FORM OF VOTING AGREEMENT

THIS VOTING AGREEMENT (this “ Agreement ”) is made and entered into as of October 30, 2006 by and between Merck & Co., Inc., a New Jersey corporation (“ Acquiror ”), and the undersigned securityholder (“ Stockholder ”) of Sirna Therapeutics, Inc., a Delaware corporation (“ Sirna ”).

A.      Acquiror, Sirna and Merger Sub (as defined below) are concurrently entering into an Agreement and Plan of Merger (the “ Merger Agreement ”), which provides for the merger (the “ Merger ”) of Spinnaker Acquisition Corp., a Delaware corporation and a wholly owned subsidiary of Acquiror (“ Merger Sub ”), with and into Sirna, pursuant to which all outstanding capital stock of Sirna will be converted into the right to receive the consideration set forth in the Merger Agreement.

B.      Stockholder is the beneficial owner (as defined in Rule 13d-3 under the Securities Exchange Act of 1934, as amended (the “ Exchange Act ”)) of such number of shares of the outstanding capital stock of Sirna, and such number of shares of capital stock of Sirna issuable upon the exercise of outstanding options and warrants, as set forth on the signature page hereof.

C.      As an inducement and a condition to entering into the Merger Agreement by Acquiror, Acquiror has requested that Stockholder agree, and Stockholder has agreed (in Stockholder’s capacity as such, and not in any other capacity, including as a director or officer of Sirna, as applicable), to enter into this Agreement in order to facilitate the consummation of the Merger.

1.       Definitions .  For the purposes of this Agreement, capitalized terms that are used but not defined herein shall have the respective meanings ascribed thereto in the Merger Agreement.

(a)      “ Expiration Date ” shall mean the earliest to occur of (i) the date and time as the Merger Agreement shall have been validly terminated according to its terms and (ii) the date and time as the Merger shall become effective in accordance with the terms and conditions set forth in the Merger Agreement.

(b)      “ Person ” shall mean any individual, corporation, partnership, limited liability company, joint venture, association, trust, unincorporated organization or other entity.

(c)      “ Shares ” shall mean: (i) all securities of Sirna (including all shares of capital stock of Sirna and all options, warrants and other rights to acquire shares of capital stock of Sirna) owned by Stockholder as of the date of this Agreement, and (ii) all additional securities of Sirna (including all additional shares of capital stock of Sirna and all additional options, warrants and other rights to acquire shares of capital stock of Sirna) which Stockholder acquires beneficial ownership

during the period commencing with the execution and delivery of this Agreement until the Expiration Date.

(d)      “ Transfer ”   shall mean, with respect to any security, the direct or indirect assignment, sale, transfer, tender, pledge, hypothecation, or the gift, placement in trust, or other disposition of such security (excluding transfers by testamentary or intestate succession or otherwise by operation of law) or any right, title or interest therein (including, but not limited to, any right or power to vote to which the holder thereof may be entitled, whether such right or power is granted by proxy or otherwise), or the record or beneficial ownership thereof, the offer to make such a sale, transfer, or other disposition, and each agreement, arrangement or understanding, whether or not in writing, to effect any of the foregoing; provided , however , the exercise of any Warrant shall not be deemed to constitute the Transfer of such Warrant or the underlying Shares.

2.       Restriction on Transfer, Proxies and Non-Interference .  At all times during the period commencing with the execution and delivery of this Agreement and continuing until the Expiration Date, Stockholder shall not, directly or indirectly, (A) cause or permit the Transfer of any of the Shares to be effected or enter into any contract, option or other agreement with respect to, or consent to, a Transfer of, any of the Shares or Stockholder’s voting or economic interest therein, (B) grant any proxies or powers of attorney with respect to any of the Shares, deposit any of the Shares into a voting trust or enter into a voting agreement or other similar commitment or arrangement with respect to any of the Shares in contravention of the obligations of Stockholder under this Agreement, (C) request that Sirna register the Transfer in contravention of this Agreement of any certificate or uncertificated interest representing any of the Shares or (D) permit any such Shares to be, or become subject to, any pledges, liens, preemptive rights, security interests, claims, charges or other encumbrances or arrangements (each, an “ Encumbrance ”). 

3.       Agreement to Vote Shares . During the period commencing on the date hereof and continuing until the Expiration Date, at every meeting of stockholders of Sirna called with respect to any of the following, and at every adjournment or postponement thereof, and on every action or approval by written consent of stockholders of Sirna with respect to any of the following, Stockholder shall vote, to the extent not voted by the Person(s) appointed as proxies under Section 4, or shall cause the record holder of any Shares on the applicable record date to appear (in Person or by proxy) and vote the Shares entitled to vote thereon:

(a)      in favor of adoption and approval of the Merger Agreement and the Merger contemplated thereby, including each other action, agreement and transaction contemplated by or in furtherance of the Merger Agreement, the Merger and this Agreement;

(b)      against approval of any proposal made in opposition to, or in competition with, consummation of the Merger and the transactions contemplated by the Merger Agreement;

(c)      except as otherwise agreed to in writing in advance by Acquiror, against any other action, proposal, transaction or agreement that would compete with or serve to interfere with, delay, discourage, adversely affect or inhibit the timely consummation of the Merger; and

(d)      against any Acquisition Proposal (other than the Acquisition Proposal contemplated by the Merger Agreement).

4.       Irrevocable Proxy . Stockholder hereby irrevocably and unconditionally revokes any and all previous proxies granted with respect to the Shares. By entering into this Agreement, Stockholder hereby irrevocably and unconditionally grants a proxy appointing Richard Kender and John Mustillo of Acquiror as such Stockholder’s attorneys-in-fact and proxies, with full power of substitution, for and in such Stockholder’s name, to vote, express, consent or dissent, or otherwise to utilize such voting power solely as specifically set forth in Section 3 as to the matters specified in Section 3. The proxy granted by Stockholder pursuant to this Section 4 is coupled with an interest and is irrevocable and is granted in consideration of Acquiror entering into this Agreement and incurring certain related fees and expenses. Notwithstanding the foregoing, the proxy granted by Stockholder shall be revoked upon termination of this Agreement in accordance with its terms. Such irrevocable proxy is executed and intended to be irrevocable in accordance with Section 212(e) of the General Corporation Law of the State of Delaware (the “ DGCL ”). Acquiror covenants and agrees that Richard Kender and John Mustillo of Acquiror shall attend any stockholder meeting called with respect to the matters in Section 3 either in person or by proxy, and shall vote all the Shares as contemplated by Section 3 at any such meeting, including any adjournment or postponement thereof.

5.       No Solicitations . From the date hereof until the Expiration Date, Stockholder agrees neither Stockholder nor any of its Affiliates (it being agreed that the Company and its Subsidiaries shall not be considered Affiliates of Stockholder for purposes of this Section 5), officers or directors shall, and Stockholder shall not permit the employees, agents or representatives, including any investment banker, attorney, consultant or accountant of the Stockholder or any of its Affiliates on its behalf to, take any action prohibited by Section 7.2 or Section 7.3 of the Merger Agreement (assuming, for purposes of this Section 5, that Stockholder is a “Representative” of the Company as defined in the Merger Agreement).

6.       Alternative Transaction Payment .

(a)      If (i) the Merger Agreement shall have been terminated (A) by Sirna pursuant to Section 9.3(a) thereof or Acquiror pursuant to Section 9.4(a) or (b) thereof or (B) by Sirna or Acquiror pursuant to Section 9.2(a) or 9.2(b) thereof, and, in either case, a proposal for an Alternative Transaction shall have been made public and not been withdrawn prior to the time of the Stockholders’ Meeting (or at any adjournment thereof) and (ii) Sirna enters into a definitive agreement with respect to an Alternative Transaction within twelve (12) months after the termination of the Merger Agreement or an Alternative Transaction is consummated within twelve (12) months after the date of such termination, then Stockholder shall pay to Acquiror, within two business days after receipt, an amount equal to 50% of the Profit (as defined in Section 6(d) below), if any, (x) received by Stockholder or (y) that would have received by Stockholder as a result of Shares held by Stockholder on the date hereof in connection with consummation of such Alternative Transaction. Any payment to Acquiror hereunder shall be made in the same form as the consideration received from such transaction (and, if the consideration so received was in more than one form, then in the same proportion as the forms of consideration so received).

(b)      If Acquiror shall have increased the Merger Consideration to an amount per share greater than $13.00 and the Merger shall have been consummated, then each Stockholder shall pay to Acquiror, within two business days after receipt, an amount equal to 50% of the Profit received by

such Stockholder in connection with consummation of the Merger. Any payment to Acquiror hereunder shall be made in the same form as the consideration received from the Merger (and, if the consideration so received was in more than one form, then in the same proportion as the forms of consideration so received).

(c)      Any payment to be made hereunder on account of Profit (i) received in cash, shall be paid by wire transfer of same day funds to an account designated by Acquiror and (ii) received in the form of securities or other property, shall be paid through delivery to Acquiror of the securities or property, suitably endorsed for transfer free and clear of all liens, charges, encumbrances, voting agreements, and commitments of every kind (other than those imposed by, through or under the Alternative Transaction or as required by law).

(d)      (i)      For purposes of this Section 6, “Profit” of a Stockholder in connection with the consummation of an Alternative Transaction (or, in the case of Section 6(b) above, the Merger) shall equal the aggregate consideration that such Stockholder received or would have received as