STOCK RESTRICTION AGREEMENT

Exhibit 10.21

Attached hereto is a form of stock restriction agreement by and among the Registrant and each of the below-named persons. The stock restriction agreement by and among the Registrant and each of the below-named persons is substantially identical in all material respects to such form, except with respect to the details that are set forth below.

The number of shares and the exercise or purchase price of each of the awards listed in the table below is presented after giving effect to the business combination between Discovery Partners International, Inc. (“Discovery Partners”) and Infinity Pharmaceuticals, Inc. (“IPI”) in accordance with the terms of the Agreement and Plan of Merger among Discovery Partners, Darwin Corp, a wholly owned subsidiary of Discovery Partners (“Darwin Corp.”), and IPI dated as of April 11, 2006, pursuant to which IPI merged with and into Darwin Corp. and became a wholly owned subsidiary of Discovery Partners and Discovery Partners changed its name to Infinity Pharmaceuticals, Inc. In addition, the number of shares and the exercise or purchase price of each of the awards listed in the table below is presented after giving effect to the Registrant’s 1-for-4 reverse stock split, which became effective on September 12, 2006.

(1) Each of these awards is currently fully vested. In accordance with the terms of each such award, the underlying shares were initially subject to a right of repurchase which right of repurchase lapsed, or “vested” as to the shares underlying the award in time-based installments.

STOCK RESTRICTION AGREEMENT

AGREEMENT made this          day of                          200  , between Infinity Pharmaceuticals, Inc., a Delaware corporation (the “Company”), and                              (the “Director”).

For valuable consideration, receipt of which is acknowledged, the parties hereto agree as follows:

1. Purchase of Shares . The Company shall issue and sell to the Director and the Director shall purchase from the Company, subject to the terms and conditions set forth in this Agreement and in the Company’s 2001 Stock Incentive Plan (the “Plan”), an aggregate of                  shares (the “Shares”) of common stock, $.0001 par value per share (“Common Stock”) of the Company at a price of $         per share (the “Option Price”), purchasable as set forth in and subject to the terms and conditions of this Agreement and the Plan.

The aggregate purchase price for the Shares shall be paid by the Director in accordance with the terms of the Plan and the Stock Option Agreement issued to the Director thereunder. Upon receipt of payment by the Company for the Shares, the Company shall issue to the Director one or more certificates in the name of the Director for that number of Shares purchased by the Director. The Director agrees that the Shares shall be subject to the Purchase Option set forth in Section 2 of this Agreement and the restrictions on transfer set forth in Section 4 of this Agreement.

2. Purchase Option .

(a) In the event that the Director ceases to provide services to the Company for any reason or no reason, with or without cause, prior to                             , the Company shall have the right and option (the “Purchase Option”) to purchase from the Director, for a sum equal to the Option Price per share, any shares then subject to the Purchase Option. All of the Shares shall be subject to the Purchase Option prior to                             . On                             ,                              of such Shares will no longer be subject to the Purchase Option and at the end of each full month thereafter,                              of such Shares shall no longer be subject to the Purchase Option until such time as all of such Shares are no longer subject to the Purchase Option.

(b) Upon the occurrence of a Change of Control Event (as hereinafter defined), the Purchase Option shall immediately lapse as to all remaining Unvested Shares, thereby rendering all Shares Vested Shares. For purposes of this subsection (c), a “Change of Control Event” shall mean:

(i) the acquisition by an individual, entity or group (within the meaning of Section 13(d)(3) or 14(d)(2) of the Exchange Act) (a “Person”) of beneficial ownership of any capital stock of the Company if, after such acquisition, such Person beneficially owns (within the meaning of Rule 13d-3 promulgated under the Exchange Act) 75% or more of either (x) the then-outstanding shares of Common Stock (the “Outstanding Company Common Stock”) or (y) the combined voting power of the then-outstanding securities of the Company entitled to vote generally in the election of directors (the “Outstanding Company Voting Securities”); provided , however , that for purposes of this subsection (i), the following acquisitions shall not constitute a Change in Control Event: (A) any acquisition directly from the Company (excluding an

acquisition pursuant to the exercise, conversion or exchange of any security exercisable for, convertible into or exchangeable for common stock or voting securities of the Company, unless the Person exercising, converting or exchanging such security acquired such security directly from the Company or an underwriter or agent of the Company), (B) any acquisition by any employee benefit plan (or related trust) sponsored or maintained by the Company or any corporation controlled by the Company, or (C) any acquisition by any corporation pursuant to a Business Combination (as defined below) which complies with clauses (x) and (y) of subsection (ii) of this definition; or

(ii) the consummation of a merger, consolidation, reorganization, recapitalization or share exchange involving the Company or a sale or other disposition of all or substantially all of the assets of the Company (a “Business Combination”), unless, immediately following such Business Combination, each of the following two conditions is satisfied: (x) all or substantially all of the individuals and entities who were the beneficial owners of the Outstanding Company Common Stock and Outstanding Company Voting Securities immediately prior to such Business Combination beneficially own, directly or indirectly, more than 50% of the then-outstanding shares of common stock and the combined voting power of the then-outstanding securities entitled to vote generally in the election of directors, respectively, of the resulting or acquiring corporation in such Business Combination (which shall include, without limitation, a corporation which as a result of such transaction owns the Company or substantially all of the Company’s assets either directly or through one or more subsidiaries) (such resulting or acquiring corporation is referred to herein as the “Acquiring Corporation”) in substantially the same proportions as their ownership of the Outstanding Company Common Stock and O