting Stock of the combined
                  company, any shares received by affiliates of such other
                  company in exchange for stock of such other company).

         For purposes of this definition of "Change in Control" the "Company"
shall include any entity that succeeds to all or substantially all of the
business of the Company and "Voting Stock" shall mean securities of any class or
classes having general voting power under ordinary circumstances, in the absence
of contingencies, to elect the directors of a corporation.

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7.       RESTRICTIVE COVENANTS

         A. Inventions. The Employee agrees to sign and honor an "Inventions
Agreement" in the form attached hereto as Exhibit A.

         B. Nonsolicitation of Employees. During the Restricted Period (as
defined in Section 7.D below), the Employee shall not, without the express prior
written approval of the Company, (i) directly or indirectly, knowingly solicit
any employee of the Company or any of its affiliates (or any employee who was
employed by the Company or any of its affiliates at any time within six (6)
months prior to the date the Employee seeks to solicit such person ("Former
Employee")) to leave the employ of the Company or its affiliates, as the case
may be.

         C. Nonsolicitation of Customers. During the Restricted Period, the
Employee agrees that other than in the ordinary course of business he shall not,
without the express prior written approval of the Company, knowingly solicit,
recruit or encourage any customer of the Company or any of its affiliates who
was a customer at, or was a customer within the six (6)-month period preceding,
the date of the Employee's termination of employment to reduce or cease its
business with the Company or any such affiliate.

         D. Noncompetition. In consideration of the consummation of the
transactions contemplated by the Purchase Agreement and in consideration of the
continued employment of the Employee by the Company under the terms of this
Agreement, the Employee shall not, during the Restricted Period, without the
express written approval of the Company, other than in the ordinary course of
performing his duties hereunder, engage in a "Competitive Business," directly or
indirectly, as an individual, partner, shareholder, director, officer,
principal, agent, employee, trustee, consultant, or in any relationship or
capacity, in any geographic location in which the Company or any of its
Affiliates is engaged in business. Anything to the contrary notwithstanding, it
shall not be a violation of this Section 7.D for the Employee to (i) own or
acquire up to two percent (2%) of the outstanding equity securities (measured by
value) of any entity, (ii) serve as a member of the board of directors or as a
member of an advisory committee of any entity on which Employee was serving
prior to the date of termination of his employment or of any entity which is not
engaged in a Competitive Business or (iii) provide services to a subsidiary,
division or affiliate of a Competitive Business if such subsidiary, division or
affiliate is not itself engaged in a Competitive Business and the Employee does
not provide services to, or have any responsibilities regarding, the Competitive
Business.

         For purposes of this Agreement, the term "Competitive Business" shall
mean a commercial, for profit entity that discovers, develops and commercializes
therapeutics for the treatment of human viral diseases in the same geographic
area that the

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therapeutics for the treatment of human viral diseases are discovered,
developed, marketed and commercialized by the Company or any of its
subsidiaries. For purposes hereof, the "Restricted Period" shall be the
Employment Period and a period of one (1) year following the termination of the
Employment Period.

         E. Enforcement. The Employee acknowledges that if he breaches any
provision of this Section 7, the Company will suffer irreparable injury. It is
therefore agreed that the Company shall have the right, if permitted by a court
of the applicable jurisdiction, to enjoin any such breach, without posting any
bond. The Employee hereby waives the adequacy of a remedy at law as a defense to
such relief. The existence of this right to injunctive, or other equitable
relief, shall not limit any other rights or remedies which the Company may have
at law or in equity including, without limitation, the right to monetary,
compensatory and punitive damages. The Employee acknowledges and agrees that the
provisions of this Section 7 are reasonable and necessary for the successful
operation of the Company and was required by the Company as condition of the
Purchase Agreement. In the event a court of competent jurisdiction determines
that the Employee has breached the Employee's obligations in any material
respect under this Section 7 (other than through the issuance of an injunction
issued without a determination on the merits), the Company, in addition to
pursuing all available remedies under this Agreement, at law or otherwise, and
without limiting its right to pursue the same, shall be entitled to cease all
payments due to the Employee under this Agreement as of the date of such
determination.

8.       WAIVER

                  All payments and benefits provided by Section 5.A hereof are
conditioned upon the Employee executing and honoring a release of claims in
favor of the Company following the Employee's termination of employment in a
form attached hereto as Exhibit B (the "Employee Release").

9.       DISPUTE RESOLUTION

                  Except as otherwise provided in Section 7.E hereof, any
controversy, dispute or claim arising out of or relating to this Agreement shall
be resolved by final and binding arbitration, to be held in Boston,
Massachusetts, in accordance with the Commercial Arbitration Rules (and not the
National Rules for the Resolution of Employment Disputes) of the American
Arbitration Association and this Section 9. Judgment upon the award rendered by
the arbitrator(s) may be entered in any court having jurisdiction thereof.
Neither Party shall be liable for punitive or exemplary damages.

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10.      WITHHOLDING OF TAXES

                  All payments required to be made by the Company to the
Employee under this Agreement shall be subject to the withholding of such
amounts relating to income tax, employment tax and such other taxes and
withholdings as the Company may reasonably determine it should withhold pursuant
to any applicable U.S. Federal, state or local law or regulation.

11.      NOTICE

                  All notices, requests and other communications pursuant to
this Agreement shall be in writing and shall be deemed to have been duly given,
if delivered in person or by courier, sent by express, registered or certified
mail, postage prepaid, or sent by facsimile transmission, addressed to the
Employee at the Employee's personal residence as reflected in the Company's
records, and to the Company at 125 CambridgePark Drive, Cambridge, Massachusetts
02140. Either party may, by written notice to the other in accordance herewith,
change the address to which notices to such party are to be delivered or mailed.

12.      GOVERNING LAW

                  This Agreement shall be construed, interpreted and enforced in
accordance with the laws of the Commonwealth of Massachusetts, without giving
effect to the choice of law principles thereof.

13.      WAIVER OF BREACH

                  Any waiver of any breach of this Agreement shall not be
construed to be a continuing waiver or consent to any subsequent breach on the
part either of the Employee or of the Company. Any waiver to be effective must
be in writing and signed by the Party against whom it is being enforced (either
the Employee or an authorized officer of the Company, as the case may be) and
must specifically refer to the provision of this Agreement which is being
waived.

14.      NON-ASSIGNMENT; SUCCESSORS

                  This Agreement shall be binding upon and inure to the benefit
of the Parties and their respective successors, heirs (in the case of the
Employee) and assigns. No rights or obligations of the Company under this
Agreement may be assigned or transferred by the Company except that such rights
or obligations may be assigned or transferred pursuant to a merger or
consolidation in which the Company is not the continuing entity, or the sale or
other disposition of all or substantially all of the assets of the Company,
provided that the assignee or transferee is the successor to all or
substantially all of the assets of the Company and such assignee or transferee
assumes

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the liabilities, obligations and duties of the Company, as contained in this
Agreement, either contractually or as a matter of law. No rights or obligations
of the Employee under this Agreement may be assigned or transferred by the
Employee other than his accrued rights to compensation and benefits, which may
be transferred only by will or operation of law, except as provided in this
Section 14 or in an applicable plan, program, grant or agreement of the Company
or any affiliate. In the event of the Employee's death or a judicial
determination of his incompetence, references in this Agreement to the Employee
shall be deemed, where appropriate, to refer to his beneficiary, estate or other
legal representative.

15.      SEVERABILITY

                  If any provision of this Agreement is determined by a court of
competent jurisdiction to be not enforceable in the manner set forth in this
Agreement, the Employee and the Company agree that it is the intention of the
parties that such provision should be enforceable to the maximum extent possible
under applicable law. If any provisions of this Agreement are held to be invalid
or unenforceable, such invalidation or unenforceability shall not affect the
validity or enforceability of any other provision of this Agreement (or any
portion thereof).

16.      ENTIRE AGREEMENT

                  This Agreement and the Inventions Agreement constitute the
entire agreement by the Company and the Employee with respect to the subject
matter hereof and except as specifically provided herein, supersedes any and all
prior agreements or understandings between the Employee and the Company with
respect to the subject matter hereof, whether written or oral, including,
without limitation, the letter agreement between the Employee and the Company,
dated December 21, 2000. Any award agreement relating to an Equity Award, to the
extent inconsistent with any provision of this Agreement, shall be treated as
amended in a manner consistent with the terms of this Agreement. This Agreement
may be amended or modified only by a written instrument (specifically
referencing the provision of this Agreement being so amended) executed by the
Employee and an authorized officer of the Company.

17.      COUNTERPARTS

                  This Agreement may be executed in two or more counterparts.

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<PAGE>

                  IN WITNESS WHEREOF, the parties have executed this Agreement
as of the date first written above and this Agreement shall be effective and
binding on the Parties from such date.

                                          IDENIX PHARMACEUTICALS, INC.

                                               /s/ Jean-Pierre Sommadossi
                                          -------------------------------------
                                          By:  Jean-Pierre Sommadossi, Ph.D.
                                          Its: Chief Executive Officer

                                          EMPLOYEE

                                               /s/ Nathaniel Brown
                                          -----------------------------------
                                          Name:  Nathaniel A. Brown, M.D.
                                          Title: Senior Vice President
                                                 Hepatitis Clinical Research

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