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                                                                    Exhibit 10.1

                          IDENIX PHARMACEUTICALS, INC.

                           1998 EQUITY INCENTIVE PLAN
                                    (Amended)

1.    Purpose.

      The purpose of this 1998 Equity Incentive Plan (the "Plan") of Idenix
Pharmaceuticals, Inc., a Delaware corporation (the "Company"), is to advance the
interests of the Company's shareholders by enhancing the Company's ability to
attract, retain and motivate persons who make (or are expected to make)
important contributions to the Company by providing such persons with equity
ownership opportunities and performance-based incentives and thereby better
aligning the interests of such persons with those of the Company's shareholders.
Except where the context otherwise requires, the term "Company" shall include
any present or future subsidiary corporations of Idenix Pharmaceuticals, Inc. as
defined in Section 424(f) of the U.S. Internal Revenue Code of 1986, as amended,
and any regulations promulgated thereunder (the "Code").

2.    Eligibility.

      All of the Company's employees, officers, directors, consultants and
advisors are eligible to be granted options, restricted shares, or other
share-based awards (each, an "Award") under the Plan. Any person who has been
granted an Award under the Plan shall be deemed a "Participant".

3.    Administration, Delegation.

      a. Administration by Board of Directors. The Plan will be administered by
the Board of Directors of the Company (the "Board"). The Board shall have
authority to grant Awards and to adopt, amend and repeal such administrative
rules, guidelines and practices relating to the Plan as it shall deem advisable.
The Board may correct any defect, supply any omission or reconcile any
inconsistency in the Plan or any Award in the manner and to the extent it shall
deem expedient to carry the Plan into effect and it shall be the sole and final
judge of such expediency. All decisions by the Board shall be made in the
Board's sole discretion and shall be final and binding on all persons having or
claiming any interest in the Plan or in any Award. No director or person acting
pursuant to the authority delegated by the Board shall be liable for any action
or determination relating to or under the Plan made in good faith.

      b. Delegation to Executive Officers. To the extent permitted by applicable
law, the Board may delegate to one or more executive officers of the Company the
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power to make Awards and exercise such other powers under the Plan as the Board
may determine, provided that the Board shall fix the maximum number of shares
subject to Awards to be made by such executive officers.

      c. Appointment of Committees. To the extent permitted by applicable law,
the Board may delegate any or all of its powers under the Plan to one or more
committees or subcommittees of the Board (a "Committee"). If and when the shares
of common stock, par value $.001 per share, of the Company (the "Shares") are
registered under the U.S. Securities Exchange Act of 1934 (the "Exchange Act"),
the Board shall appoint one such Committee of not less than two members, each
member of which shall be an "outside director" within the meaning of Section
162(m) of the Code and a "non-employee director" as defined in Rule 16b-3
promulgated under the Exchange Act. All references in the Plan to the "Board"
shall mean the Board or a Committee of the Board or the executive officer
referred to in Section 3(b) to the extent that the Board's powers or authority
under the Plan have been delegated to such Committee or executive officer.

1.    Shares Available for Awards.

      a. Number of Shares. Subject to adjustment under Section 4(b), Awards may
be made under the Plan for up to 5,068,966 Shares. If any Award expires or is
terminated, surrendered or canceled without having been fully exercised or is
forfeited in whole or in part or results in any Shares not being issued, the
unused Shares covered by such Award shall again be available for the grant of
Awards under the Plan, subject, however, in the case of Incentive Stock Options
(as hereinafter defined), to any limitation required under the Code. Shares
issued under the Plan may consist in whole or in part of authorized but unissued
shares.

      b. Adjustment to Shares. In the event of any share dividend, stock split,
recapitalization, reorganization, merger, consolidation, combination, exchange
of shares, liquidation, spin-off or other similar change in capitalization or
event, or any distribution to holders of Shares other than a normal cash
dividend, (i) the number and class of securities available under this Plan, (ii)
the number and class of security and exercise price per share subject to each
outstanding Option, (iii) the repurchase price per security subject to each
outstanding Restricted Share Award, and (iv) the terms of each other outstanding
share-based Award shall be appropriately adjusted by the Company (or substituted
Awards may be made, if applicable) to the extent the Board shall determine, in
good faith, that such an adjustment (or substitution) is necessary and
appropriate. If this Section 4(b) applies and Section 8(e)(1) also applies to
any event, Section 8(e)(1) shall be applicable to such event, and this Section
4(b) shall not be applicable.


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2.    Share Options.

      a. General. The Board may grant options to purchase Shares (each, an
"Option") and determine the number of Shares to be covered by each Option, the
exercise price of each Option and the conditions and limitations applicable to
the exercise of each Option, including conditions relating to any applicable
federal or state securities laws, as it considers necessary or advisable. An
Option which is not intended to be an Incentive Stock Option (as hereinafter
defined) shall be designated a "Nonstatutory Stock Option".

      b. Incentive Stock Options. An Option that the Board intends to be an
"incentive share option as defined in Section 422 of the Code (an "Incentive
Stock Option") shall only be granted to employees of the Company and shall be
subject to and shall be construed consistently with the requirements of Section
422 of the Code. The Company shall have no liability to a Participant, or any
other party, if an Option (or any part thereof) which is intended to be an
Incentive Stock Option is not an Incentive Stock Option.

      c. Exercise Price. The Board shall establish the exercise price at the
time each Option is granted and specify it in the applicable option agreement.

      d. Duration of Options. Each Option shall be exercisable at such times and
subject to such terms and conditions as the Board may specify in the applicable
option agreement.

      e. Exercise of Option. Options may be exercised only by delivery to the
Company of a written notice of exercise signed by the proper person together
with payment in full as specified in Section 5(f) for the number of Shares for
which the Option is exercised.

      f. Payment Upon Exercise. Shares purchased upon the exercise of an Option
granted under the Plan shall be paid for as follows:

            (1) in cash or by check, payable to the order of the Company;

            (2) except as the Board may otherwise provide in an Option
Agreement, delivery of an irrevocable and unconditional undertaking by a
creditworthy broker to deliver promptly to the Company sufficient funds to pay
the exercise price, or delivery by the Participant to the Company of a copy of
irrevocable and unconditional instructions to a creditworthy broker to deliver
promptly to the Company cash or a check sufficient to pay the exercise price;


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            (3) to the extent permitted by the Board and explicitly provided in
an Option Agreement (i) by delivery of Shares owned by the Participant valued at
their fair market value as determined by the Board in good faith ("Fair Market
Value"), which Shares were owned by the Participant at least six months prior to
such delivery, (ii) by delivery of a promissory note of the Participant to the
Company on terms determined by the Board, or (iii) by payment of such other
lawful consideration as the Board may determine; or

            (4) any combination of the above permitted forms of payment.

3.    Restricted Shares.

      a. Grants. The Board may grant Awards entitling recipients to acquire
Shares, subject to the right of the Company to repurchase all or part of such
Shares at their issue price or other stated or formula price (or to require
forfeiture of such Shares if issued at no cost) from the recipient in the event
that conditions specified by the Board in the applicable Award are not satisfied
prior to the end of the applicable restriction period or periods established by
the Board for such Award (each, "Restricted Share Award").

      b. Terms and Conditions. The Board shall determine the terms and
conditions of any such Restricted Share Award, including the conditions for
repurchase (or forfeiture) and the issue price, if any. Any share certificates
issued in respect of a Restricted Share Award shall be registered in the name of
the Participant and, unless otherwise determined by the Board, deposited by the
Participant, together with a share transfer form endorsed in blank, with the
Company (or its designee). At the expiration of the applicable restriction
periods, the Company (or such designee) shall deliver the certificates no longer
subject to such restrictions to the Participant or if the Participant has died,
to the beneficiary designated, in a manner determined by the Board, by a
Participant to receive amounts due or exercise rights of the Participant in the
event of the Participant's death (the "Designated Beneficiary"). In the absence
of an effective designation by a Participant, Designated Beneficiary shall mean
the Participant's estate.

4.    Other Share-Based Awards.

      The Board shall have the right to grant other Awards based upon the Shares
having such terms and conditions as the Board may determine, including the grant
of Shares based upon certain conditions, the grant of securities convertible
into Shares and the grant of share appreciation rights.


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5.    General Provisions Applicable to Awards.

      a. Transferability of Awards. Except as the Board may otherwise determine
or provide in an Award, Awards shall not be sold, assigned, transferred, pledged
or otherwise encumbered by the person to whom they are granted, either
voluntarily or by operation of law, except by will or the laws of descent and
distribution, and, during the life of the Participant, shall be exercisable only
by the Participant. References to a Participant, to the extent relevant in the
context, shall include references to authorized transferees.

      b. Documentation. Each Award under the Plan shall be evidenced by a
written instrument in such form as the Board shall determine. Each Award may
contain terms and conditions in addition to those set forth in the Plan.

      c. Board Discretion. Except as otherwise provided by the Plan, each type
of Award may be made alone or in addition or in relation to any other type of
Award. The terms of each type of Award need not be identical, and the Board need
not treat Participants uniformly.

      d. Termination of Status. The Board shall determine the effect on an Award
of the disability, death, retirement, authorized leave of absence or other
change in the employment or other status of a Participant and the extent to
which, and the period during which, the Participant, the Participant's legal
representative, conservator, guardian or Designated Beneficiary may exercise
rights under the Award.

      e. Acquisition Events.

            (1) Consequences of Acquisition Events. Upon the occurrence of an
Acquisition Event (as defined below), or the execution by the Company of any
agreement with respect to an Acquisition Event, the Board shall take any one or
more of the following actions with respect to then outstanding Awards: (i)
provide that outstanding Options shall be assumed, or equivalent Options shall
be substituted, by the acquiring or succeeding corporation (or an affiliate
thereof), provided that any such Options substituted for Incentive Stock Options
shall satisfy, in the determination of the Board, the requirements of Section
424(a) of the Code; (ii) upon written notice to the Participants, provide that
all then unexercised Options will become exercisable in full as of a specified
time (the "Acceleration Time") prior to the Acquisition Event and will terminate
immediately prior to the consummation of such Acquisition Event, except to the
extent exercised by the Participants between the Acceleration Time and the
consummation of such Acquisition Event; (iii) in the event of an Acquisition
Event under the terms of which holders of Shares will receive, upon consummation
thereof, a cash payment for each Share surrendered pursuant to such Acquisition
Event (the


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"Acquisition Price"), provide that all outstanding Options shall terminate upon
consummation of such Acquisition Event and each Participant shall receive, in
exchange therefor, a cash payment equal to the amount (if any) by which (A) the
Acquisition Price multiplied by the number of Shares subject to such outstanding
Options (whether or not then exercisable), exceeds (B) the aggregate exercise
price of such Options; (iv) provide that all Restricted Share Awards then
outstanding shall become free of all restrictions prior to the consummation of
the Acquisition Event; and (v) provide that any other share-based Awards
outstanding (A) shall become exercisable, realizable or vested in full, or shall
be free of all conditions or restrictions, as applicable to each such Award,
prior to the consummation of the Acquisition Event, or (B), if applicable, shall
be assumed, or equivalent Awards shall be substituted, by the acquiring or
succeeding corporation (or an affiliate thereof).

      An "Acquisition Event" shall mean: (a) any merger, amalgamation or
consolidation which results in the voting securities of the Company outstanding
immediately prior thereto representing immediately thereafter (either by
remaining outstanding or by being converted into voting securities of the
surviving or acquiring entity) less than 50% of the combined voting power of the
voting securities of the Company or such surviving or acquiring entity
outstanding immediately after such merger or consolidation; (b) any sale of all
or substantially all of the assets of the Company; or (c) the complete
liquidation of the Company.

            (2) Assumption of Options Upon Certain Events. The Board may grant
Awards under the Plan in substitution for shares and share-based awards held by
employees of another corporation who become employees of the Company as a result
of a merger, amalgamation or consolidation of the employing corporation with the
Company or the acquisition by the Company of property or shares of the employing
corporation. The substitute Awards shall be granted on such terms and conditions
as the Board considers appropriate in the circumstances.

      f. Withholding. Each Participant shall pay to the Company, or make
provision satisfactory to the Board for payment of, any taxes required by law to
be withheld in connection with Awards to such Participant no later than the date
of the event creating the tax liability. The Board may allow Participants to
satisfy such tax obligations in whole or in part in Shares, including Shares
retained from the Award creating the tax obligation, valued at their Fair Market
Value. The Company may, to the extent permitted by law, deduct any such tax
obligations from any payment of any kind otherwise due to a Participant.

      g. Amendment of Award. The Board may amend, modify or terminate any
outstanding Award, including but not limited to, substituting therefor another
Award of the same or a different type, changing the date of exercise or
realization, and


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converting an Incentive Stock Option to a Nonstatutory Stock Option, provided
that the Participant's consent to such action shall be required unless the Board
determines that the action, taking into account any related action, would not
materially and adversely affect the Participant.

      h. Conditions on Delivery of Shares. The Company will not be obligated to
deliver any Shares pursuant to the Plan or to remove restrictions from Shares
previously delivered under the Plan until (i) all conditions of the Award have
been met or removed to the satisfaction of the Company, (ii) in the opinion of
the Company's counsel, all other legal matters in connection with the issuance
and delivery of such Shares have been satisfied, including any applicable
securities laws and any applicable exchange or market rules and regulations, and
(iii) the Participant has executed and delivered to the Company such
representations or agreements as the Company may consider appropriate to satisfy
the requirements of any applicable laws, rules or regulations.

      i. Acceleration. The Board may at any time provide that any Options shall
become immediately exercisable in full or in part, that any Restricted Share
Awards shall be free of all restrictions or that any other share-based Awards
may become exercisable in full or in part or free of some or all restrictions or
conditions, or otherwise realizable in full or in part, as the case may be.

6.    Miscellaneous.

      a. No Right To Employment or Other Status. No person shall have any claim
or right to be granted an Award, and the grant of an Award shall not be
construed as giving a Participant the right to continued employment or any other
relationship with the Company. The Company expressly reserves the right at any
time to dismiss or otherwise terminate its relationship with a Participant free
from any liability or claim under the Plan, except as expressly provided in the
applicable Award.

      b. No Rights As Shareholder. Subject to the provisions of the applicable
Award, no Participant or Designated Beneficiary shall have any rights as a
shareholder with respect to any Shares to be distributed with respect to an
Award until becoming the record holder of such Shares.

      c. Effective Date and Term of Plan. The Plan shall become effective on the
date on which it is adopted by the Board. No Awards shall be granted under the
Plan after the completion of ten years from the earlier of (i) the date on which
the Plan was adopted by the Board or (ii) the date the Plan was approved by the
Company's shareholders, but Awards previously granted may extend beyond that
date.


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      d. Amendment of Plan. The Board may amend, suspend or terminate the Plan
or any portion thereof at any time.

      e. Governing Law. The provisions of the Plan and all Awards made hereunder
shall be governed by and interpreted in accordance with the laws of the
Commonwealth of Massachusetts, without regard to any applicable conflicts of
law.


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