Document:

EXHIBIT
      4.1

    

    AMENDMENT
      NUMBER TWO TO RIGHTS AGREEMENT

    BETWEEN

    SIRNA
      THERAPEUTICS, INC.

    AND

    AMERICAN
      STOCK TRANSFER AND TRUST COMPANY

    

    This
      Amendment Number Two to Rights Agreement (this “Second Amendment”), made as of
      this 30th day of October, 2006, between Sirna Therapeutics, Inc., a Delaware
      corporation (the “Company”), and American Stock Transfer & Trust Company, a
      New York banking corporation, as rights agent (the “Rights Agent”), amends the
      Rights Agreement dated as of November 22, 2000 between the Company and the
      Rights Agent (as amended to date, the “Rights Agreement”).

    

    RECITALS

    

    A.    Pursuant
      to the Rights Agreement, the Board of Directors of the Company authorized and
      declared a dividend distribution of one right (each a “Right” and collectively
      the “Rights”) for each share of the Common Stock (as defined in the Rights
      Agreement) of the Company outstanding at the close of business on December
      8,
      2000 (the “Record Date”), and authorized the issuance of one Right in respect of
      each share of common stock of the Company issued between the Record Date and
      the
      Distribution Date (as such term is defined in Section 3 of the Rights
      Agreement), each Right representing the right to purchase one one-hundredths
      of
      a share of Series AA Preferred Stock of the Company having the rights, powers
      and preferences set forth in the form of Certificate of Designation attached
      to
      the Rights Agreement as Exhibit A, upon the terms and subject to the conditions
      set forth in the Rights Agreement.

    

    B.    On
      February 11, 2003, the Company and the Rights Agent entered into an Amendment
      to
      Rights Agreement (the “First Amendment”) that amended the Rights Agreement to,
      among other matters, exempt the Investors (as defined in the First Amendment)
      from the definition of “Acquiring Person” under the Rights Agreement and to
      clarify that the announcement, approval, execution or delivery of the Stock
      Purchase Agreement (as defined in the First Amendment) shall not cause a
“Distribution Date” to have occurred under the Rights Agreement.

    

    C.    The
      Board
      of Directors of the Company has determined that it is in the best interests
      of
      the Company and its stockholders to further amend the Rights Agreement,
      including all applicable sections, to, among other things, revise the
      definitions of (i) “Acquiring Person” to exempt Merck & Co., Inc., a
      Delaware corporation (“Parent”), Spinnaker Acquisition Corp.,
      a
      Delaware corporation and a wholly-owned subsidiary of Parent (“Merger Sub”), and
      all of their respective affiliates therefrom,
      and (ii) “Expiration Date” to mean immediately prior to the Effective Time, as
      such term is defined in the Agreement and Plan of Merger, dated as of October
      30, 2006, by and among the Company, Parent and Merger
      Sub (the “Merger Agreement”), and to clarify that no “Shares Acquisition Date”
or “Distribution Date” will occur solely by reason of the approval, execution or
      delivery of the Merger Agreement or the Voting Agreements (as defined in the
      Merger Agreement) or the consummation of the transactions contemplated
      thereby.
      

    
      
         

      

      
        -1-

        
          

        

      

      
         

      

       

    

    D.    Pursuant
      to Section 27 of the Rights Agreement, the Board of Directors of the Company
      has
      determined that an amendment to the Rights Agreement as set forth herein is
      necessary and desirable in order to reflect the foregoing, and the Company
      and
      the Rights Agent desire to evidence such amendment in writing. 

    

    NOW
      THEREFORE, intending to be legally bound, the Company and the Rights Agent
      hereby agree that the Rights Agreement is hereby amended as set forth
      below:

    

    1.    Section
      1(a) of the Rights Agreement is amended to add the following sentence at the
      end
      thereof:

    

    “Notwithstanding
      anything in this Rights Agreement to the contrary, none of Parent, Merger
      Sub, or any of their respective Affiliates or Associates, individually
      or collectively, shall be deemed to be an Acquiring Person solely as a result
      of
      (i) the announcement, approval, execution or delivery of the Merger Agreement
      or
      the Voting Agreements or (ii) the consummation of the transactions contemplated
      by the Merger Agreement or the Voting Agreements.”

    

    2.    The
      following Section 1(k) is inserted into the Rights Agreement, and all subsequent
      subsections of Section 1 are renumbered accordingly, and all cross-references
      to
      such renumbered subsections are changed to refer to such subsections as if
      renumbered:

    

    “1(k)
      “Merger Agreement” shall mean the Agreement and Plan of Merger, dated as of
      October 30, 2006, by and among Merck & Co., Inc., a Delaware corporation,
      Spinnaker Acquisition Corp., a Delaware corporation and a wholly-owned
      subsidiary of Parent, and the Company, as it may be amended from time to time.
      The defined terms “Parent,” “Merger Sub,” “Effective Time” and “Voting
      Agreements” used herein shall have the meanings ascribed to such terms in the
      Merger Agreement.”

    

    3.    Section
      1(s) (after giving effect to the renumbering caused by this Second Amendment)
      of
      the Rights Agreement is hereby amended to add the following sentence at the
      end
      thereof:

    

    “Notwithstanding
      anything in this Rights Agreement to the contrary, a Stock Acquisition Date
      shall not be deemed to have occurred solely as a result of (i) the announcement,
      approval, execution or delivery of the Merger Agreement or the Voting Agreements
      or (ii) the consummation of the transactions contemplated by the Merger
      Agreement or the Voting Agreements.”

    

    4.    Section
      3(a) of the Rights Agreement is amended to add the following sentence at the
      end
      thereof: 

    

    “Notwithstanding
      anything in this Rights Agreement to the contrary, a Distribution Date shall
      not
      be deemed to have occurred solely as a result of (i) the announcement,
      approval, execution or delivery of the Merger Agreement or the Voting Agreements
      or (ii) the consummation of the transactions contemplated by the Merger
      Agreement or the Voting Agreements.”

    
      
         

      

      
        -2-

        
          

        

      

      
         

      

    

    

    5.    The
      second to last sentence of Section 7(a) of the Rights Agreement is deleted
      in
      its entirely and replaced with the following sentence: 

    

    “The
      “Expiration Date,” as used in this Rights Agreement, shall be the earliest of
      (i) the Final Expiration Date (as defined below), (ii) the time at which the
      Rights are redeemed as provided in Section 23 hereof, (iii) the time at which
      the Rights are exchanged as provided in Section 24 hereof or (iv) the time
      immediately prior to the Effective Time.”

    

    6.    Section
      11(b)(ii) of the Rights Agreement is amended to add the following sentence
      at
      the end thereof:

    

    “Notwithstanding
      anything in this Rights Agreement to the contrary, neither (i) the announcement,
      approval, execution or delivery of the Merger Agreement or the Voting Agreements
      nor (ii) the consummation of the transactions contemplated by the Merger
      Agreement or the Voting Agreements shall be deemed to be an event described
      in
      Section 11(b)(i) and shall not cause the Rights to be adjusted or exercisable
      in
      accordance with Section 11 or 12.”

    

    7.    Section
      13(a) of the Rights Agreement is amended to add the following sentence at the
      end thereof:

    

    “Notwithstanding
      anything in this Rights Agreement to the contrary, neither (i) the announcement,
      approval, execution or delivery of the Merger Agreement or the Voting Agreements
      nor (ii) the consummation of the transactions contemplated by the Merger
      Agreement or the Voting Agreements shall be deemed to be an event described
      in
      this Section 13(a) and shall not cause the Rights to be adjusted or exercisable
      in accordance with Section 12 or 13.”

    

    8.    Section
      13(b)(ii) of the Rights Agreement is amended to add the following at the end
      thereof:

    

    “Notwithstanding
      anything in this Rights Agreement to the contrary, none of Parent, Merger
      Sub, or any of their respective Affiliates or Associates, individually
      or collectively, shall be deemed to be a Principal Party solely as a result
      of
      (i) the announcement, approval, execution or delivery of the Merger Agreement
      or
      the Voting Agreements or (ii) the consummation of the transactions contemplated
      by the Merger Agreement or the Voting Agreements.”

    

    9.    The
      following Section 15 is inserted into the Rights Agreement, and all subsequent
      sections are renumbered accordingly, and all cross-references to such renumbered
      sections are changed to refer to such sections as if renumbered:

    
      
         

      

      
        -3-

        
          

        

      

      
         

      

    

     

    “15.
      TREATMENT OF MERGER AGREEMENT. Notwithstanding anything in this Rights Agreement
      to the contrary, neither (i) the announcement, approval, execution nor delivery
      of the Merger Agreement or the Voting Agreements or (ii) the consummation of
      the
      transactions contemplated by the Merger Agreement or the Voting Agreements,
      shall cause a Distribution Date, a Stock Acquisition Date, a Flip-In Event
      or a
      Flip-Over Event.”

    

    10.    EFFECTIVENESS.
      This Second Amendment shall be deemed effective immediately upon execution
      hereof by the Company and the Rights Agent. Except as amended hereby, the Rights
      Agreement shall remain in full force and effect and shall be otherwise
      unaffected hereby.

    

    11.    MISCELLANEOUS.
      This Second Amendment shall be deemed to be a contract made under the laws
      of
      the State of Colorado and for all purposes shall be governed by and construed
      in
      accordance with the laws of such state applicable to contracts made and
      performed entirely within such state. This Second Amendment may be executed
      in
      any number of counterparts, each of such counterparts shall for all purposes
      be
      deemed to be an original, and all such counterparts shall together constitute
      one and the same instrument. If any provision, covenant or restriction of this
      Second Amendment is held by a court of competent jurisdiction or other authority
      to be invalid, illegal or unenforceable, the remainder of the terms, provisions,
      covenants and restrictions of this Second Amendment shall remain in full force
      and effect and shall in no way be effected, impaired or invalidated. In the
      event of any conflict or inconsistency between this Second Amendment, on the
      one
      hand, and the First Amendment, on the other hand, this Second Amendment shall
      govern.

    

    

     

    [Signature
      Page Follows]

    
      
         

      

      
        -4-

        
          

        

      

      
         

      

    

     

    IN
      WITNESS WHEREOF, the parties hereto have executed this Second Amendment as
      of
      the date first set forth above.

     

    

      
        	
                SIRNA
                  THERAPEUTICS, INC.

              	 	
                AMERICAN
                  STOCK TRANSFER AND TRUST COMPANY

              
	 	 	 
	 	 	 
	
                By: 
                  /s/
                  Howard W. Robin

              	 	
                By: 
                  /s/
                  Wilbert Myles

              
	
                Name:
                  Howard W. Robin

              	 	
                Name:
                  Wilbert Myles

              
	
                Title:
                  President and Chief Executive Officer

              	 	
                Title:
                  Vice President

              

      

    

     

     

    
      
         

      

        -5-EXHIBIT
      10.1

     

    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”).

     

    RECITALS:

     

    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.

    

    NOW,
      THEREFORE,
      intending to be legally bound, the parties hereto agree as follows:

     

    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). 

      
        
           

        

        
          -2-

          
            

          

        

        
           

        

      

    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

    
      
         

      

      
        -3-

        
          

        

      

      
         

      

    

    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 a result of the Shares held by such Stockholder on
      the
      date hereof, directly or indirectly, as a result of such consummation, valuing
      any non-cash consideration (including any residual interest in Sirna or any
      successor whether represented by shares of Sirna Common Stock or other
      securities of Sirna or any successor to the extent that Sirna has engaged in
      a
      spin-off, recapitalization or similar transaction) at its fair market value
      as
      of the date of consummation less the amount of the aggregate consideration
      Stockholder received or would have received as a result of consummation of
      the
      Merger (assuming Merger Consideration equal to $13.00 in cash). Stockholder
      expressly agrees that Stockholder’s obligations to Acquiror under this Section 6
      are personal obligations of Stockholder and that Stockholder’s obligation to pay
      Profit to Parent under this Section 6 shall not be affected as a result of
      any
      Transfer of the Shares following the Expiration Date. Stockholder waives any
      right to any cross-claim against a future holder of the Shares in response
      to a
      claim by Parent for Profit pursuant to this Section 6.

     

    (ii)    The
      fair
      market value of any non-cash consideration consisting of (A) securities listed
      on a national securities exchange or traded on the Nasdaq National Market of
      The
      Nasdaq Stock Market, Inc. (“Nasdaq
      National Market”)
      shall
      be equal to the average of the closing price per share of such security as
      reported on such exchange or Nasdaq National Market for each of the five (5)
      trading days prior to the date of determination, provided that such securities
      are not subject by law or agreement with Acquiror to any transfer restrictions
      and such securities do not represent in the aggregate 10% or more of the
      outstanding securities of the same class of securities of which such securities
      are a part; and (B) consideration which is other than cash or securities of
      the
      type specified in subclause (A) above shall be the amount a reasonable, willing
      seller would pay a reasonable, willing buyer, taking into account the nature
      and
      terms of such property. In the event of a dispute as to the fair market value
      of
      such property, such disputed amounts shall be determined, which determination
      shall be binding on all parties to this Agreement and shall be made by a
      nationally recognized independent banking firm mutually agreed upon by the
      parties, within ten (10) business days of the event requiring selection of
      such
      investment banking firm; provided,
      however,
      that if
      Acquiror and the Stockholder are unable to agree within two (2) business days
      after the date of such event as to the investment banking firm, then Acquiror,
      on the one hand, and the Stockholder, on the other hand, shall each select
      one
      firm, and those firms shall select a third investment banking firm, which third
      firm shall make a determination; provided further,
      that
      the fees

      
        
           

        

        
          -4-

          
            

          

        

        
           

        

      

    and
      expenses of such investment banking firm shall be borne by the Stockholder.
      The
      determination of the investment banking firm shall be binding upon the parties
      hereto. 

     

    (iii)    In
      the
      event that Sirna shall declare and pay a stock or extraordinary dividend or
      other distribution, or effect a stock split, reverse stock split,
      reclassification, reorganization, recapitalization, combination or other like
      changes with respect to the shares of Sirna Common Stock, the calculations
      set
      forth in this Section 6 shall be adjusted to reflect fully such dividend,
      distribution, stock split, reverse stock split, reclassification,
      reorganization, recapitalization or combination (including any residual interest
      in Sirna or any successor whether represented by the shares of Sirna Common
      Stock or other securities of Sirna or any successor to the extent that Sirna
      has
      engaged in a spin-off, recapitalization or similar transaction) and shall be
      considered in determining the Profit as provided in this Section 6.

     

    7.    Representations
      and Warranties and Agreements of Stockholder.
      Stockholder hereby represents and warrants to Acquiror that, as of the date
      hereof and at all times until the Expiration Date:

     

    (a)    Stockholder
      is the beneficial owner of all of the Shares set forth on the signature page
      of
      this Agreement. Stockholder has sole voting power and sole power of disposition
      with respect to all of the Shares set forth on the signature page hereof, with
      no limitations, qualifications or restrictions on such rights, subject to
      applicable federal securities laws and the terms of this Agreement; provided,
      however,
      Stockholder is affiliated with a number of funds, entities and individuals
      that
      form part of [fund]1 ,
      and one
      or more of such other related funds, entities or individuals may also be deemed
      to beneficially own, solely from an economic perspective, a portion or all
      of
      such Shares. Stockholder does not beneficially own any securities of Sirna
      other
      than the Shares set forth on the signature page of this Agreement, as
      supplemented from time to time pursuant to Section 11 hereof.

     

    (b)    Stockholder
      acknowledges and agrees that all Warrants held by it or its Affiliates will,
      immediately after the Merger (to the extent not exercised prior thereto), only
      be exercisable for cash as and to the extent provided in such
      Warrants.

     

    (c)    The
      Shares are free and clear of any Encumbrances or other encumbrances of any
      kind
      or nature.

     

    (d)    Stockholder
      has the legal capacity, power and authority to enter into and perform all of
      Stockholder’s obligations under this Agreement. The execution, delivery and
      performance of this Agreement by Stockholder will not violate or breach, and
      will not give rise to any violation or breach of, Stockholder’s certificate of
      formation or limited liability company agreement or other organizational
      documents (if Stockholder is not an individual), or any law, court order,
      contract, instrument, arrangement or agreement by which such Stockholder is
      a
      party or is subject, including, without limitation, any voting agreement or
      voting trust. This Agreement has been duly and validly

    
       

       

      
        

      

    

    
      	1	
              Include
                name of applicable fund
                group.

            

    

     

    
      
        
           

        

        
          -5-

          
            

          

        

        
           

        

      

    

    

    executed
      and delivered by Stockholder and constitutes a valid and binding agreement
      of
      Stockholder, enforceable against Stockholder in accordance with its terms,
      subject to general principles of equity and as may be limited by bankruptcy,
      insolvency, moratorium or similar laws affecting creditors’ rights generally.

     

    (e)    The
      execution and delivery of this Agreement by Stockholder does not, and, to the
      best of Stockholder’s knowledge, the performance by Stockholder of his, her or
      its obligations hereunder will not, require Stockholder to obtain any consent,
      approval, authorization or permit of, or to make any filing with or notification
      to, any Governmental Entity, other than required filings under Section 13 of
      the
      Exchange Act.

     

    (f)    Each
      Stockholder will, in its capacity as a beneficial owner of the Shares,
      (i) use all reasonable efforts to cooperate with Sirna and Acquiror in
      connection with the Merger, (ii) provide any information reasonably requested
      by
      Sirna or Acquiror that Stockholder is legally and contractually permitted to
      provide for any regulatory application or filing made or approval sought for
      the
      Merger and (iii) make all filings required by Stockholder to be made with all
      third parties and Governmental Entities necessary for the consummation of the
      transactions contemplated by this Agreement and the Merger Agreement and other
      documents in connection with the Merger. 

     

    8.    Representations
      and Warranties of Acquiror.
      Acquiror hereby represents and warrants to the Stockholder that, as of the
      date
      hereof, Acquiror has the legal capacity, power and authority to enter into
      and
      perform all of its obligations under this Agreement. The execution, delivery
      and
      performance of this Agreement by Acquiror will not violate or breach, and will
      not give rise to any violation or breach of, its certificate of incorporation
      or
      any law, court order, contract, instrument, arrangement or agreement by which
      such Acquiror is a party or is subject. This Agreement has been duly and validly
      executed and delivered by Acquiror and constitutes a valid and binding agreement
      of Acquiror, enforceable against Acquiror in accordance with its terms, subject
      to general principles of equity and as may be limited by bankruptcy, insolvency,
      moratorium or similar laws affecting creditors’ rights generally.

     

    9.    Consent.
      Stockholder consents and authorizes Acquiror and Sirna to publish and disclose
      in the Proxy Materials (including all documents filed with the SEC in connection
      therewith) its identity and ownership of the Shares and the nature of its
      commitments, arrangements and understandings under this Agreement.

     

    10.    No
      Ownership Interest. 
      Nothing contained in this Agreement shall be deemed to vest in Acquiror any
      direct or indirect ownership or incidence of ownership of or with respect to
      any
      Shares.  Except as provided in this Agreement, all rights, ownership and
      economic benefits relating to the Shares shall remain vested in and belong
      to
      Stockholder.

     

    11.    Stockholder
      Notification of Acquisition of Additional Shares. 
      At all times during the period commencing with the execution and delivery of
      this Agreement and continuing until the Expiration Date, Stockholder shall
      promptly notify Acquiror of the number of any additional Shares and the number
      and type of any other voting securities of Sirna acquired by Stockholder, if
      any, after the date hereof.

      
        
           

        

        
          -6-

          
            

          

        

        
           

        

      

    12.    Directors
      and Officers. 
      Notwithstanding anything in this Agreement to the contrary, if Stockholder
      or
      any affiliate thereof is a director or officer of Sirna, nothing contained
      in
      this Agreement shall prohibit such director or officer from (i) acting in
      his/her capacity as such or from taking such action as a director or officer
      of
      Sirna that may be required on the part of such Person as a director or officer
      of Sirna, including, without limitation, acting in compliance with Sections
      7.2
      and 7.3 of the Merger Agreement, but only to the extent that Sirna is permitted
      to take such actions under the aforementioned Sections or (ii) complying with
      such director or officer’s fiduciary duties under applicable law (in the context
      of, and in the manner permitted by, Sections 7.2 and 7.3 of the Merger
      Agreement).

     

    13.    Termination.
      Except
      as otherwise provided in this Section 13, this Agreement shall terminate and
      be
      of no further force or effect as of the Expiration Date. Notwithstanding
      anything to the contrary contained herein, the provisions of Section 6 and
      Section 15 shall survive the expiration or sooner termination of this Agreement.
      

     

    14.    Appraisal
      Rights.
      Stockholder irrevocably waives and agrees not to exercise any rights (including,
      without limitation, under Section 262 of the DGCL) to demand appraisal of any
      of
      the Shares which may arise with respect to the Merger. 

     

    15.    Miscellaneous.
      

     

    (a)    Entire
      Agreement. 
      This Agreement constitutes the entire agreement between the parties hereto
      with
      respect to the subject matter hereof and supersedes all other prior agreements
      and understandings, both written and oral, between the parties with respect
      to
      the subject matter hereof.

     

    (b)    Certain
      Events. 
      This Agreement and the obligations hereunder shall attach to all of the Shares
      and shall be binding upon any Person to whom legal or beneficial ownership
      of
      any of the Shares shall pass, whether by operation of law or
      otherwise.   

     

    (c)    Assignment. 
      Neither
      this Agreement nor any of the rights, interests or obligations hereunder shall
      be assigned by any of the parties hereto (whether by operation of law or
      otherwise) without the prior written consent of the other parties.

    

    (d)    Amendment. 
      This Agreement may not be amended except by an instrument in writing signed
      on
      behalf of each of the parties hereto.

    

    (e)    Notices.
      Any
      notice, request, instruction or other document to be given hereunder by any
      party to the others shall be in writing and delivered personally or sent by
      registered or certified mail, postage prepaid, facsimile or by overnight
      courier:

     

    If
      to
      Aquiror:

    

    Merck
      & Co., Inc.

    One
      Merck
      Drive

    P.O.
      Box
      100, WS3A-65

    Whitehouse
      Station, NJ 08889-0100

    Attention:
       Office
      of
      the Secretary

    Facsimile:
       (908)
      735-1246

    
      
         

      

      
        -7-

        
          

        

      

      
         

      

    

    

    with
      a
      copy, which will not constitute notice, to:

     

    Fried,
      Frank, Harris, Shriver & Jacobson LLP

    One
      New
      York Plaza

    New
      York,
      NY 10004

    Attention:
       David
      N.
      Shine/Brian T. Mangino

    Facsimile:
       (212)
      859-4000

    

    If
      to
      Stockholder, to the address for notice set forth on the signature
      page hereof.

     

    with
      a
      copy, which will not constitute notice, to:

     

    O’Melveny
      & Myers LLP

    2765
      Sand
      Hill Road

    Menlo
      Park, California 94025

    Attention:
       Sam
      Zucker

    Facsimile:
       (650)
      473-2601

     

    or
      to
      such other persons or addresses as may be designated in writing by the Person
      to
      receive such notice as provided above. Any
      notice, request, instruction or other document given as provided above shall
      be
      deemed given to the receiving party upon actual receipt, if delivered
      personally; three (3) business days after deposit in the mail, if sent by
      registered or certified mail; upon confirmation of successful transmission
      if
      sent by facsimile; or on the next business day after deposit with an
      internationally recognized overnight courier, if sent by such a
      courier.

     

    (f)    Severability. 
      The provisions of this Agreement shall be deemed severable and the invalidity
      or
      unenforceability of any provision shall not affect the validity or
      enforceability of the other provisions hereof. If any provision of this
      Agreement, or the application thereof to any Person or any circumstance is
      determined by a court of competent jurisdiction to be invalid, void or
      unenforceable the remaining provisions hereof, shall, subject to the following
      sentence, remain in full force and effect and shall in no way be affected,
      impaired or invalidated thereby, so long as the economic or legal substance
      of
      the transactions contemplated hereby is not affected in any manner adverse
      to
      either party. Upon such determination, the parties shall negotiate in good
      faith
      in an effort to agree upon such a suitable and equitable provision to effect
      the
      original intent of the parties.

     

    (g)    No
      Waiver. 
      At any time prior to the Effective Time, the parties hereto may, to the extent
      legally allowed, waive compliance with any of the obligations contained herein.
      Any agreement on the part of a party hereto to any such waiver shall be valid
      only if set forth in a written instrument signed on behalf of such party, but
      such waiver or failure to insist on strict compliance with an obligation
      contained herein shall not operate as a waiver of, or estoppel with respect
      to,
      any subsequent or other failure.

    
      
         

      

      
        -8-

        
          

        

      

      
         

      

    

    (h)    Governing
      Law. 
      This Agreement shall be governed and construed in accordance with the laws
      of
      the State of Delaware, without regard to any applicable conflicts of law rules.
      

     

    (i)    Enforcement
      of Agreement.
      The
      parties hereto agree that irreparable damage would occur in the event that
      the
      provisions of this Agreement were not performed in accordance with its specific
      terms or were otherwise breached. It is accordingly agreed that the parties
      shall be entitled to seek, without the posting of a bond, an injunction or
      injunctions to prevent breaches of this Agreement and to enforce specifically
      the terms and provisions thereof in any court of the United States or any state
      having jurisdiction, this being in addition to any other remedy to which they
      are entitled at law or in equity.

     

    (j)    Counterparts;
      Signatures.
      This
      Agreement may be executed in two or more counterparts, each of which shall
      be
      deemed an original but all of which together shall be considered one and the
      same agreement and shall become effective when counterparts have been signed
      by
      each of the parties hereto and delivered to the other parties, it being
      understood that all parties need not sign the same counterpart. This Agreement
      may be executed and delivered by facsimile transmission.

     

    (k)    Expenses.
      Whether
      or not the Merger is consummated, all costs and expenses incurred in connection
      with this Agreement shall be paid by the party incurring such expense. The
      Acquiror will not object if Sirna reimburses up to $10,000 of Stockholder’s
      incurred legal fees and expenses in connection with this Agreement and related
      matters.

     

     

     

    [SIGNATURE
      PAGE FOLLOWS]

    
      
         

      

      
        -9-

        
          

        

      

      
         

      

    

     

    IN
      WITNESS WHEREOF,
      the
      undersigned have executed, or caused this Voting Agreement to be executed by
      a
      duly authorized officer, as of the date first written above.

     

    
      	
              MERCK
                & CO., INC.

            	 	
              STOCKHOLDER:

            
	 	 	 
	
              By:
                ______________________________________

            	 	
              By:
                _________________________________

            
	
              Signature
                of Authorized Signatory

            	 	
              Signature

            
	
              Name:
                ____________________________________

            	 	
              Name:
                _______________________________

            
	
              Title:
                _____________________________________

            	 	
              Title:
                ________________________________

            
	 	 	________________________________
	 	 	________________________________
	 	 	
              Print
                Address

            
	 	 	 
	 	 	
              Shares
                beneficially owned:

            
	 	 	 
	 	 	
              __________
                shares of Sirna Common Stock

            
	 	 	 
	 	 	
              __________
                shares of Sirna Common Stock issuable upon the exercise of outstanding
                options, warrants or other rights

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