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    EXHIBIT
      4.18 

     

    SIRNA
      THERAPEUTICS, INC. 

    Issuer

     

    and

     

    

     

    AS
      DEPOSITARY 

     

    and

     

    HOLDERS
      OF DEPOSITARY RECEIPTS

     

    

     

    DEPOSIT
      AGREEMENT 

     

    

     

    Dated
      as of 

     

    [
      __________________ ]

    
    

    

    TABLE
      OF CONTENTS 

     

    
      	
            	
            	
            	
            	
            
	
              ARTICLE
                I

            	  	
              DEFINITIONS

            	  	1
	
            	
            	
            
	
              ARTICLE
                II

            	  	
              FORM
                OF RECEIPTS, DEPOSIT OF
                STOCK, EXECUTION AND DELIVERY, TRANSFER, SURRENDER AND REDEMPTION
                AND
                REPURCHASE OF RECEIPTS

            	  	2
	
              Section
                2.1

            	  	
              Form
                and Transfer of
                Receipts

            	  	2
	
              Section
                2.2

            	  	
              Deposit
                of Stock; Execution
                and Delivery of Receipts in Respect Thereof

            	  	3
	
              Section
                2.3

            	  	
              Redemption
                and Repurchase of
                Stock

            	  	4
	
              Section
                2.4

            	  	
              Register
                of Transfer of
                Receipts

            	  	5
	
              Section
                2.5

            	  	
              Combination
                and Split-ups of
                Receipts

            	  	5
	
              Section
                2.6

            	  	
              Surrender
                of Receipts and
                Withdrawal of Stock

            	  	5
	
              Section
                2.7

            	  	
              Limitations
                on Execution and
                Delivery, Transfer, Split-up, Combination and Surrender of Receipts
                and
                Withdrawal or Deposit of Stock

            	  	
              6

            
	
              Section
                2.8

            	  	
              Lost
                Receipts,
                etc

            	  	6
	
              Section
                2.9

            	  	
              Cancellation
                and Destruction
                of Surrendered Receipts

            	  	6
	
              Section 2.10

            	  	
              Conversion

            	  	6
	
            	
            	
            
	
              ARTICLE
                III

            	  	
              CERTAIN
                OBLIGATIONS OF HOLDERS
                OF RECEIPTS AND THE COMPANY

            	  	7
	
              Section
                3.1

            	  	
              Filing
                Proofs, Certificates
                and Other Information

            	  	7
	
              Section
                3.2

            	  	
              Payment
                of Taxes or Other
                Governmental Charges

            	  	7
	
              Section
                3.3

            	  	
              Withholding

            	  	8
	
              Section
                3.4

            	  	
              Representations
                and Warranties
                as to Stock

            	  	8
	
            	
            	
            
	
              ARTICLE IV

            	  	
              THE
                STOCK,
                NOTICES

            	  	8
	
              Section
                4.1

            	  	
              Cash
                Distributions

            	  	8
	
              Section
                4.2

            	  	
              Distributions
                Other Than
                Cash

            	  	8
	
              Section
                4.3

            	  	
              Subscription
                Rights,
                Preferences or Privileges

            	  	9
	
              Section
                4.4

            	  	
              Notice
                of Dividends, Fixing of
                Record Date for Holders of Receipts

            	  	9
	
              Section
                4.5

            	  	
              Voting
                Rights

            	  	9
	
              Section
                4.6

            	  	
              Changes
                Affecting Stock and
                Reclassifications, Recapitalizations, etc

            	  	10
	
              Section
                4.7

            	  	
              Reports

            	  	10
	
              Section
                4.8

            	  	
              Lists
                of Receipt
                Holders

            	  	10
	
            	
            	
            
	
              ARTICLE
                V

            	  	
              THE
                DEPOSITARY, THE
                DEPOSITARY’S AGENTS, THE REGISTRAR AND THE COMPANY

            	  	10
	
              Section
                5.1

            	  	
              Maintenance
                of Offices,
                Agencies, Transfer Books by the Depositary; the Registrar

            	  	10
	
              Section
                5.2

            	  	
              Prevention
                or Delay in
                Performance by the Depositary, the Depositary’s Agents, the Registrar or
                the Company

            	  	11
	
              Section
                5.3

            	  	
              Obligations
                of the Depositary,
                the Depositary’s Agents, the Registrar and the Company

            	  	11
	
              Section
                5.4

            	  	
              Resignation
                and Removal of the
                Depositary, Appointment of Successor Depositary

            	  	12
	
              Section
                5.5

            	  	
              Corporate
                Notices and
                Reports

            	  	12
	
              Section
                5.6

            	  	
              Deposit
                of Stock by the
                Company

            	  	12
	
              Section
                5.7

            	  	
              Indemnification
                by the
                Company

            	  	13

    

    
    

    

    
      	
            	
            	
            	
            	
            
	
              Section
                5.8

            	  	
              Fees,
                Charges and
                Expenses

            	  	13
	
            	
            	
            
	
              ARTICLE VI

            	  	
              AMENDMENT
                AND
                TERMINATION

            	  	13
	
              Section
                6.1

            	  	
              Amendment

            	  	13
	
              Section
                6.2

            	  	
              Termination

            	  	13
	
            	
            	
            
	
              ARTICLE VII

            	  	
              MISCELLANEOUS

            	  	13
	
              Section
                7.1

            	  	
              Counterparts

            	  	13
	
              Section
                7.2

            	  	
              Exclusive
                Benefits of
                Parties

            	  	14
	
              Section
                7.3

            	  	
              Invalidity
                of
                Provisions

            	  	14
	
              Section
                7.4

            	  	
              Notices

            	  	14
	
              Section
                7.5

            	  	
              Depositary’s
                Agents

            	  	14
	
              Section
                7.6

            	  	
              Holders
                of Receipts Are
                Parties

            	  	14
	
              Section
                7.7

            	  	
              Governing
                Law

            	  	14
	
              Section
                7.8

            	  	
              Headings

            	  	15

    

    
    

    

    DEPOSIT
      AGREEMENT 

     

    DEPOSIT
      AGREEMENT, dated as of
      [                    ]
      among Sirna Therapeutics, Inc., a Delaware corporation,
      [                    ],
      a under the laws of the State of
      [                    ],
      as Depositary, and all holders from time to time of Receipts issued hereunder.
      

     

    W
      I T N E S S E T H: 

     

    WHEREAS,
      the Company desires to
      provide as hereinafter set forth in this Deposit Agreement, for the deposit
      of
      shares of the Stock with the Depositary, as agent for the holders of the
      Receipts evidencing Depositary Shares representing an interest in the Stock
      so
      deposited, for the purposes set forth in this Deposit Agreement and for the
      issuance hereunder of such Receipts; and 

     

    WHEREAS,
      the Receipts are to be
      substantially in the form annexed as Exhibit A to this Deposit Agreement, with
      appropriate insertions, modifications and omissions to reflect the terms of
      any
      Certificate of Designation and otherwise, as hereinafter provided in this
      Deposit Agreement. 

     

    NOW,
      THEREFORE, in consideration of
      the premises contained herein, it is agreed by and among the parties hereto
      as
      follows: 

     

    ARTICLE
      I 

    DEFINITIONS

     

    The
      following definitions shall
      apply to the respective terms (in the singular and plural forms of such terms)
      used in this Deposit Agreement and the Receipts: 

     

    “CERTIFICATE
      OF DESIGNATION” shall
      mean the Certificate of Designation establishing and setting forth the rights,
      preferences, privileges, limitations and restrictions of the Stock, as filed
      with the Secretary of State of the State of Delaware. 

     

    “CERTIFICATE
      OF INCORPORATION” shall
      mean the Amended and Restated Certificate of Incorporation, as amended or as
      amended and restated from time to time, of the Company. 

     

    “COMPANY”
      shall mean Sirna
      Therapeutics, Inc., a Delaware corporation, and its successors. 

     

    “CORPORATE
      OFFICE” shall mean the
      office of the Depositary in
      [                    ],
      at which at any particular time its business in respect of matters governed
      by
      this Deposit Agreement shall be administered, which at the date of this Deposit
      Agreement is located at
      [                    ]
      . 

     

    “DEPOSIT
      AGREEMENT” shall mean this
      agreement, as the same may be amended, modified or supplemented from time to
      time to reflect the terms of any Certificate of Designation or otherwise in
      accordance with the provisions hereof. 

     

    “DEPOSITARY”
      shall mean
      [                    ],
      as Depositary hereunder, and any successor as Depositary hereunder. 

     

    “DEPOSITARY
      SHARE” shall mean the
      rights evidenced by the Receipts executed and delivered hereunder, including
      the
      interests in Stock granted to holders of Receipts pursuant to the terms and
      conditions of the Deposit Agreement. Each Depositary Share shall represent
      an
      interest in
      [                    ]
      of one share of Stock deposited with the Depositary hereunder and the same
      proportionate interest in any and all other property received by the Depositary
      in respect of such share of Stock and held under this Deposit Agreement. Subject
      to the terms of this Deposit Agreement, each record holder of a Receipt
      evidencing a Depositary Share or Shares is entitled, 

     

    1

    
    

    

    proportionately,
      to all the rights,
      preferences and privileges, and subject to all the qualifications and
      restrictions, of the Stock represented by such Depositary Share or Shares,
      including any dividend, voting, conversion, redemption, liquidation and sinking
      fund rights contained in the Certificate of Designation, and to the benefits
      of
      all obligations and duties of the Company in respect of the Stock under the
      Certificate of Designation and the Certificate of Incorporation. 

     

    “DEPOSITARY’S
      AGENT” shall mean an
      agent appointed by the Depositary as provided, and for the purposes specified,
      in Section 7.5. 

     

    “RECEIPT”
      shall mean a Depositary
      Receipt executed and delivered hereunder, in substantially the form of Exhibit
      A
      hereto, evidencing a Depositary Share or Shares, as the same may be amended
      from
      time to time to reflect the terms of any Certificate of Designation or otherwise
      in accordance with the provisions hereof. 

     

    “RECORD
      HOLDER” or “HOLDER” as
      applied to a Receipt shall mean the person in whose name a Receipt is registered
      on the books maintained by or on behalf of the Depositary for such purpose.
      

     

    “REGISTRAR”
      shall mean any company
      appointed to register ownership and transfers of Receipts as herein provided.
      

     

    “SECURITIES
      ACT” shall mean the
      Securities Act of 1933, as amended. 

     

    “STOCK”
      shall mean shares of the
      Company’s Preferred Stock, Series
      [                    ],
      par value $0.01 per share. 

     

     

    ARTICLE
      II 

    FORM
      OF RECEIPTS, DEPOSIT OF STOCK,
      EXECUTION AND DELIVERY, 

    TRANSFER,
      SURRENDER AND REDEMPTION
      AND REPURCHASE OF RECEIPTS 

     

    SECTION
      2.1 FORM AND TRANSFER OF
      RECEIPTS. Receipts shall be engraved or printed or lithographed unless they
      are
      evidenced by a global receipt held by a depositary for a clearing system and
      shall be substantially in the form set forth in Exhibit A annexed to this
      Deposit Agreement, with appropriate insertions, modifications and omissions
      to
      reflect the terms of any Certificate of Designation or otherwise, as hereinafter
      provided. Receipts shall be executed by the Depositary by the manual signature
      of a duly authorized officer of the Depositary; provided, however, that such
      signature may be a facsimile if a Registrar (other than the Depositary) shall
      have countersigned the Receipts by manual signature of a duly authorized officer
      of the Registrar. Pending the preparation of definitive Receipts, the
      Depositary, upon the written order of the Company delivered in accordance with
      Section 2.2, shall execute and deliver temporary Receipts which shall
      be
      printed, lithographed, typewritten, or otherwise reproduced substantially of
      the
      tenor of the definitive Receipts in lieu of which they are issued and with
      appropriate insertions, modifications, omissions, substitutions and other
      variations as the persons executing such Receipts may determine are necessary
      for such temporary Receipts, as evidenced by their execution of such temporary
      Receipts. If temporary Receipts are issued, the Company and the Depositary
      will
      cause definitive Receipts to be prepared without unreasonable delay; provided
      that if such temporary Receipts are global Receipts held by a depositary for
      a
      clearing system, definitive Receipts need not be prepared until the Receipts
      cease to be so held. After the preparation of definitive Receipts, the temporary
      Receipts shall be exchangeable for definitive Receipts upon surrender of the
      temporary Receipts at the Corporate Office or such other office as the
      Depositary may designate, without charge to the holder. Upon surrender for
      cancellation of any one or more temporary Receipts, the Depositary shall execute
      and deliver in exchange therefore definitive Receipts representing the same
      number of Depositary Shares as represented by the surrendered temporary Receipt
      or Receipts. Such exchange shall be made at the Company’s expense and without
      any charge therefore. Until so exchanged, the temporary Receipts shall in all
      respects be entitled to the same benefits under this Deposit Agreement, and
      with
      respect to the Stock, as definitive Receipts. 

     

    No
      Receipt shall be entitled to any
      benefits under this Deposit Agreement or be valid or obligatory for any purpose
      unless it shall have been executed as provided in the preceding paragraph.
      

     

    2

    
    

    

    The
      Depositary shall record on its
      books each Receipt executed as provided above and delivered as hereinafter
      provided. Receipts bearing the facsimile signature of anyone who was at any
      time
      a duly authorized officer of the Depositary shall bind the Depositary,
      notwithstanding that such officer has ceased to hold such office prior to the
      delivery of such Receipts. 

     

    Receipts
      may be issued in
      denominations of any number of whole Depositary Shares. All Receipts shall
      be
      dated the date of their execution. 

     

    Receipts
      may be endorsed with or
      have incorporated in the text thereof such legends or recitals or changes not
      inconsistent with the provisions of this Deposit Agreement as may be required
      by
      the Depositary or required to comply with any applicable law or regulation
      or
      with the rules and regulations of any securities exchange upon which the Stock
      or the Depositary Shares may be listed or to conform with any usage with respect
      thereto, or to indicate any special limitations or restrictions to which any
      particular Receipts are subject by reason of the date of issuance of the Stock
      or otherwise. 

     

    Title
      to any Receipt (and to the
      Depositary Shares evidenced by such Receipt) that is properly endorsed or
      accompanied by a properly executed instrument of transfer shall be transferable
      by delivery with the same effect as in the case of investment securities in
      general; provided, however, that the Depositary may, notwithstanding any notice
      to the contrary, treat the record holder thereof at such time as the absolute
      owner thereof for the purpose of determining the person entitled to
      distributions of dividends or other distributions or to any notice provided
      for
      in this Deposit Agreement and for all other purposes. 

     

    SECTION
      2.2 DEPOSIT OF STOCK;
      EXECUTION AND DELIVERY OF RECEIPTS IN RESPECT THEREOF. Subject to the terms
      and
      conditions of this Deposit Agreement, the Company or any holder of Stock may
      deposit such Stock under this Deposit Agreement by delivery to the Depositary
      of
      a certificate or certificates for the Stock to be deposited, properly endorsed
      or accompanied, if required by the Depositary, by a properly executed instrument
      of transfer in form satisfactory to the Depositary, together with (i) all
      such certifications as may be required by the Depositary in accordance with
      the
      provisions of this Deposit Agreement and (ii) a written order of the
      Company or such holder, as the case may be, directing the Depositary to execute
      and deliver to or upon the written order of the person or persons stated in
      such
      order a Receipt or Receipts for the number of Depositary Shares representing
      such deposited Stock. 

     

    Upon
      receipt by the Depositary of a
      certificate or certificates for Stock to be deposited hereunder, together with
      the other documents specified above, the Depositary shall, as soon as transfer
      and registration can be accomplished, present such certificate or certificates
      to the registrar and transfer agent of the Stock for transfer and registration
      in the name of the Depositary or its nominee of the Stock being deposited.
      Deposited Stock shall be held by the Depositary in an account to be established
      by the Depositary at the Corporate Office. 

     

    Upon
      receipt by the Depositary of a
      certificate or certificates for Stock to be deposited hereunder, together with
      the other documents specified above, the Depositary, subject to the terms and
      conditions of this Deposit Agreement, shall execute and deliver, to or upon
      the
      order of the person or persons named in the written order delivered to the
      Depositary referred to in the first paragraph of this Section 2.2, a
      Receipt or Receipts for the number of whole Depositary Shares representing
      the
      Stock so deposited and registered in such name or names as may be requested
      by
      such person or persons. The Depositary shall execute and deliver such Receipt
      or
      Receipts at the Corporate Office, except that, at the request, risk and expense
      of any person requesting such delivery and for such person’s account or, upon
      the order of such person, any other person’s account, such delivery may be made
      at such other place as may be designated by such person. In each case, delivery
      will be made only upon payment to the Depositary of all taxes and other
      governmental charges and any fees payable in connection with such deposit and
      the transfer of the deposited Stock. 

     

    The
      Company shall deliver to the
      Depositary from time to time such quantities of Receipts as the Depositary
      may
      request to enable the Depositary to perform its obligations under this Deposit
      Agreement. 

     

    3

    
    

    

    SECTION
      2.3 REDEMPTION AND
      REPURCHASE OF STOCK. Whenever the Company shall redeem shares of Stock in
      accordance with a Certificate of Designation, it shall (unless otherwise agreed
      in writing with the Depositary) give the Depositary in its capacity as
      Depositary not less than 3 business days’ prior notice of the proposed date of
      the mailing of a notice of redemption of Stock and the simultaneous redemption
      of the Depositary Shares representing the Stock to be redeemed and of the number
      of such shares of Stock held by the Depositary to be redeemed. Unless the
      Certificate of Designation for a specific series of Stock provides for a
      different notice period with respect to that Stock in the event of its
      redemption, the Depositary shall, as directed by the Company in writing, mail,
      first class postage prepaid, notice of the redemption of Stock and the proposed
      simultaneous redemption of the Depositary Shares representing the Stock to
      be
      redeemed not less than 30 and not more than 60 days prior to the date fixed
      for
      redemption of such Stock and Depositary Shares, to the record holders of the
      Receipts evidencing the Depositary Shares to be so redeemed at the addresses
      of
      such holders as the same appear on the records of the Depositary or any
      Depositary’s Agent or Registrar. Notwithstanding the foregoing, neither failure
      to mail or publish any such notice to one or more such holders nor any defect
      in
      any notice shall affect the sufficiency of the proceedings for redemption.
      The
      Company shall provide the Depositary with such notice, and each such notice
      shall state the method for determining the amount payable per Depositary Share,
      the redemption date, and the number of Depositary Shares to be redeemed, and
      such notice shall call upon each holder of Depositary Shares to surrender,
      on
      the redemption date and at the place or places designated by the Company, the
      Receipts evidencing Depositary Shares to be redeemed. On the date of any such
      redemption the Depositary shall surrender the certificate or certificates held
      by the Depositary evidencing the number of shares of Stock to be redeemed in
      the
      manner specified in the notice of redemption of Stock provided by the Company
      pursuant to the applicable Certificate of Designation. The Depositary shall,
      thereafter, redeem the number of Depositary Shares representing such redeemed
      Stock upon the surrender of Receipts evidencing such Depositary Shares in the
      manner provided in the notice sent to record holders of Receipts. 

     

    Notice
      having been mailed by the
      Depositary as aforesaid, from and after the redemption date (unless the Company
      shall have failed to redeem the shares of Stock to be redeemed by it upon the
      surrender of the certificate or certificates therefore by the Depositary as
      described in the preceding paragraph), the Depositary Shares called for
      redemption shall be deemed no longer to be outstanding and all rights of the
      holders of Receipts evidencing such Depositary Shares (except the right to
      receive the cash, securities or other property payable upon redemption upon
      surrender of such Receipts) shall, to the extent of such Depositary Shares,
      cease and terminate. The foregoing shall be subject further to the terms and
      conditions of the applicable Certificate of Designation. 

     

    If
      fewer than all the Depositary
      Shares are to be redeemed, the Depositary Shares to be redeemed will be selected
      by lot or proportionately, as may be determined by the Depositary. If fewer
      than
      all of the Depositary Shares evidenced by a Receipt are called for redemption,
      the Depositary will deliver to the holder of such Receipt upon its surrender
      to
      the Depositary, cash, securities or other property payable upon redemption
      in
      respect of the Depositary Shares called for redemption and a new Receipt
      evidencing the Depositary Shares evidenced by such prior Receipt and not called
      for redemption. 

     

    The
      Depositary shall not be required
      to transfer or exchange for another Receipt any Receipt evidencing Depositary
      Shares called or being called for redemption, in whole or in part except as
      provided in the immediately preceding paragraph of this Section 2.3.
      

     

    Whenever
      the Company shall be
      required to make an offer to repurchase Depositary Shares representing Stock
      in
      accordance with a Certificate of Designation, it shall (unless otherwise agreed
      in writing with the Depositary) give the Depositary in its capacity as
      Depositary not less than 3 business days’ prior notice of the required date of
      the mailing of a notice of the repurchase offer. The Depositary shall, as
      directed by the Company in writing, mail, first class postage prepaid, notice
      of
      the relevant terms of the repurchase offer, as provided by the Company, to
      the
      record holders of the Receipts at the addresses of such holders as the same
      appear on the records of the Depositary or any Depositary’s Agent or Registrar,
      including: (i) that such notice is being given pursuant to a repurchase
      offer, (ii) the number of Depositary Shares and Stock for which the
      offer
      is being made, (iii) the method for determining the amount payable per
      Depositary Share, (iv) the last date, which, unless the Certificate
      of
      Designation for a specific series of Stock provides for a different period
      with
      respect to that Stock in the event that the Company is required to make an
      offer
      to repurchase it, shall not be less than 30 nor more than 60 days after the
      date
      of such notice, by which a holder must elect to accept the repurchase offer,
      (v) the procedures that such holder must follow to exercise its rights,
      and
      (vi) the procedures for withdrawing an election. 

     

    4

    
    

    

    The
      Depositary shall, thereafter,
      receive from each holder electing to have Depositary Shares repurchased pursuant
      to the repurchase offer in accordance with the instructions in the notice,
      the
      holder’s Receipts, with an appropriate form duly completed prior to the
      repurchase date. Holders will be entitled to withdraw an election by a written
      notice of withdrawal delivered to the Depositary prior to the close of business
      on the repurchase date. The notice of withdrawal shall state the number of
      Depositary Shares and the Receipt numbers to which the notice of withdrawal
      relates and the number of Depositary Shares and Receipt numbers, if any, which
      remain subject to election. In case the aggregate number of Depositary Shares
      offered for repurchase by the holders exceeds the amount of Depositary Shares
      which the Company has offered to repurchase pursuant to the repurchase offer,
      the Depositary Shares to be repurchased shall be selected by the Depositary
      by
      lot or proportionately, as may be determined by the Depositary. The Depositary
      shall, at the direction of the Company, cause payment to be mailed or delivered
      to each tendering holder as promptly as reasonably practicable after the
      repurchase date, in the amount of the repurchase price for the Depositary Shares
      tendered, and any unpurchased Depositary Shares to be returned to the holder
      thereof. The foregoing is subject further to the terms and conditions of the
      applicable Certificate of Designation. 

     

    SECTION
      2.4 REGISTER OF TRANSFER OF
      RECEIPTS. Subject to the terms and conditions of this Deposit Agreement, the
      Depositary shall register on its books from time to time transfers of Receipts
      upon any surrender thereof at the Corporate Office, or such other office as
      the
      Depositary may designate for such purpose, by the record holder in person or
      by
      a duly authorized attorney, properly endorsed or accompanied by a properly
      executed instrument of transfer, together with evidence of the payment of any
      transfer taxes as may be required by law. Upon such surrender, the Depositary
      shall execute a new Receipt or Receipts and deliver the same to or upon the
      order of the person entitled thereto evidencing the same aggregate number of
      Depositary Shares evidenced by the Receipt or Receipts surrendered. 

     

    SECTION
      2.5 COMBINATION AND
      SPLIT-UPS OF RECEIPTS. Upon surrender of a Receipt or Receipts at the Corporate
      Office, or such other office as the Depositary may designate for the purpose
      of
      effecting a split-up or combination of Receipts, subject to the terms and
      conditions of this Deposit Agreement, the Depositary shall execute and deliver
      a
      new Receipt or Receipts in the authorized denominations requested evidencing
      the
      same aggregate number of Depositary Shares evidenced by the Receipt or Receipts
      surrendered; provided, however, that the Depositary shall not issue any Receipt
      evidencing a fractional Depositary Share. 

     

    SECTION
      2.6 SURRENDER OF RECEIPTS
      AND WITHDRAWAL OF STOCK. Any holder of a Receipt, including the Company, shall
      have the right, upon payment of any amount due to the Depositary with respect
      to
      the Receipt, to withdraw any or all of the Stock (but only in whole shares
      of
      Stock) represented by the Depositary Shares and all money and other property,
      if
      any, represented by such Depositary Shares by surrendering the Receipt or
      Receipts evidencing such Depositary Shares at the Corporate Office, or at such
      other office as the Depositary may designate for such withdrawals (and
      cancellation of the surrendered Receipts as provided in Section 2.9).
      After
      such surrender, without unreasonable delay, the Depositary shall deliver to
      the
      holder the whole number of shares of Stock and all such money and other
      property, if any, represented by the Depositary Shares evidenced by the Receipt
      or Receipts so surrendered for withdrawal. If the Receipt or Receipts delivered
      by the holder to the Depositary in connection with such withdrawal shall
      evidence a number of Depositary Shares in excess of the number of whole
      Depositary Shares representing the whole number of shares of Stock to be
      withdrawn, the Depositary shall at the same time, in addition to such whole
      number of shares of Stock and such money and other property, if any, to be
      withdrawn, deliver to the holder, or (subject to Section 2.4) upon its
      order, a new Receipt or Receipts evidencing such excess number of whole
      Depositary Shares. 

     

    Delivery
      of the Stock and such money
      and other property being withdrawn may be made by the delivery of such
      certificates, documents of title and other instruments as the Depositary may
      deem appropriate, which, if required by the Depositary, shall be properly
      endorsed or accompanied by proper instruments of transfer. 

     

    The
      Depositary shall deliver the
      Stock and the money and other property, if any, represented by the Depositary
      Shares evidenced by Receipts surrendered for withdrawal, without unreasonable
      delay, at the office at which such Receipts were surrendered, except that,
      at
      the request, risk and expense of the Company such delivery may be made, without
      unreasonable delay, at such other place as may be designated by the Company.
      

     

    5

    
    

    

    For
      purposes of determining the
      number of Depositary Shares outstanding on any dividend payment date, the
      Receipts representing Depositary Shares acquired by the Company on or prior
      to
      such dividend payment date and not theretofore delivered to the Depositary
      for
      withdrawal and cancellation shall be deemed to be outstanding. 

     

    SECTION
      2.7 LIMITATIONS ON EXECUTION
      AND DELIVERY, TRANSFER, SPLIT-UP, COMBINATION AND SURRENDER OF RECEIPTS AND
      WITHDRAWAL OR DEPOSIT OF STOCK. As a condition precedent to the execution and
      delivery, registration of transfer, split-up, combination, or surrender of
      any
      Receipt, the delivery of any distribution thereon or withdrawal or deposit
      of
      Stock, or the exercise of any conversion right referred to in Section 2.10,
      the Depositary, any of the Depositary’s Agents, the Registrar or the Company may
      require any or all of the following: (i) payment to it of a sum sufficient
      for the payment (or, in the event that the Depositary or the Company shall
      have
      made such payment, the reimbursement to it) of any tax or other governmental
      charge or fee with respect thereto (including any such tax or charge or fee
      with
      respect to the Stock being deposited or the Stock being withdrawn or with
      respect to property of the Company being issued upon redemption or conversion);
      (ii) production of proof satisfactory to it as to the identity and
      genuineness of any signature; and (iii) compliance with such reasonable
      regulations, if any, as the Depositary or the Company may establish not
      inconsistent with the provisions of this Deposit Agreement. 

     

    The
      deposit of Stock may be refused,
      or the registration of transfer, split-up, combination or surrender of
      outstanding Receipts and the withdrawal of deposited Stock or the exercise
      of
      any conversion right referred to in Section 2.10 may be suspended
      (i) during any period when the register of stockholders of the Company
      is
      closed, (ii) if any such action is deemed necessary or advisable by
      the
      Depositary, any of the Depositary’s Agents or the Company at any time or from
      time to time because of any requirement of law or of any government or
      governmental body or commission, or under any provision of this Deposit
      Agreement, or (iii) with the approval of the Company, for any other
      reason.
      Without limitation of the foregoing, the Depositary shall not knowingly accept
      for deposit under this Deposit Agreement any shares of Stock that are required
      to be registered under the Securities Act unless a registration statement under
      the Securities Act is in effect as to such shares of Stock. 

     

    SECTION
      2.8 LOST RECEIPTS, ETC. In
      case any Receipt shall be mutilated or destroyed or lost or stolen, the
      Depositary shall execute and deliver a Receipt of like form and tenor in
      exchange and substitution for such mutilated Receipt or in lieu of and in
      substitution for such destroyed, lost or stolen Receipt unless the Depositary
      has notice that such Receipt has been acquired by a bona fide purchaser;
      provided, however, that the holder thereof provides the Depositary with
      (i) evidence satisfactory to the Depositary of such destruction, loss
      or
      theft of such Receipt, of the authenticity thereof and of his ownership thereof,
      (ii) reasonable indemnification satisfactory to the Depositary or the
      payment of any charges incurred by the Depositary in obtaining insurance in
      lieu
      of such indemnification and (iii) payment of any expense (including
      fees,
      charges and expenses of the Depositary) in connection with such execution and
      delivery. 

     

    SECTION
      2.9 CANCELLATION AND
      DESTRUCTION OF SURRENDERED RECEIPTS. All Receipts surrendered to the Depositary
      or any Depositary’s Agent shall be cancelled by the Depositary. Except as
      prohibited by applicable law or regulation, the Depositary is authorized to
      destroy such Receipts so canceled. 

     

    SECTION
      2.10 CONVERSION. In the
      event that the Stock, in accordance with its Certificate of Designation, is
      convertible into the Company’s common stock or other securities, subject to the
      terms and conditions of this Deposit Agreement, a holder of a Receipt or
      Receipts may surrender such Receipt or Receipts at the Corporate Office or
      at
      such other office or to a Depositary’s Agent that the Depositary may designate
      for such purpose, together with a notice of conversion duly completed and
      executed, thereby directing the Depositary or such Depositary’s Agent to
      instruct the Company to cause the conversion of the number of shares of Stock
      specified in such notice of conversion into shares of the Company’s common stock
      or other securities at the rate specified in the applicable Certificate of
      Designation, and an assignment of such Receipt or Receipts to the Company or
      in
      blank, duly completed and executed, along with any other documents or
      instruments and any amounts required by the applicable Certificate of
      Designation. 

     

    Upon
      receipt by the Depositary or a
      Depositary’s Agent of a Receipt or Receipts, together with a notice of
      conversion, duly completed and executed, directing the Depositary or such
      Depositary’s Agent to instruct the Company to cause the conversion of a
      specified number of shares of Stock at the rate specified in the applicable
      Certificate of Designation, and an assignment of such Receipt or Receipts to
      the
      Company or in blank, duly 

     

    6

    
    

    

    completed
      and executed, along with
      any other documents or instruments or amounts referred to in the preceding
      paragraph, the Depositary or such Depositary’s Agent shall instruct the Company,
      subject to any adjustment provided for in the applicable Certificate of
      Designation, (i) to cause the conversion at the rate specified in the
      applicable Certificate of Designation of the number of shares of Stock
      represented by the Depositary Shares evidenced by the Receipt or Receipts so
      surrendered for conversion as specified in the written notice to the Depositary
      or such Depositary’s Agent and (ii) to cause the delivery to the holder of
      such Receipt or Receipts of (a) a certificate or certificates evidencing
      the number of whole shares of the Company’s common stock or other securities
      into which such Stock has been converted, and (b) the amount of cash
      or
      other property, if any, to which such holder is entitled in lieu of fractional
      shares of, or fractional interests in, the Company’s common stock or other
      securities otherwise deliverable by the Company upon such conversion, calculated
      in accordance with the applicable Certificate of Designation. The Company shall
      as promptly as practicable after receipt thereof cause the delivery of the
      certificate or certificates and cash or other property, if any, referred to
      in
      clauses (a) and (b) above, and such conversion shall be deemed
      to have
      been effected immediately prior to the close of business on the date of such
      receipt and shall occur at the rate specified in the Certificate of Designation
      in effect at such time and on such date. Upon such conversion, the Depositary
      or
      such Depositary’s Agent (i) shall deliver to the holder a Receipt
      evidencing the number of Depositary Shares evidenced by the surrendered Receipt
      or Receipts in excess of the number of Depositary Shares evidenced by such
      Receipt or Receipts that have been so converted, (ii) shall cancel the
      Receipts surrendered for conversion and (iii) shall deliver to the Company
      for cancellation the number of shares of Stock evidenced by the Receipts so
      surrendered and so converted. Upon the delivery of the shares of Stock to be
      cancelled due to such conversion by the Depositary or such Depositary’s Agent to
      the Company, the Company shall deliver to the Depositary or such Depositary’s
      Agent, as applicable, a certificate or certificates evidencing the number of
      shares of Stock, if any, that equals the excess of the number of shares
      evidenced by the surrendered certificate over the number of shares evidenced
      by
      that certificate that have been so converted. Depositary Shares converted in
      connection with conversion of the Stock represented thereby shall only be
      converted in whole, and not in part. 

     

    Upon
      the conversion of any Stock for
      which a notice of conversion has been provided to the Depositary or a
      Depositary’s Agent by the holder of the Receipt or Receipts representing such
      Stock, the Depositary Shares evidenced by such Receipt or Receipts shall be
      deemed no longer outstanding, all rights of the holder of the Receipt or
      Receipts evidencing such Depositary Shares (except the right to receive
      (i) the Company’s common stock or other securities to which such holder is
      entitled upon conversion in accordance with the applicable Certificate of
      Designation, (ii) any cash or other property payable in accordance with
      the
      applicable Certificate of Designation with respect to any fractional shares
      or
      other fractional interests in the Company’s common stock or other securities
      otherwise deliverable by the Company upon conversion, (iii) any Receipts
      evidencing Depositary Shares representing Stock which was not so converted
      and
      (iv) any other securities, property or cash to which such holder is
      entitled under this Deposit Agreement) shall cease and terminate, and the
      Receipt or Receipts evidencing such Depositary Shares shall be cancelled. No
      fractional shares or other fractional interests in the Company’s common stock or
      other securities shall be deliverable by the Company upon conversion of the
      Stock represented by the Depositary Shares. 

     

     

    ARTICLE
      III 

    CERTAIN
      OBLIGATIONS OF HOLDERS OF
      RECEIPTS AND THE COMPANY 

     

    SECTION
      3.1 FILING PROOFS,
      CERTIFICATES AND OTHER INFORMATION. Any person presenting Stock for deposit
      or
      any holder of a Receipt may be required from time to time to file such proof
      of
      residence or other information, to execute such certificates and to make such
      representations and warranties as the Depositary or the Company may reasonably
      deem necessary or proper. The Depositary or the Company may withhold or delay
      the delivery of any Receipt, the registration of transfer or redemption of
      any
      Receipt, the withdrawal of the Stock represented by the Depositary Shares
      evidenced by any Receipt, the distribution of any dividend or other distribution
      or the exercise of any conversion right referred to in Section 2.10,
      or
      refuse to accept Receipts that are delivered for surrender, until such proof
      or
      other information is filed, such certificates are executed or such
      representations and warranties are made. 

     

    SECTION
      3.2 PAYMENT OF TAXES OR
      OTHER GOVERNMENTAL CHARGES. If any tax or other governmental charge or fee
      shall
      become payable by or on behalf of the Depositary with respect to (i) any
      Receipt, (ii) the Depositary Shares evidenced by such Receipt,
      (iii) the Stock (or fractional interest therein) or other property
      represented by such Depositary Shares, or (iv) any transaction referred
      to
      in Section 4.6, such tax (including transfer, 

     

    7

    
    

    

    issuance
      or acquisition taxes, if
      any) or governmental charge or fee shall be payable by the holder of such
      Receipt, who shall pay the amount thereof to the Depositary. Until such payment
      is made, registration or transfer of any Receipt or any split-up or combination
      thereof or any withdrawal of the Stock or money or other property, if any,
      represented by the Depositary Shares evidenced by such Receipt or Receipts
      delivered for surrender or the exercise of any conversion right referred to
      in
      Section 2.10 may be refused, any dividend or other distribution may
      be
      withheld and any part or all of the Stock or other property represented by
      the
      Depositary Shares evidenced by such Receipt may be sold for the account of
      the
      holder thereof (after attempting by reasonable means to notify such holder
      prior
      to such sale). Any dividend or other distribution so withheld and the proceeds
      of any such sale may be applied to any payment of such tax or other governmental
      charge or fee, the holder of such Receipt remaining liable for any deficiency.
      

     

    SECTION
      3.3 WITHHOLDING. The
      Depositary shall act as the tax withholding agent for any payments,
      distributions made with respect to the Depositary Shares and Receipts, and
      the
      Stock. The Depositary shall be responsible with respect to the Depositary
      Shares, Receipts and Stock for the timely (i) collection and deposit
      of any
      required withholding or backup withholding tax, and (ii) filing of any
      information returns or other documents with federal (and other applicable)
      taxing authorities. 

     

    SECTION
      3.4 REPRESENTATIONS AND
      WARRANTIES AS TO STOCK. In the case of the initial deposit of the Stock, the
      Company and, in the case of subsequent deposits thereof, each person so
      depositing Stock under this Deposit Agreement shall be deemed thereby to
      represent and warrant that such Stock and each certificate therefore are valid
      and that the person making such deposit is duly authorized to do so. Such
      representations and warranties shall survive the deposit of the Stock and the
      issuance of Receipts therefor. 

     

     

    ARTICLE
      IV 

    THE
      STOCK, NOTICES 

     

    SECTION
      4.1 CASH DISTRIBUTIONS.
      Whenever the Depositary shall receive any cash dividend or other cash
      distribution on the Stock (other than cash dividends or cash distributions
      paid
      by the Company in lieu of fractional shares or other fractional interests in
      the
      Company’s common stock or other securities otherwise deliverable by the Company
      in accordance with the applicable Certificate of Designation), the Depositary
      shall, subject to Section 3.2, distribute to record holders of Receipts
      on
      the record date fixed pursuant to Section 4.4 such amounts of such sum
      as
      are, as nearly as practicable, in proportion to the respective numbers of
      Depositary Shares evidenced by the Receipts held by such holders; provided,
      however, that in case the Company or the Depositary shall be required by law
      to
      withhold and does withhold from any cash dividend or other cash distribution
      in
      respect of the Stock an amount on account of taxes, the amount made available
      for distribution or distributed in respect of Depositary Shares shall be reduced
      accordingly. The Depositary shall distribute or make available for distribution,
      as the case may be, only such amount, however, as can be distributed without
      attributing to any holder of Depositary Shares a fraction of one cent and any
      balance not so distributable shall be held by the Depositary (without liability
      for interest thereon) and shall be added to and be treated as part of the next
      sum received by the Depositary for distribution to record holders of Receipts
      then outstanding. 

     

    SECTION
      4.2 DISTRIBUTIONS OTHER THAN
      CASH. Whenever the Depositary shall receive any distribution other than cash,
      rights, preferences or privileges upon the Stock, the Depositary shall, subject
      to Section 3.2, distribute to record holders of Receipts on the record
      date
      fixed pursuant to Section 4.4 such amounts of the securities or property
      received by it as are, as nearly as practicable, in proportion to the respective
      numbers of Depositary Shares evidenced by the Receipts held by such holders,
      in
      any manner that the Depositary and the Company may deem equitable and
      practicable for accomplishing such distribution. If, in the opinion of the
      Company after consultation with the Depositary, such distribution cannot be
      made
      proportionately among such record holders, or if for any other reason (including
      any tax withholding or securities law requirement), the Depositary deems, after
      consultation with the Company, such distribution not to be feasible, the
      Depositary may, with the approval of the Company which approval shall not be
      unreasonably withheld, adopt such method as it deems equitable and practicable
      for the purpose of effecting such distribution, including the sale (at public
      or
      private sale) of the securities or property thus received, or any part thereof,
      at such place or places and upon such terms as it may deem proper. The net
      proceeds of any such sale shall, subject to Section 3.2, be distributed
      or
      made available for distribution, as the case may be, by the Depositary to record
      holders of Receipts as provided by Section 4.1 in the case of a
      distribution received in cash. 

     

    8

    
    

    

    SECTION
      4.3 SUBSCRIPTION RIGHTS,
      PREFERENCES OR PRIVILEGES. If the Company shall at any time offer or cause
      to be
      offered to the persons in whose names Stock is registered on the books of the
      Company any rights, preferences or privileges to subscribe for or to purchase
      any securities or any rights, preferences or privileges of any other nature,
      such rights, preferences or privileges shall in each such instance be made
      available by the Depositary to the record holders of Receipts in such manner
      as
      the Company shall instruct (including by the issue to such record holders of
      warrants representing such rights, preferences or privileges); provided,
      however, that (i) if at the time of issue or offer of any such rights,
      preferences or privileges the Company determines and instructs the Depositary
      that it is not lawful or feasible to make such rights, preferences or privileges
      available to some or all holders of Receipts (by the issue of warrants or
      otherwise) or (ii) if and to the extent instructed by holders of Receipts
      who do not desire to exercise such rights, preferences or privileges, the
      Depositary shall then, in each case, and if applicable laws or the terms of
      such
      rights, preferences or privileges so permit, sell such rights, preferences
      or
      privileges of such holders at public or private sale, at such place or places
      and upon such terms as it may deem proper. The net proceeds of any such sale
      shall be distributed by the Depositary to the record holders of Receipts
      entitled thereto as provided by Section 4.1 in the case of a distribution
      received in cash. 

     

    If
      registration under the Securities
      Act of the securities to which any rights, preferences or privileges relate
      is
      required in order for holders of Receipts to be offered or sold such securities,
      the Company shall promptly file a registration statement pursuant to the
      Securities Act with respect to such rights, preferences or privileges and
      securities and use its reasonable best efforts and take all steps reasonably
      available to it to cause such registration statement to become effective
      sufficiently in advance of the expiration of such rights, preferences or
      privileges to enable such holders to exercise such rights, preferences or
      privileges. In no event shall the Depositary make available to the holders
      of
      Receipts any right, preference or privilege to subscribe for or to purchase
      any
      securities unless and until such registration statement shall have become
      effective or unless the offering and sale of such securities to such holders
      are
      exempt from registration under the provisions of the Securities Act. 

     

    If
      any other action under the law of
      any jurisdiction or any governmental or administrative authorization, consent
      or
      permit is required in order for such rights, preferences or privileges to be
      made available to holders of Receipts, the Company agrees with the Depositary
      that the Company will use its reasonable best efforts to take such action or
      obtain such authorization, consent or permit sufficiently in advance of the
      expiration of such rights, preferences or privileges to enable such holders
      to
      exercise such rights, preferences or privileges. 

     

    SECTION
      4.4 NOTICE OF DIVIDENDS,
      FIXING OF RECORD DATE FOR HOLDERS OF RECEIPTS. Whenever (i) any cash
      dividend or other cash distribution shall become payable, or any distribution
      other than cash shall be made, or any rights, preferences or privileges shall
      at
      any time be offered, with respect to the Stock, or (ii) the Depositary
      shall receive notice of any meeting at which holders of Stock are entitled
      to
      vote or of which holders of Stock are entitled to notice, or (iii) of
      any
      mandatory conversion of, or any election on the part of the Company to call
      for
      the redemption or exchange of, any shares of Stock, in accordance with the
      provisions of the applicable Certificate of Designation or otherwise, the
      Depositary shall in each such instance fix a record date (which shall be the
      same date as the record date fixed by the Company with respect to the Stock)
      for
      the determination of the holders of Receipts (x) who shall be entitled
      to
      receive such dividend, distribution, rights, preferences or privileges or the
      net proceeds of the sale thereof, or (y) who shall be entitled to give
      instructions for the exercise of voting rights at any such meeting or to receive
      notice of such meeting or of such conversion, exchange or redemption.

     

    SECTION
      4.5 VOTING RIGHTS. Upon
      receipt of notice of any meeting at which the holders of Stock are entitled
      to
      vote, the Depositary shall, as soon as practicable thereafter, mail to the
      record holders of Receipts a notice, which shall be provided by the Company
      and
      which shall contain (i) such information as is contained in such notice
      of
      meeting, (ii) a statement that the holders of Receipts at the close
      of
      business on a specified record date fixed pursuant to Section 4.4 will
      be
      entitled, subject to any applicable provision of law, the Certificate of
      Incorporation or the applicable Certificate of Designation, to instruct the
      Depositary as to the exercise of the voting rights pertaining to the Stock
      represented by their respective Depositary Shares and (iii) a brief
      statement as to the manner in which such instructions may be given. Upon the
      written request of a holder of a Receipt on such record date, the Depositary
      shall endeavor insofar as practicable to vote or cause to be voted the Stock
      represented by the Depositary Shares evidenced by such Receipt in accordance
      with the instructions set forth in such request. The 

     

    9

    
    

    

    Company
      hereby agrees to take all
      reasonable action that may be deemed necessary by the Depositary in order to
      enable the Depositary to vote such Stock or cause such Stock to be voted. In
      the
      absence of specific instructions from the holder of a Receipt, the Depositary
      will abstain from voting to the extent of the Stock represented by the
      Depositary Shares evidenced by such Receipt. 

     

    SECTION
      4.6 CHANGES AFFECTING STOCK
      AND RECLASSIFICATIONS, RECAPITALIZATIONS, ETC. Upon any split-up, consolidation
      or any other reclassification of Stock, or upon any recapitalization,
      reorganization, merger, amalgamation or consolidation affecting the Company
      or
      to which it is a party or sale of all or substantially all of the Company’s
      assets, the Depositary shall treat any shares of Stock or other securities
      or
      property (including cash) that shall be received by the Depositary in exchange
      for or in conversion of or in respect of the Stock as new deposited property
      under this Deposit Agreement, and Receipts then outstanding shall thenceforth
      represent the proportionate interests of holders thereof in the new deposited
      property so received in exchange for or in respect of such Stock. In any such
      case the Depositary may, in its discretion, with the approval of the Company,
      execute and deliver additional Receipts, or may call for the surrender of all
      outstanding Receipts to be exchanged for new Receipts specifically describing
      such new deposited property. 

     

    SECTION
      4.7 REPORTS. The Company or,
      at the option of the Company, the Depositary shall forward to the holders of
      Receipts any reports and communications received from the Company that are
      received by the Depositary as the holder of Stock. 

     

    SECTION
      4.8 LISTS OF RECEIPT
      HOLDERS. Promptly upon request from time to time by the Company, the Depositary
      shall furnish to it a list, as of a recent date, of the names, addresses and
      holdings of Depositary Shares of all persons in whose names Receipts are
      registered on the books of the Depositary, any Depositary’s Agent or the
      Registrar. At the expense of the Company, the Company shall have the right
      to
      inspect transfer and registration records of the Depositary, any Depositary’s
      Agent or the Registrar, take copies thereof and require the Depositary, any
      Depositary’s Agent or the Registrar to supply copies of such portions of such
      records as the Company may request. 

     

     

    ARTICLE
      V 

    THE
      DEPOSITARY, THE DEPOSITARY’S
      AGENTS, THE REGISTRAR AND THE COMPANY 

     

    SECTION
      5.1 MAINTENANCE OF OFFICES,
      AGENCIES, TRANSFER BOOKS BY THE DEPOSITARY; THE REGISTRAR. Upon execution of
      this Deposit Agreement in accordance with its terms, the Depositary shall
      maintain (i) at the Corporate Office facilities for the execution and
      delivery, registration, registration of transfer, surrender, split-up,
      combination, redemption and conversion of Receipts and deposit and withdrawal
      of
      Stock, and (ii) at the offices of the Depositary’s Agents, if any,
      facilities for the delivery, registration, registration of transfer, surrender,
      split-up, combination, redemption and conversion of Receipts and deposit and
      withdrawal of Stock, all in accordance with the provisions of this Deposit
      Agreement. 

     

    The
      Depositary, acting as transfer
      agent and Registrar, shall keep books at the Corporate Office for the
      registration and transfer of Receipts, which books at all reasonable times
      shall
      be open for inspection by the record holders of Receipts; provided that any
      such
      holder requesting to exercise such right shall certify to the Depositary that
      such inspection shall be for a proper purpose reasonably related to such
      person’s interest as an owner of Depositary Shares. The Depositary shall consult
      with the Company upon receipt of any request for inspection. The Depositary
      may
      close such books, at any time or from time to time, when deemed expedient by
      it
      in connection with the performance of its duties hereunder. 

     

    If
      the Receipts or the Depositary
      Shares evidenced thereby or the Stock represented by such Depositary Shares
      shall be listed on one or more securities exchanges, the Depositary shall,
      with
      the approval of the Company, appoint a Registrar for registry of such Receipts
      or Depositary Shares in accordance with the requirements of such exchange or
      exchanges. Such Registrar (which may be the Depositary if so permitted by the
      requirements of such exchange or exchanges) may be removed and a substitute
      Registrar appointed by the Depositary upon the request or with the approval
      of
      the Company. In addition, if the Receipts, such Depositary Shares or such Stock
      are listed on one or more securities exchanges, the Depositary will, at the
      request of the Company, arrange such facilities for the delivery, registration,
      registration of transfer, surrender, split-up, combination, redemption and
      conversion of such Receipts, such Depositary Shares or such Stock as may be
      required by law or applicable security exchange regulations. 

     

    10

    
    

    

    SECTION
      5.2 PREVENTION OR DELAY IN
      PERFORMANCE BY THE DEPOSITARY, THE DEPOSITARY’S AGENTS, THE REGISTRAR OR THE
      COMPANY. Neither the Depositary nor any Depositary’s Agent nor the Registrar nor
      the Company shall incur any liability to any holder of any Receipt, if by reason
      of any provision of any present or future law or regulation thereunder of the
      United States of America or of any other governmental authority or of any
      provision, present or future, of the Certificate of Incorporation or the
      applicable Certificate of Designation or by reason of any act of god or war
      or
      other circumstances beyond the control of the relevant party, the Depositary,
      any Depositary’s Agent, the Registrar or the Company shall be prevented or
      forbidden from doing or performing any act or thing that the terms of this
      Deposit Agreement provide shall be done or performed; nor shall the Depositary,
      any Depositary’s Agent, the Registrar or the Company incur any liability to any
      holder of a Receipt (i) by reason of any nonperformance or delay, caused
      as
      aforesaid, in the performance of any act or thing that the terms of this Deposit
      Agreement provide shall or may be done or performed, or (ii) by reason
      of
      any exercise of, or failure to exercise, any discretion provided for in this
      Deposit Agreement except, in the case of the Depositary, any Depositary’s Agent
      or the Registrar, if any such exercise or failure to exercise discretion is
      caused by its negligence or willful misconduct. 

     

    SECTION
      5.3 OBLIGATIONS OF THE
      DEPOSITARY, THE DEPOSITARY’S AGENTS, THE REGISTRAR AND THE COMPANY. The Company
      assumes no obligation and shall be subject to no liability under this Deposit
      Agreement or the Receipts to holders or other persons, except to perform its
      obligations as are specifically set forth and undertaken by it to perform in
      this Deposit Agreement without negligence or bad faith. Each of the Depositary,
      the Depositary’s Agents and the Registrar assumes no obligation and shall be
      subject to no liability under this Deposit Agreement or the Receipts to holders
      or other persons, except to perform such obligations as are specifically set
      forth and undertaken by it to perform in this Deposit Agreement without
      negligence or willful misconduct. 

     

    Neither
      the Depositary nor any
      Depositary’s Agent nor the Registrar nor the Company shall be liable for any
      action or any failure to act by it in reliance upon the advice of or information
      from legal counsel, accountants, any person presenting Stock for deposit, any
      holder of a Receipt or any other person believed by it in good faith to be
      competent to give such advice or information. The Depositary, any Depositary’s
      Agent, the Registrar and the Company may each rely and shall each be protected
      in acting upon any written notice, request, direction or other document believed
      by it to be genuine and to have been signed or presented by the proper party
      or
      parties. 

     

    The
      Depositary, the Registrar and
      any Depositary’s Agent may own and deal in any class of securities of the
      Company and its affiliates and in Receipts or Depositary Shares. The Depositary
      may also act as transfer agent or Registrar of any of the securities of the
      Company and its affiliates. 

     

    It
      is intended that neither the
      Depositary nor any Depositary’s Agent nor the Registrar shall be deemed to be an
“issuer” of the Stock, the Depositary Shares, or the Receipts or other
      securities issued upon conversion, exchange or redemption of the Stock under
      the
      federal securities laws or applicable state securities laws, it being expressly
      understood and agreed that the Depositary and any Depositary’s Agent and the
      Registrar are acting only in a ministerial capacity; provided, however, that
      the
      Depositary and any Depositary’s Agent agree to comply with all information
      reporting and withholding requirements applicable to each of them under law
      or
      this Deposit Agreement in their capacity as such. 

     

    Neither
      the Depositary (or its
      officers, directors, employees or agents) nor any Depositary’s Agent nor the
      Registrar makes any representation or has any responsibility as to the validity
      of the registration statement pursuant to which the Depositary Shares are
      registered under the Securities Act, the Stock, the Depositary Shares or any
      instruments referred to therein or herein, or as to the correctness of any
      statement made therein or herein; provided, however, that the Depositary is
      responsible for its representations in this Deposit Agreement. 

     

    The
      Depositary assumes no
      responsibility for the correctness of the description that appears in the
      Receipts, which can be taken as a statement of the Company summarizing certain
      provisions of this Deposit Agreement. Notwithstanding any other provision herein
      or in the Receipts, the Depositary makes no warranties or representations as
      to
      the validity, genuineness or sufficiency of any Stock at any time deposited
      with
      the Depositary 

     

    11

    
    

    

    hereunder
      or of the Depositary
      Shares, as to the validity or sufficiency of this Deposit Agreement, as to
      the
      value of the Depositary Shares or as to any right, title or interest of the
      record holders of Receipts in and to the Depositary Shares except that the
      Depositary hereby represents and warrants as follows: (i) the Depositary
      has been duly organized and is validly existing and in good standing under
      the
      laws of the State of
      [                        ],
      with full power, authority and legal right under such law to execute, deliver
      and carry out the terms of this Deposit Agreement; (ii) this Deposit
      Agreement has been duly authorized, executed and delivered by the Depositary;
      and (iii) this Deposit Agreement constitutes, and when executed and
      delivered, each Receipt will constitute, a valid and binding obligation of
      the
      Depositary, enforceable against the Depositary in accordance with its terms,
      except as enforcement thereof may be limited by bankruptcy, insolvency,
      reorganization or other similar laws affecting enforcement of creditors’ rights
      generally and except as enforcement thereof is subject to general principles
      of
      equity (regardless of whether enforcement is considered in a proceeding in
      equity or at law). The Depositary shall not be accountable for the use or
      application by the Company of its proceeds from the Depositary Shares or the
      Receipts. 

     

    SECTION
      5.4 RESIGNATION AND REMOVAL
      OF THE DEPOSITARY, APPOINTMENT OF SUCCESSOR DEPOSITARY. The Depositary may
      at
      any time resign as Depositary hereunder by written notice to the Company of
      its
      election to do so, such resignation to take effect upon the appointment of
      a
      successor Depositary and its acceptance of such appointment as hereinafter
      provided. 

     

    The
      Depositary may at any time be
      removed by the Company by written notice to the Depositary of such removal,
      such
      removal to take effect upon the appointment of a successor depositary and its
      acceptance of such appointment as hereinafter provided. 

     

    In
      case at any time the Depositary
      acting hereunder shall resign or be removed, the Company shall, within 60 days
      after the delivery of the notice of resignation or removal, as the case may
      be,
      appoint a successor depositary, which shall be a bank or trust company, or
      an
      affiliate of a bank or trust company, having its principal office in the United
      States of America and having a combined capital and surplus of at least
      $50,000,000. If a successor depositary shall not have been appointed in 60
      days,
      the resigning or removed Depositary may petition a court of competent
      jurisdiction to appoint a successor depositary. Every successor depositary
      shall
      execute and deliver to its predecessor and to the Company an instrument in
      writing accepting its appointment hereunder, and thereupon such successor
      depositary, without any further act or deed, shall become fully vested with
      all
      the rights, powers, duties and obligations of its predecessor and for all
      purposes shall be the Depositary under this Deposit Agreement, and such
      predecessor, upon payment of all sums due it and on the written request of
      the
      Company, shall promptly execute and deliver an instrument transferring to such
      successor all rights and powers of such predecessor hereunder, shall duly
      assign, transfer and deliver all rights, title and interest in the Stock and
      any
      moneys or property held hereunder to such successor and shall deliver to such
      successor a list of the record holders of all outstanding Receipts. Any
      successor Depositary shall promptly mail notice of its appointment to the record
      holders of Receipts. 

     

    Any
      corporation into or with which
      the Depositary may be merged, consolidated or converted shall be the successor
      of such Depositary without the execution or filing of any document or any
      further act. Such successor Depositary may execute the Receipts either in the
      name of the predecessor Depositary or in the name of the successor Depositary.
      

     

    SECTION
      5.5 CORPORATE NOTICES AND
      REPORTS. The Company agrees that it will deliver to the Depositary, and the
      Depositary will, promptly after receipt thereof, transmit to the record holders
      of Receipts, in each case at the address recorded in the Depositary’s books or
      the books of any Depositary’s Agent or the Registrar, copies of all notices and
      reports (including financial statements) required by law, by the rules of any
      national securities exchange upon which the Stock, the Depositary Shares or
      the
      Receipts are listed or by the Certificate of Incorporation or the applicable
      Certificate of Designation to be furnished by the Company to holders of Stock.
      Such transmission will be at the Company’s expense and the Company will provide
      the Depositary with such number of copies of such documents as the Depositary
      may reasonably request. In addition, the Depositary will transmit to the record
      holders of Receipts at the Company’s expense such other documents as may be
      requested by the Company. 

     

    SECTION
      5.6 DEPOSIT OF STOCK BY THE
      COMPANY. The Company agrees with the Depositary that neither the Company nor
      any
      Company controlled by the Company will at any time deposit any Stock if such
      Stock is required to be registered under the provisions of the Securities Act
      and no registration statement is at such time in effect as to such Stock.

     

    12

    
    

    

    SECTION
      5.7 INDEMNIFICATION BY THE
      COMPANY. The Company shall indemnify the Depositary for, and hold it harmless
      against, any loss, liability, claim or expense (“Loss”) arising out of or in
      connection with its duties under this Deposit Agreement, including the
      reasonable costs and expenses of defending itself against Loss, unless such
      Loss
      shall have been determined by a court of competent jurisdiction to be a result
      of the Depositary’s negligence or willful misconduct. 

     

    SECTION
      5.8 FEES, CHARGES AND
      EXPENSES. No fees, charges and expenses of the Depositary or any Depositary’s
      Agent hereunder or of any Registrar shall be payable by any person other than
      the Company, except for any taxes and other governmental charges and except
      as
      provided in this Deposit Agreement. If, at the request of a holder of a Receipt,
      the Depositary incurs fees, charges or expenses for which it is not otherwise
      liable hereunder, such holder or other person will be liable for such fees,
      charges and expenses. All other fees, charges and expenses of the Depositary
      and
      any Depositary’s Agent hereunder and of any Registrar (including, in each case,
      reasonable fees and expenses of counsel) incident to the performance of their
      respective obligations hereunder will be paid from time to time upon
      consultation and agreement between the Depositary and the Company as to the
      amount and nature of such fees, charges and expenses. 

     

     

    ARTICLE
      VI 

    AMENDMENT
      AND TERMINATION

     

    SECTION
      6.1 AMENDMENT. The form of
      the Receipts and any provision of this Deposit Agreement may at any time and
      from time to time be amended by agreement between the Company and the Depositary
      in any respect that they may deem necessary or desirable, including to reflect
      the terms of any Certificate of Designation; provided, however, that no such
      amendment that shall materially and adversely alter the rights of the holders
      of
      Receipts shall be effective as to outstanding Receipts until the expiration
      of
      90 days after notice of such amendment shall have been given to the record
      holders of outstanding Receipts. Each holder of an outstanding Receipt at the
      time any such amendment becomes effective shall be deemed, by continuing to
      hold
      such Receipt, to consent and agree to such amendment and to be bound by this
      Deposit Agreement as amended thereby. In no event shall any amendment impair
      the
      right, subject to the provisions of this Deposit Agreement, of any holder of
      any
      Depositary Shares to surrender the Receipt evidencing such Depositary Shares
      with instructions to the Depositary to deliver to the holder the Stock or to
      cause the conversion of such Stock into the Company’s common stock or other
      securities in accordance with the applicable Certificate of Designation and
      to
      deliver all securities, money and other property, if any, represented thereby,
      except in order to comply with mandatory provisions of applicable law.

     

    SECTION
      6.2 TERMINATION. This
      Deposit Agreement may be terminated by either the Company or the Depositary,
      upon notice to the other, only if (i) all of the outstanding Depositary
      Shares have been redeemed or converted for any other securities into which
      the
      Stock is convertible, or (ii) there has been a final distribution of
      the
      Stock to the holders of Receipts in connection with the Company’s liquidation,
      dissolution or winding up. The Depositary will mail notice of such termination
      to the record holders of all Receipts then outstanding at least 30 days prior
      to
      the date fixed in such notice for such termination. If any Receipts shall remain
      outstanding after the date of termination of this Deposit Agreement, the
      Depositary thereafter shall discontinue the transfer of Receipts, and shall
      not
      give any further notices or perform any further acts under this Deposit
      Agreement, except that the Depositary shall continue to deliver the Stock and
      any money and other property represented by Receipts, without liability for
      interest thereon, upon surrender thereof by the holders thereof, and the
      Depositary shall be discharged from all obligations under this Deposit Agreement
      except to account for such Stock, money and other property. Upon the termination
      of this Deposit Agreement, the Company shall be discharged from all obligations
      under this Deposit Agreement except for its obligations to the Depositary,
      any
      Depositary’s Agent and any Registrar under Sections 5.7 and 5.8. 

     

     

    ARTICLE
      VII 

    MISCELLANEOUS

     

    SECTION
      7.1 COUNTERPARTS. This
      Deposit Agreement may be executed by the Company and the Depositary in separate
      counterparts, each of which counterparts, when so executed and delivered, shall
      be deemed 

     

    13

    
    

    

    an
      original, but all such
      counterparts taken together shall constitute one and the same instrument.
      Delivery of an executed counterpart of a signature page to this Deposit
      Agreement by telecopier shall be effective as delivery of a manually executed
      counterpart of this Deposit Agreement. Copies of this Deposit Agreement shall
      be
      filed with the Depositary and the Depositary’s Agents and shall be open to
      inspection during business hours at the Corporate Office and the respective
      offices of the Depositary’s Agents, if any, by any holder of a Receipt.

     

    SECTION
      7.2 EXCLUSIVE BENEFITS OF
      PARTIES. This Deposit Agreement is for the exclusive benefit of the parties
      hereto, and their respective successors hereunder, and shall not be deemed
      to
      give any legal or equitable right, remedy or claim to any other person
      whatsoever. 

     

    SECTION
      7.3 INVALIDITY OF
      PROVISIONS. In case any one or more of the provisions contained in this Deposit
      Agreement or in the Receipts should be or become invalid, illegal or
      unenforceable in any respect, the validity, legality and enforceability of
      the
      remaining provisions contained herein or therein shall in no way be affected,
      prejudiced or disturbed thereby. 

     

    SECTION
      7.4 NOTICES. Any notices to
      be given to the Company hereunder or under the Receipts shall be in writing
      and
      shall be deemed to have been duly given if personally delivered or sent by
      mail
      (first class postage prepaid), by a nationally recognized overnight courier
      service, or by telecopier confirmed by letter, addressed to the Company at
      185
      Berry Street, Suite 6504, San Francisco, California 94107, Attention: General
      Counsel, or at any other place to which the Company may have transferred its
      principal executive office. 

     

    Any
      notices to be given to the
      Depositary hereunder or under the Receipts shall be in writing and shall be
      deemed to have been duly given if personally delivered or sent by mail (first
      class postage), by a nationally recognized overnight courier service, or by
      telecopier confirmed by letter, addressed to the Depositary at the Corporate
      Office. 

     

    Except
      as provided in the next
      paragraph, any notices given to any record holder of a Receipt hereunder or
      under the Receipts shall be in writing and shall be deemed to have been duly
      given if personally delivered or sent by mail (first class postage), by a
      nationally recognized overnight courier service or by telecopier confirmed
      by
      letter, addressed to such record holder at the address of such record holder
      as
      it appears on the books of the Depositary or, if such holder shall have filed
      with the Depositary a written request that notices intended for such holder
      be
      mailed to some other address, at the address designated in such request.

     

    In
      addition, whenever the
      Certificate of Designation requires any notice to be published, the Depositary
      will, if requested by the Company, cause such notice to be published in the
      manner directed by the Company. 

     

    Delivery
      of a notice sent by mail,
      by overnight courier or by telecopier shall be deemed to be effected at the
      time
      when a duly addressed letter containing the same (or a duly addressed letter
      confirming an earlier notice in the case of a telecopier message) is deposited,
      postage prepaid, in a post office letter box or with the overnight courier
      service. The Depositary or the Company may, however, act upon any telecopier
      message received by it from the other or from any holder of a Receipt,
      notwithstanding that such telecopier message shall not subsequently be confirmed
      by letter as aforesaid. 

     

    SECTION
      7.5 DEPOSITARY’S AGENTS. The
      Depositary may, with the approval of the Company which approval shall not be
      unreasonably withheld, from time to time appoint one or more Depositary’s Agents
      to act in any respect for the Depositary for the purposes of this Deposit
      Agreement and may vary or terminate the appointment of such Depositary’s Agents.

     

    SECTION
      7.6 HOLDERS OF RECEIPTS ARE
      PARTIES. Notwithstanding that holders of Receipts have not executed and
      delivered this Deposit Agreement or any counterpart thereof, the holders of
      Receipts from time to time shall be deemed to be parties to this Deposit
      Agreement and shall be bound by all of the terms and conditions, and be entitled
      to all of the benefits, hereof and of the Receipts by acceptance of delivery
      of
      Receipts. 

     

    SECTION
      7.7 GOVERNING LAW. This
      Deposit Agreement and the Receipts and all rights hereunder and thereunder
      and
      provisions hereof and thereof shall be governed by, and construed in accordance
      with, the law of the State of New York without giving effect to principles
      of
      conflict of laws. 

     

    14

    
    

    

    SECTION
      7.8 HEADINGS. The headings
      of articles and sections in this Deposit Agreement and in the form of the
      Receipt set forth in Exhibit A hereto have been inserted for convenience only
      and are not to be regarded as a part of this Deposit Agreement or to have any
      bearing upon the meaning or interpretation of any provision contained herein
      or
      in the Receipts. 

     

    IN
      WITNESS WHEREOF, Sirna
      Therapeutics, Inc. and
      [            ]
      have duly executed this Deposit Agreement as of the day and year first above
      set
      forth and all holders of Receipts shall become parties hereto by and upon
      acceptance by them of delivery of Receipts issued in accordance with the terms
      hereof. 

     

    Sirna
      Therapeutics, Inc. 

     

    By:                                      
                                        

    Authorized
      Officer 

     

                                          
            , AS DEPOSITARY 

     

    By:                                      
                                        

    Authorized
      Officer 

     

     

    15

    
    

    

    Exhibit
      A to Deposit Agreement

     

    [FORM
      OF DEPOSITARY RECEIPT]

     

    [Unless
      this certificate is
      presented by an authorized representative of The Depository Trust Company,
      a New
      York corporation (“DTC”), to the Company or its agent for registration of
      transfer, conversion, exchange or payment, and any certificate issued is
      registered in the name of Cede & Co. or in such other name as is
      requested by an authorized representative of DTC (and any payment is made to
      Cede & Co. or to such other entity as is requested by an authorized
      representative of DTC), ANY TRANSFER, PLEDGE OR OTHER USE HEREOF FOR VALUE
      OR
      OTHERWISE BY OR TO ANY PERSON IS WRONGFUL in as much as the registered owner
      hereof, Cede & Co., has an interest herein.] 

     

    DEPOSITARY
      RECEIPT 

     

    FOR

     

    DEPOSITARY
      SHARES 

     

    EACH
      REPRESENTING
      [            ]
      OF A SHARE OF 

     

    PREFERRED
      STOCK, SERIES
      [            ]

     

    OF

     

    Sirna
      Therapeutics, Inc. 

     

    No.
      CUSIP: 

     

    [            ](the
      “Depositary”) hereby certifies that [Cede & Co.] 

     

    [            ]is
      the registered holder of
      [            ]
      Depositary Shares (the “Depositary Shares”), each Depositary Share representing
      [            ]
      of a share of
      [            ]
      Preferred Stock, Series
      [            ],
      $0.01 par value (the “Stock”), of Sirna Therapeutics, Inc., a corporation duly
      organized and existing under the laws of the State of Delaware (the “Company”),
      deposited with the Depositary and the same proportionate interest in any and
      all
      other property received by the Depositary in respect of such shares of Stock
      and
      held by the Depositary under the Deposit Agreement (as defined below). Subject
      to the terms of the Deposit Agreement, each owner of a Depositary Share is
      entitled, proportionately, to all the rights, preferences and privileges, and
      subject to all the limitations and restrictions, of the Stock represented
      thereby, including any dividend, voting, conversion, redemption, liquidation
      and
      sinking fund rights contained in the Certificate of Designation establishing
      the
      rights, preferences, privileges, limitations and restrictions of the Stock
      (the
“Certificate of Designation”), copies of which are on file at the office of the
      Depositary in the City of New York, Borough of Manhattan
      [            ]
      at which at any particular time its business in respect of matters governed
      by
      the Deposit Agreement shall be administered, which at the time of the execution
      of the Deposit Agreement is located at
      [            ]
      (the “Corporate Office”). 

     

    16

    
    

    

    This
      Depositary Receipt (“Receipt”)
      shall not be entitled to any benefits under the Deposit Agreement or be valid
      or
      obligatory for any purpose unless this Receipt shall have been executed manually
      or, if a Registrar for the Receipts (other than the Depositary) shall have
      been
      appointed, by facsimile by the Depositary by the signature of a duly authorized
      officer and, if executed by facsimile signature of the Depositary, shall have
      been countersigned manually by such Registrar by the signature of a duly
      authorized officer. 

     

    THE
      DEPOSITARY IS NOT RESPONSIBLE
      FOR THE VALIDITY OF ANY DEPOSITED STOCK. THE DEPOSITARY ASSUMES NO
      RESPONSIBILITY FOR THE CORRECTNESS OF THE DESCRIPTION SET FORTH IN THIS RECEIPT,
      WHICH CAN BE TAKEN AS A STATEMENT OF THE COMPANY SUMMARIZING CERTAIN PROVISIONS
      OF THE DEPOSIT AGREEMENT. UNLESS EXPRESSLY SET FORTH IN THE DEPOSIT AGREEMENT,
      THE DEPOSITARY MAKES NO WARRANTIES OR REPRESENTATIONS AS TO THE VALIDITY,
      GENUINENESS OR SUFFICIENCY OF ANY STOCK AT ANY TIME DEPOSITED WITH THE
      DEPOSITARY UNDER THE DEPOSIT AGREEMENT OR OF THE DEPOSITARY SHARES, AS TO THE
      VALIDITY OR SUFFICIENCY OF THE DEPOSIT AGREEMENT, AS TO THE VALUE OF THE
      DEPOSITARY SHARES OR AS TO ANY RIGHT, TITLE OR INTEREST OF THE RECORD HOLDERS
      OF
      THE RECEIPTS IN AND TO THE DEPOSITARY SHARES. 

     

    The
      Company will furnish to any
      holder of this Receipt without charge, upon request addressed to its executive
      office, a full statement of the designation, relative rights, preferences,
      privileges, limitations and restrictions of the shares of each authorized class,
      and of each class of preferred stock authorized to be issued, so far as the
      same
      may have been fixed, and a statement of the authority of the board of directors
      of the Company to designate and fix the relative rights, preferences,
      privileges, limitations and restrictions of other classes. 

     

    This
      Receipt is continued on the
      reverse hereof and the additional provisions therein set forth for all purposes
      have the same effect as if set forth at this place. 

     

    Dated:                                     
                                     

     

    as
      Depositary and Registrar

     

    By:                                      
                                        

     

    Authorized
      Officer 

     

    Further
      Conditions and Agreements
      Forming Part of this Receipt Appear on the Reverse Side. 

     

    17

    
    

    

    [REVERSE
      OF DEPOSITARY RECEIPT]

     

    1.
      THE DEPOSIT AGREEMENT. Receipts,
      of which this Receipt is one, are made available upon the terms and conditions
      set forth in the Deposit Agreement, dated as of
      [            ]
      (the
“Deposit Agreement”), among the Company, the Depositary and all holders from
      time to time of Receipts. The Deposit Agreement (copies of which are on file
      at
      the Corporate Office and at the office of any agent of the Depositary) sets
      forth the rights of holders of Receipts and the rights and duties of the
      Depositary. The statements made on the face and the reverse of this Receipt
      are
      summaries of certain provisions of the Deposit Agreement and are subject to
      the
      detailed provisions thereof, to which reference is hereby made. In the event
      of
      any conflict between the provisions of this Receipt and the provisions of the
      Deposit Agreement, the provisions of the Deposit Agreement will govern.

     

    2.
      DEFINITIONS. Unless otherwise
      expressly herein provided, all defined terms used herein shall have the meanings
      ascribed thereto in the Deposit Agreement. 

     

    3.
      REDEMPTION BY THE COMPANY;
      REPURCHASE BY THE COMPANY. Whenever the Company shall redeem shares of Stock
      in
      accordance with a Certificate of Designation, it shall (unless otherwise agreed
      in writing with the Depositary) give the Depositary in its capacity as
      Depositary not less than 3 business days’ prior notice of the proposed date of
      the mailing of a notice of redemption of Stock and the simultaneous redemption
      of the Depositary Shares representing the Stock to be redeemed and of the number
      of such shares of Stock held by the Depositary to be redeemed. Unless the
      Certificate of Designation for a specific series of Stock provides for a
      different notice period with respect to that Stock in the event of its
      redemption, the Depositary shall, as directed by the Company in writing, mail,
      first class postage prepaid, notice of the redemption not less than 30 and
      not
      more than 60 days prior to the date fixed for redemption of such Stock and
      Depositary Shares, to the record holders of the Receipts evidencing the
      Depositary Shares to be so redeemed, at the addresses of such holders as the
      same appear on the records of the Depositary or any Depositary’s Agent or
      Registrar. Notwithstanding the foregoing, neither failure to mail or publish
      any
      such notice to one or more such holders nor any defect in any notice shall
      affect the sufficiency of the proceedings for redemption. On the date of any
      such redemption, the Depositary shall surrender the certificate or certificates
      held by the Depositary evidencing the number of shares of Stock to be redeemed
      in the manner specified in the notice of redemption. The Depositary shall,
      thereafter, redeem the number of Depositary Shares representing such redeemed
      Stock upon the surrender of Receipts evidencing such Depositary Shares in the
      manner provided in the notice sent to record holders of Receipts. Notice having
      been mailed as aforesaid, from and after the redemption date (unless the Company
      shall have failed to redeem the shares of Stock to be redeemed by it upon the
      surrender of the certificate or certificates therefore by the Depositary as
      described above), the Depositary Shares called for redemption shall be deemed
      no
      longer to be outstanding and all rights of the holders of Receipts evidencing
      such Depositary Shares (except the right to receive the cash, securities or
      other property payable upon redemption upon surrender of such Receipts) shall,
      to the extent of such Depositary Shares, cease and terminate. The foregoing
      shall be subject further to the terms and conditions of the applicable
      Certificate of Designation. 

     

    Whenever
      the Company shall be
      required to make an offer to repurchase Depositary Shares in accordance with
      a
      Certificate of Designation, it shall (unless otherwise agreed in writing with
      the Depositary) give the Depositary in its capacity as Depositary not less
      than
      3 business days’ prior notice of the required date of the mailing of a notice of
      the repurchase offer. The Depositary shall, as directed by the Company in
      writing, mail, first class postage prepaid, notice of the relevant terms of
      the
      repurchase offer, as provided by the Company, to the record holders of the
      Receipts at the addresses of such holders as the same appear on the records
      of
      the Depositary or any Depositary’s Agent or Registrar. The Depositary shall,
      thereafter, receive from each holder electing to have Depositary Shares
      repurchased pursuant to the repurchase offer in accordance with the instructions
      in the notice, the holder’s Receipts, with an appropriate form duly completed
      prior to the repurchase date. In case the aggregate number of Depositary Shares
      offered for repurchase by the holders exceeds the amount of Depositary Shares
      which the Company has offered to repurchase pursuant to the repurchase offer,
      the Depositary Shares to be repurchased shall be selected by the Depositary
      by
      lot or proportionately, as may be determined by the Depositary. The foregoing
      is
      subject further to the terms and conditions of the applicable Certificate of
      Designation. 

     

    4.
      CONVERSION. In the event that the
      Stock, in accordance with its Certificate of Designation, is convertible into
      the Company’s common stock or other securities, subject to the terms and
      conditions of the Deposit Agreement, a holder of a Receipt or Receipts may
      surrender such Receipt or Receipts at the Corporate Office or at such other
      office or to a Depositary’s Agent that the Depositary may designate for such
      purpose, together with a notice of 

     

    18

    
    

    

    conversion
      duly completed and
      executed, thereby directing the Depositary or such Depositary’s Agent to
      instruct the Company to cause the conversion of the number of shares of Stock
      specified in such notice of conversion into shares of the Company’s common stock
      or other securities at the rate specified in the applicable Certificate of
      Designation, and an assignment of such Receipt or Receipts to the Company or
      in
      blank, duly completed and executed, along with any other documents or
      instruments and any amounts required by the applicable Certificate of
      Designation. 

     

    Upon
      receipt by the Depositary or a
      Depositary’s Agent of a Receipt or Receipts of such notice, assignment and other
      documents, instruments or amounts, the Depositary or such Depositary’s Agent
      shall instruct the Company, subject to any adjustment provided for in the
      applicable Certificate of Designation, (i) to cause the conversion at
      the
      rate specified in the applicable Certificate of Designation of the number of
      shares of Stock evidenced by the Receipt or Receipts so surrendered for
      conversion as specified in the written notice to the Depositary or such
      Depositary’s Agent and (ii) to cause the delivery to the holder of such
      Receipt or Receipts of (a) a certificate or certificates evidencing
      the
      number of whole shares of the Company’s common stock or other securities into
      which such Stock has been converted, and (b) the amount of cash or other
      property, if any, to which such holder is entitled in lieu of fractional shares
      of, or fractional interests in, the Company’s common stock or other securities
      otherwise deliverable by the Company upon such conversion, calculated in
      accordance with the applicable Certificate of Designation. Upon such conversion,
      the Depositary or such Depositary’s Agent (i) shall deliver to the holder a
      Receipt evidencing the number of Depositary Shares evidenced by the surrendered
      Receipt or Receipts in excess of the number of Depositary Shares evidenced
      by
      such Receipt or Receipts that have been so converted, (ii) shall cancel
      the
      Receipts surrendered for conversion and (iii) shall deliver to the Company
      for cancellation the number of shares of Stock evidenced by the Receipts so
      surrendered and so converted. 

     

    Upon
      the conversion of any Stock for
      which a notice of conversion has been provided to the Depositary or a
      Depositary’s Agent by the holder of the Receipt or Receipts representing such
      Stock, the Depositary Shares evidenced by such Receipt or Receipts shall be
      deemed no longer outstanding, all rights of the holder of the Receipt or
      Receipts evidencing such Depositary Shares (except the right to receive
      (i) the Company’s common stock or other securities to which such holder is
      entitled upon conversion in accordance with the applicable Certificate of
      Designation, (ii) any cash or other property payable in accordance with
      the
      applicable Certificate of Designation with respect to any fractional shares
      or
      other fractional interests in the Company’s common stock or other securities
      otherwise deliverable by the Company upon conversion, (iii) any Receipts
      evidencing Depositary Shares representing Stock which was not so converted
      and
      (iv) any other securities, property or cash to which such holder is
      entitled under this Deposit Agreement) shall cease and terminate, and the
      Receipt or Receipts evidencing such Depositary Shares shall be cancelled.

     

    No
      fractional shares or other
      fractional interests in the Company’s common stock or other securities shall be
      deliverable by the Company upon conversion of the Stock represented by the
      Depositary Shares. 

     

    5.
      WITHDRAWAL OF STOCK. Holders of
      Receipts shall have the right, upon payment of any amount due to the Depositary
      with respect to the Receipts, to withdraw any or all of the Stock (but only
      in
      whole shares of Stock) represented by the Depositary Shares and all money and
      other property, if any, represented by such Depositary Shares by surrendering
      the Receipt or Receipts evidencing such Depositary Shares at the Corporate
      Office, or at such other offices as the Depositary may designate for such
      withdrawal (and cancellation of the surrendered Receipts as provided in the
      Deposit Agreement). 

     

    6.
      TRANSFERS, SPLIT-UPS,
      COMBINATIONS. Subject to Paragraphs 7, 8 and 9 below, this Receipt is
      transferable on the books of the Depositary upon surrender of this Receipt
      to
      the Depositary at the Corporate Office or at such other offices as the
      Depositary may designate for such purchase, properly endorsed or accompanied
      by
      a properly executed instrument of transfer, together with evidence of the
      payment of any transfer taxes as may be required by law, and upon such transfer
      the Depositary shall execute and deliver a Receipt or Receipts to or upon the
      order of the person entitled thereto, all as provided in and subject to the
      Deposit Agreement. Subject to the Deposit Agreement, this Receipt may be split
      into other Receipts or combined with other Receipts into one Receipt evidencing
      the same aggregate number of Depositary Shares evidenced by the Receipt or
      Receipts surrendered; provided, however, that the Depositary shall not issue
      any
      Receipt evidencing a fractional Depositary Share. 

     

    7.
      CONDITIONS TO SIGNING AND
      DELIVERY, TRANSFER, ETC., OF RECEIPTS. 

    Prior
      to the execution and delivery,
      registration of transfer, split-up, combination, or surrender of this Receipt,
      the delivery of any distribution hereon or withdrawal or deposit of the Stock
      evidenced hereby, or the exercise of any 

     

    19

    
    

    

    applicable
      conversion right, the
      Depositary, any of the Depositary’s Agents, the Registrar or the Company may
      require any or all of the following: 

     

    (i)
      payment to it of a sum
      sufficient for the payment (or, in the event that the Depositary or the Company
      shall have made such payment, the reimbursement to it) of any tax or other
      governmental charge or fee with respect thereto (including any such tax or
      charge or fee with respect to Stock being deposited or withdrawn or with respect
      to other property of the Company being issued upon redemption or conversion);
      (ii) production of proof satisfactory to it as to the identity and
      genuineness of any signature; and (iii) compliance with such reasonable
      regulations, if any, as the Depositary or the Company may establish not
      inconsistent with the Deposit Agreement. Any person presenting Stock for
      deposit, or any holder of this Receipt, may be required to file such proof
      of
      residence or other information, to execute such certificates and to make such
      representations and warranties as the Depositary or the Company may reasonably
      deem necessary or proper. The Depositary or the Company may withhold or delay
      the delivery of this Receipt, the registration of transfer or redemption of
      this
      Receipt, the withdrawal of the Stock represented by the Depositary Shares
      evidenced by this Receipt, the distribution of any dividend or other
      distribution or the exercise of any applicable conversion right, until such
      proof or other information is filed, such certificates are executed or such
      representations and warranties are made. 

     

    8.
      SUSPENSION OF DELIVERY, TRANSFER,
      ETC. The registration of transfer, split-up, combination or surrender of this
      Receipt and the withdrawal of deposited Stock or the exercise of any applicable
      conversion right may be suspended (i) during any period when the register
      of stockholders of the Company is closed, (ii) if any such action is
      deemed
      necessary or advisable by the Depositary, any of the Depositary’s Agents or the
      Company at any time or from time to time because of any requirement of law
      or of
      any government or governmental body or commission, or under any provision of
      the
      Deposit Agreement, or (iii) with the approval of the Company, for any
      other
      reason. 

     

    9.
      PAYMENT OF TAXES OR OTHER
      GOVERNMENTAL CHARGES. If any tax or other governmental charge or fee shall
      become payable by or on behalf of the Depositary with respect to (i) this
      Receipt, (ii) the Depositary Shares evidenced by this Receipt,
      (iii) the Stock (or fractional interest therein) or other property
      represented by such Depositary Shares, or (iv) any transaction referred
      to
      in Section 4.6 of the Deposit Agreement, such tax (including transfer,
      issuance or acquisition taxes, if any) or governmental charge or fee shall
      be
      payable by the holder of this Receipt, who shall pay the amount thereof to
      the
      Depositary. Until such payment is made, registration or transfer of this Receipt
      or any split-up or combination of this Receipt or any withdrawal of the Stock
      or
      money or other property, if any, represented by the Depositary Shares evidenced
      by this Receipt upon its surrender or the exercise of any applicable conversion
      right may be refused, any dividend or other distribution may be withheld and
      any
      part or all of the Stock or other property represented by the Depositary Shares
      evidenced by this Receipt may be sold for the account of the holder hereof
      (after attempting by reasonable means to notify such holder prior to such sale).
      Any dividend or other distribution so withheld and the proceeds of any such
      sale
      may be applied to any payment of such tax or other governmental charge or fee,
      the holder of this Receipt remaining liable for any deficiency. 

     

    10.
      AMENDMENT. The form of the
      Receipts and any provision of the Deposit Agreement may at any time and from
      time to time be amended by agreement between the Company and the Depositary
      in
      any respect that they may deem necessary or desirable, including to reflect
      the
      terms of any Certificate of Designation; provided, however, that no such
      amendment that shall materially and adversely alter the rights of the holders
      of
      Receipts shall be effective as to outstanding Receipts until the expiration
      of
      90 days after notice of such amendment shall have been given to the record
      holders of outstanding Receipts. Each holder of an outstanding Receipt at the
      time any such amendment becomes effective shall be deemed, by continuing to
      hold
      such Receipt, to consent and agree to such amendment and to be bound by the
      Deposit Agreement as amended thereby. In no event shall any amendment impair
      the
      right, subject to the provisions of the Deposit Agreement, of the holder of
      the
      Depositary Shares evidenced by this Receipt to surrender this Receipt with
      instructions to the Depositary to deliver to the holder the Stock or to cause
      the conversion of such Stock into the Company’s common stock or other securities
      in accordance with the applicable Certificate of Designation and to deliver
      all
      securities, money and other property, if any, represented thereby, except in
      order to comply with mandatory provisions of applicable law. 

     

    11.
      FEES, CHARGES AND EXPENSES. The
      Company will pay all fees, charges and expenses of the Depositary, except for
      taxes and other governmental charges and such charges as are otherwise expressly
      provided for in the Deposit Agreement. 

     

    20

    
    

    

    12.
      TITLE TO RECEIPTS. It is a
      condition of this Receipt, and every successive holder hereof by accepting
      or
      holding the same consents and agrees, that title to this Receipt (and to the
      Depositary Shares evidenced hereby), when properly endorsed or accompanied
      by a
      properly executed instrument of transfer, is transferable by delivery with
      the
      same effect as in the case of investment securities in general; provided,
      however, that the Depositary may, notwithstanding any notice to the contrary,
      treat the record holder hereof at such time as the absolute owner hereof for
      the
      purpose of determining the person entitled to distribution of dividends or
      other
      distributions or to any notice provided for in the Deposit Agreement and for
      all
      other purposes. 

     

    13.
      DIVIDENDS AND DISTRIBUTIONS.
      Whenever the Depositary shall receive any cash dividend or other cash
      distribution on the Stock (other than cash dividends or cash distributions
      paid
      by the Company in lieu of fractional shares or other fractional interests in
      the
      Company’s common stock or other securities otherwise deliverable by the Company
      in accordance with the applicable Certificate of Designation), the Depositary
      shall, subject to the provisions of the Deposit Agreement, distribute to record
      holders of Receipts on the record date fixed pursuant to paragraph 15 such
      amounts of such sums as are, as nearly as practicable, in proportion to the
      respective numbers of Depositary Shares evidenced by the Receipts held by such
      holders; provided, however, that in case the Company or the Depositary shall
      be
      required by law to withhold and does withhold from any cash dividend or other
      cash distribution in respect of the Stock an amount on account of taxes, the
      amount made available for distribution or distributed in respect of Depositary
      Shares shall be reduced accordingly. The Depositary shall distribute or make
      available for distribution, as the case may be, only such amount, however,
      as
      can be distributed without attributing to any holder of Depositary Shares a
      fraction of one cent and any balance not so distributable shall be held by
      the
      Depositary (without liability for interest thereon) and shall be added to and
      be
      treated as part of the next sum received by the Depositary for distribution
      to
      record holders of Receipts then outstanding. 

     

    14.
      SUBSCRIPTION RIGHTS, PREFERENCES
      OR PRIVILEGES. If the Company shall at any time offer or cause to be offered
      to
      the persons in whose name Stock is registered on the books of the Company any
      rights, preferences or privileges to subscribe for or to purchase any securities
      or any rights, preferences or privileges of any other nature, such rights,
      preferences or privileges shall in each such instance, subject to the provisions
      of the Deposit Agreement, be made available by the Depositary to the record
      holders of Receipts in such manner as the Company shall instruct. 

     

    15.
      NOTICE OF DIVIDENDS, FIXING OF
      RECORD DATE. Whenever (i) any cash dividend or other cash distribution
      shall become payable, or any distribution other than cash shall be made, or
      any
      rights, preferences or privileges shall at any time be offered, with respect
      to
      the Stock, or (ii) the Depositary shall receive notice of any meeting
      at
      which holders of Stock are entitled to vote or of which holders of Stock are
      entitled to notice, or (iii) of any mandatory conversion of, or any
      election on the part of the Company to call for redemption or exchange of,
      any
      shares of Stock, in accordance with the applicable Certificate of Designation
      or
      otherwise, the Depositary shall in each such instance fix a record date (which
      shall be the same date as the record date fixed by the Company with respect
      to
      the Stock) for the determination of the holders of Receipts (x) who
      shall
      be entitled to receive such dividend, distribution, rights, preferences or
      privileges or the net proceeds of the sale thereof, or (y) who shall
      be
      entitled to give instructions for the exercise of voting rights at any such
      meeting or to receive notice of such meeting or of such conversion, exchange
      or
      redemption. 

     

    16.
      VOTING RIGHTS. Upon receipt of
      notice of any meeting at which the holders of Stock are entitled to vote, the
      Depositary shall, as soon as practicable thereafter, mail to the record holders
      of Receipts a notice, which shall contain (i) such information as is
      contained in such notice of meeting, (ii) a statement that the holders
      of
      Receipts at the close of business on a specified record date determined as
      provided in paragraph 15 will be entitled, subject to any applicable provision
      of law, the Certificate of Incorporation or the applicable Certificate of
      Designation, to instruct the Depositary as to the exercise of the voting rights
      pertaining to the Stock represented by their respective Depositary Shares,
      and
      (iii) a brief statement as to the manner in which such instructions
      may be
      given. Upon the written request of a holder of this Receipt on such record
      date,
      the Depositary shall endeavor insofar as practicable to vote or cause to be
      voted the Stock represented by the Depositary Shares evidenced by this Receipt
      in accordance with the instructions set forth in such request. The Company
      hereby agrees to take all reasonable action that may be deemed necessary by
      the
      Depositary in order to enable the Depositary to vote such Stock or cause such
      Stock to be voted. In the absence of specific instructions from the holder
      of
      this Receipt, the Depositary will abstain from voting to the extent of the
      Stock
      represented by the Depositary Shares evidenced by this Receipt. 

     

    21

    
    

    

    17.
      REPORTS, INSPECTION OF TRANSFER
      BOOKS. The Company agrees that it will deliver to the Depositary, and the
      Depositary will, promptly after receipt thereof, transmit to the record holders
      of Receipts, in each case at the address recorded in the Depositary’s books or
      the books of any Depositary’s Agent or the Registrar, copies of all notices and
      reports (including financial statements) required by law, by the rules of any
      national securities exchange upon which the Stock, the Depositary Shares or
      the
      Receipts are listed or by the Certificate of Incorporation or the applicable
      Certificate of Designation to be furnished by the Company to holders of Stock.
      The Depositary, acting as transfer agent and Registrar, shall keep books at
      the
      Corporate Office for the registration and transfer of Receipts, which books
      at
      all reasonable times shall be open for inspection by the record holders of
      Receipts; provided that any such holder requesting to exercise such right shall
      certify to the Depositary that such inspection shall be for a proper purpose
      reasonably related to such person’s interest as an owner of Depositary Shares.

     

    18.
      LIABILITY OF THE DEPOSITARY, THE
      DEPOSITARY’S AGENTS, THE REGISTRAR AND THE COMPANY. Neither the Depositary nor
      any Depositary’s Agent nor the Registrar nor the Company shall incur any
      liability to any holder of this Receipt, if by reason of any provision of any
      present or future law or regulation thereunder of any governmental authority
      or
      of any provision, present or future, of the Certificate of Incorporation or
      the
      applicable Certificate of Designation or by reason of any act of god or war
      or
      other circumstances beyond the control of the relevant party, the Depositary,
      any Depositary’s Agent, the Registrar or the Company shall be prevented or
      forbidden from doing or performing any act or thing that the terms of the
      Deposit Agreement provide shall be done or performed; nor shall the Depositary,
      any Depositary’s Agent, the Registrar or the Company incur any liability to any
      holder of this Receipt (i) by reason of any nonperformance or delay,
      caused
      as aforesaid, in the performance of any act or thing that the terms of the
      Deposit Agreement provide shall or may be done or performed or (ii) by
      reason of any exercise of, or failure to exercise, any discretion provided
      for
      in the Deposit Agreement except, in the case of the Depositary, any Depositary’s
      Agent or the Registrar, if such exercise or failure to exercise discretion
      is
      caused by its negligence or willful misconduct. 

     

    19.
      OBLIGATIONS OF THE DEPOSITARY,
      THE DEPOSITARY’S AGENT, THE REGISTRAR AND THE COMPANY. The Company assumes no
      obligation and shall be subject to no liability under the Deposit Agreement
      or
      this Receipt to the holder hereof or other persons, except to perform its
      obligations as are specifically set forth and undertaken by it to perform in
      the
      Deposit Agreement without negligence or bad faith. Each of the Depositary,
      the
      Depositary’s Agents and the Registrar assumes no obligation and shall be subject
      to no liability under the Deposit Agreement or this Receipt to the holder hereof
      or other persons, except to perform such obligations as are specifically set
      forth and undertaken by it to perform in the Deposit Agreement without
      negligence or willful misconduct. 

     

    Neither
      the Depositary nor any
      Depositary’s Agent nor the Registrar nor the Company shall be liable for any
      action or failure to act by it in reliance upon the advice of or information
      from legal counsel, accountants, any person presenting Stock for deposit, any
      holder of this Receipt or any other person believed by it in good faith to
      be
      competent to give such advice or information. 

     

    20.
      TERMINATION OF DEPOSIT
      AGREEMENT. The Deposit Agreement may be terminated only if (i) all of
      the
      outstanding Depositary Shares have been redeemed or converted for any other
      securities into which the Stock is convertible or (ii) there has been
      a
      final distribution of the Stock to the holders of Receipts in connection with
      the Company’s liquidation, dissolution or winding up. The Depositary will mail
      notice of such termination to the record holders of all Receipts then
      outstanding at least 30 days prior to the date fixed in such notice for such
      termination. Upon the termination of the Deposit Agreement, the Company shall
      be
      discharged from all obligations thereunder except for its obligations to the
      Depositary, any Depositary’s Agent and any Registrar under Sections 5.7 and 5.8
      of the Deposit Agreement. 

     

    If
      any Receipts remain outstanding
      after the date of termination of the Deposit Agreement, the Depositary
      thereafter shall discontinue all functions and be discharged from all
      obligations as provided in the Deposit Agreement, except as specifically
      provided therein. 

     

    21.
      GOVERNING LAW. The Deposit
      Agreement and this Receipt and all rights thereunder and hereunder and
      provisions thereof and hereof shall be governed by, and construed in accordance
      with, the law of the State of New York without giving effect to principles
      of
      conflict of laws. 

     

    22

    
    

    

    FORM
      OF ASSIGNMENT 

     

    FOR
      VALUE RECEIVED, the undersigned
      hereby sells, assigns and transfers unto the within Receipt and all rights
      and
      interests represented by the Depositary Shares evidenced thereby, and hereby
      irrevocably constitutes and appoints
      [            ]
      attorney, to transfer the same on the books of the within-named Depositary,
      with
      full power of substitution in the premises. 

     

    Dated:                                     
                 
      Signature:                                      
            

     

    NOTE:
      The signature to this
      assignment 

    must
      correspond with the name as
      written 

    upon
      the face of the Receipt in
      every 

    particular,
      without alteration or

    enlargement,
      or any change whatever.

     

    23EXHIBIT 4.20 
  
 SIRNA THERAPEUTICS, INC. 
  
 AND 
  
 [                        ], AS WARRANT AGENT 
  
 FORM OF COMMON STOCK 
 WARRANT AGREEMENT 
  
 DATED AS OF 
  
 [----------------—] 

 TABLE OF CONTENTS 
  

					
	 ARTICLE I
	  	 ISSUANCE OF WARRANTS AND EXECUTION AND DELIVERY OF WARRANT CERTIFICATES
	  	1
	 Section 1.1
	  	 Issuance of Warrants
	  	1
	 Section 1.2
	  	 Execution and Delivery of Warrant Certificates
	  	1
	 Section 1.3
	  	 Issuance of Warrant Certificates
	  	2
			
	 ARTICLE II
	  	 WARRANT PRICE, DURATION AND EXERCISE OF WARRANTS
	  	2
	 Section 2.1
	  	 Warrant Price
	  	2
	 Section 2.2
	  	 Duration of Warrants
	  	2
	 Section 2.3
	  	 Exercise of Warrants
	  	2
			
	 ARTICLE III
	  	 OTHER PROVISIONS RELATING TO RIGHTS OF HOLDERS OF WARRANT CERTIFICATES
	  	3
	 Section 3.1
	  	 No Rights as Warrant Securityholder Conferred by Warrants or Warrant Certificates
	  	3
	 Section 3.2
	  	 Lost, Stolen, Mutilated or Destroyed Warrant Certificates
	  	4
	 Section 3.3
	  	 Holder of Warrant Certificate May Enforce Rights
	  	4
	 Section 3.4
	  	 Adjustments
	  	4
	 Section 3.5
	  	 Notice to Warrantholders
	  	6
	 Section 3.6
	  	 [If the Warrants are subject to acceleration by the Company, insert — Acceleration of Warrants by the Company
	  	6
			
	 ARTICLE IV
	  	 EXCHANGE AND TRANSFER OF WARRANT CERTIFICATES
	  	7
	 Section 4.1
	  	 Exchange and Transfer of Warrant Certificates
	  	7
	 Section 4.2
	  	 Treatment of Holders of Warrant Certificates
	  	8
	 Section 4.3
	  	 Cancellation of Warrant Certificates
	  	8
			
	 ARTICLE V
	  	 CONCERNING THE WARRANT AGENT
	  	8
	 Section 5.1
	  	 Warrant Agent
	  	8
	 Section 5.2
	  	 Conditions of Warrant Agent’s Obligations
	  	8
	 Section 5.3
	  	 Resignation, Removal and Appointment of Successor
	  	10
			
	 ARTICLE VI
	  	 MISCELLANEOUS
	  	11
	 Section 6.1
	  	 Amendment
	  	11
	 Section 6.2
	  	 Notices and Demands to the Company and Warrant Agent
	  	11
	 Section 6.3
	  	 Addresses
	  	11
	 Section 6.4
	  	 Governing Law
	  	11
	 Section 6.5
	  	 Delivery of Prospectus
	  	11
	 Section 6.6
	  	 Obtaining of Governmental Approvals
	  	11
	 Section 6.7
	  	 Persons Having Rights Under Warrant Agreement
	  	11
	 Section 6.8
	  	 Headings
	  	11
	 Section 6.9
	  	 Counterparts
	  	11
	 Section 6.10
	  	 Inspection of Agreement
	  	12

 SIRNA THERAPEUTICS, INC. 
 Form of Common Stock Warrant Agreement 
  
 COMMON STOCK WARRANT AGREEMENT, dated as of [            ]between SIRNA THERAPEUTICS, INC., a Delaware corporation (the “COMPANY”) and
[            ], a [corporation] [national banking association] organized and existing under the laws of
[            ] and having a corporate trust office in [            ], as warrant agent (the “WARRANT
AGENT”). 
  
 WHEREAS, the Company proposes to sell [if
Warrants are sold with other securities — [title of such other securities being offered] (the “OTHER SECURITIES”) with] warrant certificates evidencing one or more warrants (the “WARRANTS” or individually a
“WARRANT”) representing the right to purchase Common Stock of the Company, par value $0.01 per share (the “WARRANT SECURITIES”), such warrant certificates and other warrant certificates issued pursuant to this Agreement being
herein called the “WARRANT CERTIFICATES”; and 
  
 WHEREAS, the Company desires the Warrant Agent to act on behalf of the Company, and the Warrant Agent is willing so to act, in connection with the issuance, registration, transfer, exchange, exercise and replacement of the Warrant
Certificates, and in this Agreement wishes to set forth, among other things, the form and provisions of the Warrant Certificates and the terms and conditions on which they may be issued, registered, transferred, exchanged, exercised and replaced;

  
 NOW THEREFORE, in consideration of the premises and of the
mutual agreements herein contained, the parties hereto agree as follows: 
  
  
 ARTICLE I 
 ISSUANCE OF WARRANTS AND EXECUTION
AND 
 DELIVERY OF WARRANT CERTIFICATES 
  
 SECTION 1.1 ISSUANCE OF WARRANTS. [If Warrants alone — Upon issuance, each Warrant Certificate shall evidence one or more Warrants.] [If Other
Securities and Warrants — Warrants shall be [initially] issued in connection with the issuance of the Other Securities [but shall be separately transferable on and after
[            ] (the “Detachable Date”)] [and shall not be separately transferable] and each Warrant Certificate shall evidence one or more Warrants.] Each Warrant evidenced
thereby shall represent the right, subject to the provisions contained herein and therein, to purchase one Warrant Security. [If Other Securities and Warrants — Warrant Certificates shall be initially issued in units with the Other Securities
and each Warrant Certificate included in such a unit shall evidence [            ] Warrants for each [$
[            ]principal amount] [             shares] of Other Securities included in such unit.] 
  
 SECTION 1.2 EXECUTION AND DELIVERY OF WARRANT CERTIFICATES. Each Warrant
Certificate, whenever issued, shall be in registered form substantially in the form set forth in Exhibit A hereto, shall be dated the date of its countersignature by the Warrant Agent and may have such letters, numbers, or other marks of
identification or designation and such legends or endorsements printed, lithographed or engraved thereon as the officers of the Company executing the same may approve (execution thereof to be conclusive evidence of such approval) and as are not
inconsistent with the provisions of this Agreement, or as may be required to comply with any law or with any rule or regulation made pursuant thereto or with any rule or regulation of any securities exchange on which the Warrants may be listed, or
to conform to usage. The Warrant Certificates shall be signed on behalf of the Company by any of its present or future chief executive officers, presidents, senior vice presidents, vice presidents, chief financial officers, chief legal officers,
treasurers, assistant treasurers, controllers, assistant controllers, secretaries or assistant secretaries under its corporate seal reproduced thereon. Such signatures may be manual or facsimile signatures of such authorized officers and may be
imprinted or otherwise reproduced on the Warrant Certificates. The seal of the Company may be in the form of a facsimile thereof and may be impressed, affixed, imprinted or otherwise reproduced on the Warrant Certificates. 
  
 No Warrant Certificate shall be valid for any purpose, and no Warrant
evidenced thereby shall be exercisable, until such Warrant Certificate has been countersigned by the manual signature of the Warrant Agent. Such signature by the Warrant Agent upon any Warrant Certificate executed by the Company shall be conclusive
evidence that the Warrant Certificate so countersigned has been duly issued hereunder. 
  

 1 

 In case any officer of the Company who shall have signed any of the Warrant Certificates either manually
or by facsimile signature shall cease to be such officer before the Warrant Certificates so signed shall have been countersigned and delivered by the Warrant Agent, such Warrant Certificates may be countersigned and delivered notwithstanding that
the person who signed Warrant Certificates ceased to be such officer of the Company; and any Warrant Certificate may be signed on behalf of the Company by such persons as, at the actual date of the execution of such Warrant Certificate, shall be the
proper officers of the Company, although at the date of the execution of this Agreement any such person was not such officer. 
  
 The term “holder” or “holder of a Warrant Certificate” as used herein shall mean any person in whose name at the time any Warrant
Certificate shall be registered upon the books to be maintained by the Warrant Agent for that purpose [If Other Securities and Warrants are not immediately detachable — or upon the registration of the Other Securities prior to the Detachable
Date. Prior to the Detachable Date, the Company will, or will cause the registrar of the Other Securities to, make available at all times to the Warrant Agent such information as to holders of the Other Securities as may be necessary to keep the
Warrant Agent’s records up to date]. 
  
 SECTION 1.3 ISSUANCE
OF WARRANT CERTIFICATES. Warrant Certificates evidencing the right to purchase Warrant Securities may be executed by the Company and delivered to the Warrant Agent upon the execution of this Warrant Agreement or from time to time thereafter. The
Warrant Agent shall, upon receipt of Warrant Certificates duly executed on behalf of the Company, countersign such Warrant Certificates and shall deliver such Warrant Certificates to or upon the order of the Company. 
  
  
 ARTICLE II 
 WARRANT PRICE, DURATION AND EXERCISE OF WARRANTS 
  
 SECTION 2.1 WARRANT PRICE. During the period specified in Section 2.2, each Warrant shall, subject to the terms of this
Warrant Agreement and the applicable Warrant Certificate, entitle the holder thereof to initially purchase the number of Warrant Securities specified in the applicable Warrant Certificate at an initial exercise price of $
[            ] per Warrant Security, subject to adjustment upon the occurrence of certain events, as hereinafter provided. Such purchase price per Warrant Security is referred to in
this Agreement as the “WARRANT PRICE.” 
  
 SECTION 2.2
DURATION OF WARRANTS. Each Warrant may be exercised in whole or in part at any time, as specified herein, on or after [the date thereof] [            ] and at or before
[            ] p.m., [City] time, on              or such later date as the Company may designate by notice to the
Warrant Agent and the holders of Warrant Certificates mailed to their addresses as set forth in the record books of the Warrant Agent (the “EXPIRATION DATE”). Each Warrant not exercised at or before
[            ] p.m., [City] time, on the Expiration Date shall become void, and all rights of the holder of the Warrant Certificate evidencing such Warrant under this Agreement shall
cease. 
  
 SECTION 2.3 EXERCISE OF WARRANTS. 
  
 (a) During the period specified in Section 2.2, the Warrants may be
exercised to purchase a whole number of Warrant Securities in registered form by providing certain information as set forth on the reverse side of the Warrant Certificate and by paying in full, in lawful money of the United States of America, [in
cash or by certified check or official bank check in New York Clearing House funds] [by bank wire transfer in immediately available funds] the Warrant Price for each Warrant Security with respect to which a Warrant is being exercised to the Warrant
Agent at its corporate trust office, provided that such exercise is subject to receipt within five business days of such payment by the Warrant Agent of the Warrant Certificate with the form of election to purchase Warrant Securities set forth on
the reverse side of the Warrant Certificate properly completed and duly executed. The date on which payment in full of the Warrant Price is received by the Warrant Agent shall, subject to receipt of the Warrant Certificate as aforesaid, be deemed to

  

 2 

 
be the date on which the Warrant is exercised; provided, however, that if, at the date of receipt of such Warrant Certificates and payment in full of the
Warrant Price, the transfer books for the Warrant Securities purchasable upon the exercise of such Warrants shall be closed, no such receipt of such Warrant Certificates and no such payment of such Warrant Price shall be effective to constitute the
person so designated to be named as the holder of record of such Warrant Securities on such date, but shall be effective to constitute such person as the holder of record of such Warrant Securities for all purposes at the opening of business on the
next succeeding day on which the transfer books for the Warrant Securities purchasable upon the exercise of such Warrants shall be opened, and the certificates for the Warrant Securities in respect of which such Warrants are then exercised shall be
issuable as of the date on such next succeeding day on which the transfer books shall next be opened, and until such date the Company shall be under no duty to deliver any certificate for such Warrant Securities. The Warrant Agent shall deposit all
funds received by it in payment of the Warrant Price in an account of the Company maintained with it and shall advise the Company by telephone at the end of each day on which a payment for the exercise of Warrants is received of the amount so
deposited to its account. The Warrant Agent shall promptly confirm such telephone advice to the Company in writing. 
  
 (b) The Warrant Agent shall, from time to time, as promptly as practicable, advise the Company of (i) the number of Warrant Securities with respect
to which Warrants were exercised, (ii) the instructions of each holder of the Warrant Certificates evidencing such Warrants with respect to delivery of the Warrant Securities to which such holder is entitled upon such exercise,
(iii) delivery of Warrant Certificates evidencing the balance, if any, of the Warrants for the remaining Warrant Securities after such exercise, and (iv) such other information as the Company shall reasonably require. 
  
 (c) As soon as practicable after the exercise of any Warrant, the Company
shall issue to or upon the order of the holder of the Warrant Certificate evidencing such Warrant the Warrant Securities to which such holder is entitled, in fully registered form, registered in such name or names as may be directed by such holder.
If fewer than all of the Warrants evidenced by such Warrant Certificate are exercised, the Company shall execute, and an authorized officer of the Warrant Agent shall manually countersign and deliver, a new Warrant Certificate evidencing Warrants
for the number of Warrant Securities remaining unexercised. 
  
 (d) The Company shall not be required to pay any stamp or other tax or other governmental charge required to be paid in connection with any transfer involved in the issue of the Warrant Securities, and in the event that any such transfer is
involved, the Company shall not be required to issue or deliver any Warrant Security until such tax or other charge shall have been paid or it has been established to the Company’s satisfaction that no such tax or other charge is due.

  
 (e) Prior to the issuance of any Warrants there shall have
been reserved, and the Company shall at all times through the Expiration Date keep reserved, out of its authorized but unissued Warrant Securities, a number of shares sufficient to provide for the exercise of the Warrants. 
  
  
 ARTICLE III 
 OTHER PROVISIONS RELATING TO RIGHTS 
 OF HOLDERS OF WARRANT CERTIFICATES 
  
 SECTION 3.1 NO RIGHTS AS WARRANT SECURITYHOLDER CONFERRED BY WARRANTS OR WARRANT CERTIFICATES. No Warrant Certificate or Warrant evidenced thereby shall entitle the holder thereof to any of the rights of a holder of Warrant Securities,
including, without limitation, the right to receive the payment of dividends or distributions, if any, on the Warrant Securities or to exercise any voting rights, except to the extent expressly set forth in this Agreement or the applicable Warrant
Certificate. 
  

 3 

 SECTION 3.2 LOST, STOLEN, MUTILATED OR DESTROYED WARRANT CERTIFICATES. Upon receipt by the Warrant Agent
of evidence reasonably satisfactory to it and the Company of the ownership of and the loss, theft, destruction or mutilation of any Warrant Certificate and/or indemnity reasonably satisfactory to the Warrant Agent and the Company and, in the case of
mutilation, upon surrender of the mutilated Warrant Certificate to the Warrant Agent for cancellation, then, in the absence of notice to the Company or the Warrant Agent that such Warrant Certificate has been acquired by a bona fide purchaser, the
Company shall execute, and an authorized officer of the Warrant Agent shall manually countersign and deliver, in exchange for or in lieu of the lost, stolen, destroyed or mutilated Warrant Certificate, a new Warrant Certificate of the same tenor and
evidencing Warrants for a like number of Warrant Securities. Upon the issuance of any new Warrant Certificate under this Section 3.2, the Company may require the payment of a sum sufficient to cover any tax or other governmental charge that may
be imposed in relation thereto and any other expenses (including the fees and expenses of the Warrant Agent) in connection therewith. Every substitute Warrant Certificate executed and delivered pursuant to this Section 3.2 in lieu of any lost,
stolen or destroyed Warrant Certificate shall represent an additional contractual obligation of the Company, whether or not the lost, stolen or destroyed Warrant Certificate shall be at any time enforceable by anyone, and shall be entitled to the
benefits of this Agreement equally and proportionately with any and all other Warrant Certificates duly executed and delivered hereunder. The provisions of this Section 3.2 are exclusive and shall preclude (to the extent lawful) all other
rights and remedies with respect to the replacement of mutilated, lost, stolen or destroyed Warrant Certificates. 
  
 SECTION 3.3 HOLDER OF WARRANT CERTIFICATE MAY ENFORCE RIGHTS. Notwithstanding any of the provisions of this Agreement, any holder of a Warrant
Certificate, without the consent of the Warrant Agent, the holder of any Warrant Securities or the holder of any other Warrant Certificate, may, in such holder’s own behalf and for such holder’s own benefit, enforce, and may institute and
maintain any suit, action or proceeding against the Company suitable to enforce, or otherwise in respect of, such holder’s right to exercise the Warrants evidenced by such holder’s Warrant Certificate in the manner provided in such
holder’s Warrant Certificate and in this Agreement. 
  
 SECTION 3.4 ADJUSTMENTS. 
  
 (a) In case the Company
shall at any time subdivide its outstanding shares of Common Stock into a greater number of shares, the Warrant Price in effect immediately prior to such subdivision shall be proportionately reduced and the number of Warrant Securities purchasable
under the Warrants shall be proportionately increased. Conversely, in case the outstanding shares of Common Stock of the Company shall be combined into a smaller number of shares, the Warrant Price in effect immediately prior to such combination
shall be proportionately increased and the number of Warrant Securities purchasable under the Warrants shall be proportionately decreased. 
  
 (b) If at any time or from time to time the holders of Common Stock (or any shares of stock or other securities at the time receivable upon the exercise
of the Warrants) shall have received or become entitled to receive, without payment therefore, 
  
 (i) Common Stock or any shares of stock or other securities which are at any time directly or indirectly convertible into or exchangeable for Common
Stock, or any rights or options to subscribe for, purchase or otherwise acquire any of the foregoing by way of dividend or other distribution; 
  
 (ii) any cash paid or payable otherwise than as a cash dividend paid or payable out of the Company’s current or retained earnings; 
  
 (iii) any evidence of the Company’s indebtedness or rights to
subscribe for or purchase the Company’s indebtedness; or 
  
 (iv) Common Stock or additional stock or other securities or property (including cash) by way of spinoff, split-up, reclassification, combination of shares or similar corporate rearrangement (other than shares of Common Stock issued as a
stock split or adjustments in respect of which shall be covered by the terms of Section 3.4(a) above), 
  

 4 

 then and in each such case, the holder of each Warrant shall, upon the exercise of the
Warrant, be entitled to receive, in addition to the number of Warrant Securities receivable thereupon, and without payment of any additional consideration therefore, the amount of stock and other securities and property (including cash and
indebtedness (or rights to subscribe for or purchase indebtedness) which such holder would hold on the date of such exercise had he been the holder of record of such Warrant Securities as of the date on which holders of Common Stock received or
became entitled to receive such shares or all other additional stock and other securities and property. 
  
 (c) In case of (i) any reclassification, capital reorganization, or change in the Common Stock of the Company (other than as a result of a
subdivision, combination, or stock dividend provided for in Section 3.4(a) or Section 3.4(b) above), (ii) share exchange, merger or similar transaction of the Company with or into another person or entity (other than a share exchange,
merger or similar transaction in which the Company is the acquiring or surviving corporation and which does not result in any change in the Common Stock other than the issuance of additional shares of Common Stock) or (iii) the sale, exchange,
lease, transfer or other disposition of all or substantially all of the properties and assets of the Company as an entirety (in any such case, a “REORGANIZATION EVENT”), then, as a condition of such Reorganization Event, lawful provisions
shall be made, and duly executed documents evidencing the same from the Company or its successor shall be delivered to the holders of the Warrants, so that the holders of the Warrants shall have the right at any time prior to the expiration of the
Warrants to purchase, at a total price equal to that payable upon the exercise of the Warrants, the kind and amount of shares of stock and other securities and property receivable in connection with such Reorganization Event by a holder of the same
number of Warrant Securities as were purchasable by the holders of the Warrants immediately prior to such Reorganization Event. In any such case appropriate provisions shall be made with respect to the rights and interests of the holders of the
Warrants so that the provisions hereof shall thereafter be applicable with respect to any shares of stock or other securities and property deliverable upon exercise the Warrants, and appropriate adjustments shall be made to the Warrant Price payable
hereunder provided the aggregate purchase price shall remain the same. In the case of any transaction described in clauses (ii) and (iii) above, the Company shall thereupon be relieved of any further obligation hereunder or under the
Warrants, and the Company as the predecessor corporation may thereupon or at any time thereafter be dissolved, wound up or liquidated. Such successor or assuming entity thereupon may cause to be signed, and may issue either in its own name or in the
name of the Company, any or all of the Warrants issuable hereunder which heretofore shall not have been signed by the Company, and may execute and deliver securities in its own name, in fulfillment of its obligations to deliver Warrant Securities
upon exercise of the Warrants. All the Warrants so issued shall in all respects have the same legal rank and benefit under this Agreement as the Warrants theretofore or thereafter issued in accordance with the terms of this Agreement as though all
of such Warrants had been issued at the date of the execution hereof. In any case of any such Reorganization Event, such changes in phraseology and form (but not in substance) may be made in the Warrants thereafter to be issued as may be
appropriate. 
  
 The Warrant Agent may receive a written opinion
of legal counsel as conclusive evidence that any such Reorganization Event complies with the provisions of this Section 3.4. 
  
 (d) The Company may, at its option, at any time until the Expiration Date, reduce the then current Warrant Price to any amount deemed appropriate by the
Board of Directors of the Company for any period not exceeding twenty consecutive days (as evidenced in a resolution adopted by such Board of Directors), but only upon giving the notices required by Section 3.5 at least ten days prior to taking
such action. 
  
 (e) Except as herein otherwise expressly
provided, no adjustment in the Warrant Price shall be made by reason of the issuance of shares of Common Stock, or securities convertible into or exchangeable for shares of Common Stock, or securities carrying the right to purchase any of the
foregoing or for any other reason whatsoever. 
  

 5 

 (f) No fractional Warrant Securities shall be issued upon the exercise of Warrants. If more than one
Warrant shall be exercised at one time by the same holder, the number of full Warrant Securities which shall be issuable upon such exercise shall be computed on the basis of the aggregate number of Warrant Securities purchased pursuant to the
Warrants so exercised. Instead of any fractional Warrant Security which would otherwise be issuable upon exercise of any Warrant, the Company shall pay a cash adjustment in respect of such fraction in an amount equal to the same fraction of the last
sales price (or bid price if there were no sales) per Warrant Security, in either case as reported on the New York Stock Exchange Composite Tape on the business day which next precedes the day of exercise or, if the Warrant Securities are not then
listed or admitted to trading on the New York Stock Exchange, on the principal national securities exchange on which the Warrant Securities are listed or admitted to trading or, if not listed or admitted to trading on any national securities
exchange, on the National Market System of the National Association of Securities Dealers, Inc. Automated Quotations System (“NASDAQ”), or if the Warrant Securities are not then listed or admitted to trading on any national securities
exchange or quoted on the National Market System of NASDAQ, the average of the closing high bid and low asked prices in the over-the-counter market, as reported by NASDAQ, or such other system then in use, or if on any such date the Warrant
Securities are not quoted by any such organization, an amount equal to the same fraction of the average of the closing bid and asked prices as furnished by any New York Stock Exchange firm selected from time to time by the Company for that purpose
at the close of business on the business day which next precedes the day of exercise. 
  
 (g) Whenever the Warrant Price then in effect is adjusted as herein provided, the Company shall mail to each holder of the Warrants at such holder’s address as it shall appear on the books of the Company a
statement setting forth the adjusted Warrant Price then and thereafter effective under the provisions hereof, together with the facts, in reasonable detail, upon which such adjustment is based. 
  
 SECTION 3.5 NOTICE TO WARRANTHOLDERS. In case the Company shall
(a) effect any dividend or distribution described in Section 3.4(b), (b) effect any Reorganization Event, (c) make any distribution on or in respect of the Common Stock in connection with the dissolution, liquidation or winding
up of the Company, or (d) reduce the then current Warrant Price pursuant to Section 3.4(d), then the Company shall mail to each holder of Warrants at such holder’s address as it shall appear on the books of the Warrant Agent, at least
ten days prior to the applicable date hereinafter specified, a notice stating (x) the record date for such dividend or distribution, or, if a record is not to be taken, the date as of which the holders of record of Common Stock that will be
entitled to such dividend or distribution are to be determined, (y) the date on which such Reorganization Event, dissolution, liquidation or winding up is expected to become effective, and the date as of which it is expected that holders of
Common Stock of record shall be entitled to exchange their shares of Common Stock for securities or other property deliverable upon such Reorganization Event, dissolution, liquidation or winding up, or (z) the first date on which the then
current Warrant Price shall be reduced pursuant to Section 3.4(d). No failure to mail such notice nor any defect therein or in the mailing thereof shall affect any such transaction or any adjustment in the Warrant Price required by
Section 3.4. 
  
 SECTION 3.6 [IF THE WARRANTS ARE SUBJECT TO
ACCELERATION BY THE COMPANY, INSERT—ACCELERATION OF WARRANTS BY THE COMPANY. 
  
 (a) At any time on or after[            ], the Company shall have the right to accelerate any or all Warrants at any time by causing them to expire at
the close of business on the day next preceding a specified date (the “ACCELERATION DATE”), if the Market Price (as hereinafter defined) of the Common Stock equals or
exceeds[            ] percent (        %) of the then effective Warrant Price on any twenty Trading Days (as hereinafter defined)
within a period of thirty consecutive Trading Days ending no more than five Trading Days prior to the date on which the Company gives notice to the Warrant Agent of its election to accelerate the Warrants. 
  

 6 

 (b) Market Price” for each Trading Day shall be, if the Common Stock is listed or admitted for
trading on the New York Stock Exchange, the last reported sale price, regular way (or, if no such price is reported, the average of the reported closing bid and asked prices, regular way) of Common Stock, in either case as reported on the New York
Stock Exchange Composite Tape or, if the Common Stock is not listed or admitted to trading on the New York Stock Exchange, on the principal national securities exchange on which the Common Stock is listed or admitted to trading or, if not listed or
admitted to trading on any national securities exchange, on the National Market System of NASDAQ or, if not listed or admitted to trading on any national securities exchange or quoted on the National Market System of NASDAQ, the average of the
closing high bid and low asked prices in the over-the-counter market, as reported by NASDAQ, or such other system then in use, or if on any such date the shares of Common Stock are not quoted by any such organization, the average of the closing bid
and asked prices as furnished by any New York Stock Exchange firm selected from time to time by the Company for that purpose. “Trading Day” shall be each Monday through Friday, other than any day on which securities are not traded in the
system or on the exchange that is the principal market for the Common Stock, as determined by the Board of Directors of the Company. 
  
 (c) In the event of an acceleration of less than all of the Warrants, the Warrant Agent shall select the Warrants to be accelerated by lot, pro rata or in
such other manner as it deems, in its discretion, to be fair and appropriate. 
  
 (d) Notice of an acceleration specifying the Acceleration Date shall be sent by mail first class, postage prepaid, to each registered holder of a Warrant Certificate representing a Warrant accelerated at such
holder’s address appearing on the books of the Warrant Agent not more than sixty days nor less than thirty days before the Acceleration Date. Such notice of an acceleration also shall be given no more than twenty days, and no less than ten
days, prior to the mailing of notice to registered holders of Warrants pursuant to this Section 3.6, by publication at least once in a newspaper of general circulation in the City of New York. 
  
 (e) Any Warrant accelerated may be exercised until
[        ] p.m., [City] time, on the business day next preceding the Acceleration Date. The Warrant Price shall be payable as provided in Section 2.] 
  
  
 ARTICLE IV 
 EXCHANGE AND TRANSFER OF WARRANT CERTIFICATES 
  
 SECTION 4.1 EXCHANGE AND TRANSFER OF WARRANT CERTIFICATES. [If Other Securities with Warrants which are immediately
detachable—Upon] [If Other Securities with Warrants which are not immediately detachable—Prior to the Detachable Date, a Warrant Certificate may be exchanged or transferred only together with the Other Security to which the Warrant
Certificate was initially attached, and only for the purpose of effecting or in conjunction with an exchange or transfer of such Other Security. Prior to any Detachable Date, each transfer of the Other Security shall operate also to transfer the
related Warrant Certificates. After the Detachable Date, upon] surrender at the corporate trust office of the Warrant Agent, Warrant Certificates evidencing Warrants may be exchanged for Warrant Certificates in other denominations evidencing such
Warrants or the transfer thereof may be registered in whole or in part; provided that such other Warrant Certificates evidence Warrants for the same aggregate number of Warrant Securities as the Warrant Certificates so surrendered. The Warrant Agent
shall keep, at its corporate trust office, books in which, subject to such reasonable regulations as it may prescribe, it shall register Warrant Certificates and exchanges and transfers of outstanding Warrant Certificates, upon surrender of the
Warrant Certificates to the Warrant Agent at its corporate trust office for exchange or registration of transfer, properly endorsed or accompanied by appropriate instruments of registration of transfer and written instructions for transfer, all in
form satisfactory to the Company and the Warrant Agent. No service charge shall be made for any exchange or registration of transfer of Warrant Certificates, but the Company may require payment of a sum sufficient to cover any stamp or other tax or
other governmental charge that may be imposed in connection with any such exchange or registration of transfer. Whenever any Warrant Certificates are so surrendered for exchange or 

  

 7 

 
registration of transfer, an authorized officer of the Warrant Agent shall manually countersign and deliver to the person or persons entitled thereto a
Warrant Certificate or Warrant Certificates duly authorized and executed by the Company, as so requested. The Warrant Agent shall not be required to effect any exchange or registration of transfer which will result in the issuance of a Warrant
Certificate evidencing a Warrant for a fraction of a Warrant Security or a number of Warrants for a whole number of Warrant Securities and a fraction of a Warrant Security. All Warrant Certificates issued upon any exchange or registration of
transfer of Warrant Certificates shall be the valid obligations of the Company, evidencing the same obligations and entitled to the same benefits under this Agreement as the Warrant Certificate surrendered for such exchange or registration of
transfer. 
  
 SECTION 4.2 TREATMENT OF HOLDERS OF WARRANT
CERTIFICATES. [If Other Securities and Warrants are not immediately detachable—Prior to the Detachable Date, the Company, the Warrant Agent and all other persons may treat the owner of the Other Security as the owner of the Warrant Certificates
initially attached thereto for any purpose and as the person entitled to exercise the rights represented by the Warrants evidenced by such Warrant Certificates, any notice to the contrary notwithstanding. After the Detachable Date and prior to due
presentment of a Warrant Certificate for registration of transfer, the] [T]he Company, the Warrant Agent and all other persons may treat the registered holder of a Warrant Certificate as the absolute owner thereof for any purpose and as the person
entitled to exercise the rights represented by the Warrants evidenced thereby, any notice to the contrary notwithstanding. 
  
 SECTION 4.3 CANCELLATION OF WARRANT CERTIFICATES. Any Warrant Certificate surrendered for exchange, registration of transfer or exercise of the Warrants
evidenced thereby shall, if surrendered to the Company, be delivered to the Warrant Agent and all Warrant Certificates surrendered or so delivered to the Warrant Agent shall be promptly canceled by the Warrant Agent and shall not be reissued and,
except as expressly permitted by this Agreement, no Warrant Certificate shall be issued hereunder in exchange therefor or in lieu thereof. The Warrant Agent shall deliver to the Company from time to time or otherwise dispose of canceled Warrant
Certificates in a manner satisfactory to the Company. 
  
  
 ARTICLE V 
 CONCERNING THE WARRANT AGENT

  
 SECTION 5.1 WARRANT AGENT. The Company hereby appoints
[            ] as Warrant Agent of the Company in respect of the Warrants and the Warrant Certificates upon the terms and subject to the conditions herein set forth, and
[            ] hereby accepts such appointment. The Warrant Agent shall have the powers and authority granted to and conferred upon it in the Warrant Certificates and hereby and such
further powers and authority to act on behalf of the Company as the Company may hereafter grant to or confer upon it. All of the terms and provisions with respect to such powers and authority contained in the Warrant Certificates are subject to and
governed by the terms and provisions hereof. 
  
 SECTION 5.2
CONDITIONS OF WARRANT AGENT’S OBLIGATIONS. The Warrant Agent accepts its obligations herein set forth upon the terms and conditions hereof, including the following to all of which the Company agrees and to all of which the rights hereunder of
the holders from time to time of the Warrant Certificates shall be subject: 
  
 (a) COMPENSATION AND INDEMNIFICATION. The Company agrees promptly to pay the Warrant Agent the compensation to be agreed upon with the Company for all services rendered by the Warrant Agent and to reimburse the
Warrant Agent for reasonable out-of-pocket expenses (including reasonable counsel fees) incurred without negligence, bad faith or willful misconduct by the Warrant Agent in connection with the services rendered hereunder by the Warrant Agent. The
Company also agrees to indemnify the Warrant Agent for, and to hold it harmless against, any loss, liability or expense incurred without negligence, bad faith or willful misconduct on the part of the Warrant Agent, arising out of or in connection
with its acting as Warrant Agent hereunder, including the reasonable costs and expenses of defending against any claim of such liability. 
  
 (b) AGENT FOR THE COMPANY. In acting under this Warrant Agreement and in connection with the Warrant Certificates, the Warrant Agent is acting solely as
agent of the Company and does not assume any obligations or relationship of agency or trust for or with any of the holders of Warrant Certificates or beneficial owners of Warrants. 
  

 8 

 (c) COUNSEL. The Warrant Agent may consult with counsel satisfactory to it, which may include counsel for
the Company, and the written advice of such counsel shall be full and complete authorization and protection in respect of any action taken, suffered or omitted by it hereunder in good faith and in accordance with the advice of such counsel.

  
 (d) DOCUMENTS. The Warrant Agent shall be protected and shall
incur no liability for or in respect of any action taken or omitted by it in reliance upon any Warrant Certificate, notice, direction, consent, certificate, affidavit, statement or other paper or document reasonably believed by it to be genuine and
to have been presented or signed by the proper parties. 
  
 (e)
CERTAIN TRANSACTIONS. The Warrant Agent, and its officers, directors and employees, may become the owner of, or acquire any interest in, Warrants, with the same rights that it or they would have if it were not the Warrant Agent hereunder, and, to
the extent permitted by applicable law, it or they may engage or be interested in any financial or other transaction with the Company and may act on, or as depositary, trustee or agent for, any committee or body of holders of Warrant Securities or
other obligations of the Company as freely as if it were not the Warrant Agent hereunder. Nothing in this Warrant Agreement shall be deemed to prevent the Warrant Agent from acting as trustee under any indenture to which the Company is a party.

  
 (f) NO LIABILITY FOR INTEREST. Unless otherwise agreed with
the Company, the Warrant Agent shall have no liability for interest on any monies at any time received by it pursuant to any of the provisions of this Agreement or of the Warrant Certificates. 
  
 (g) NO LIABILITY FOR INVALIDITY. The Warrant Agent shall have no liability
with respect to any invalidity of this Agreement or any of the Warrant Certificates (except as to the Warrant Agent’s countersignature thereon). 
  
 (h) NO RESPONSIBILITY FOR REPRESENTATIONS. The Warrant Agent shall not be responsible for any of the recitals or representations herein or in the Warrant
Certificates (except as to the Warrant Agent’s countersignature thereon), all of which are made solely by the Company. 
  
 (i) NO IMPLIED OBLIGATIONS. The Warrant Agent shall be obligated to perform only such duties as are herein and in the Warrant Certificates specifically
set forth and no implied duties or obligations shall be read into this Agreement or the Warrant Certificates against the Warrant Agent. The Warrant Agent shall not be under any obligation to take any action hereunder which may tend to involve it in
any expense or liability, the payment of which within a reasonable time is not, in its reasonable opinion, assured to it. The Warrant Agent shall not be accountable or under any duty or responsibility for the use by the Company of any of the Warrant
Certificates authenticated by the Warrant Agent and delivered by it to the Company pursuant to this Agreement or for the application by the Company of the proceeds of the Warrant Certificates. The Warrant Agent shall have no duty or responsibility
in case of any default by the Company in the performance of its covenants or agreements contained herein or in the Warrant Certificates or in the case of the receipt of any written demand from a holder of a Warrant Certificate with respect to such
default, including, without limiting the generality of the foregoing, any duty or responsibility to initiate or attempt to initiate any proceedings at law or otherwise or, except as provided in Section 6.2 hereof, to make any demand upon the
Company. 
  

 9 

 SECTION 5.3 RESIGNATION, REMOVAL AND APPOINTMENT OF SUCCESSOR. 
  
 (a) The Company agrees, for the benefit of the holders from time to time of
the Warrant Certificates, that there shall at all times be a Warrant Agent hereunder until all the Warrants have been exercised or are no longer exercisable. 
  

(b) The Warrant Agent may at any time resign as agent by giving written notice to the Company of such intention on its part, specifying the date on
which its desired resignation shall become effective; provided that such date shall not be less than three months after the date on which such notice is given unless the Company otherwise agrees. The Warrant Agent hereunder may be removed at any
time by the filing with it of an instrument in writing signed by or on behalf of the Company and specifying such removal and the intended date when it shall become effective. Such resignation or removal shall take effect upon the appointment by the
Company, as hereinafter provided, of a successor Warrant Agent (which shall be a bank or trust company authorized under the laws of the jurisdiction of its organization to exercise corporate trust powers) and the acceptance of such appointment by
such successor Warrant Agent. The obligation of the Company under Section 5.2(a) shall continue to the extent set forth therein notwithstanding the resignation or removal of the Warrant Agent. 
  
 (c) In case at any time the Warrant Agent shall resign, or shall be
removed, or shall become incapable of acting, or shall be adjudged a bankrupt or insolvent, or shall commence a voluntary case under the Federal bankruptcy laws, as now or hereafter constituted, or under any other applicable Federal or state
bankruptcy, insolvency or similar law or shall consent to the appointment of or taking possession by a receiver, custodian, liquidator, assignee, trustee, sequestrator (or other similar official) of the Warrant Agent or its property or affairs, or
shall make an assignment for the benefit of creditors, or shall admit in writing its inability to pay its debts generally as they become due, or shall take corporate action in furtherance of any such action, or a decree or order for relief by a
court having jurisdiction in the premises shall have been entered in respect of the Warrant Agent in an involuntary case under the Federal bankruptcy laws, as now or hereafter constituted, or any other applicable Federal or state bankruptcy,
insolvency or similar law, or a decree or order by a court having jurisdiction in the premises shall have been entered for the appointment of a receiver, custodian, liquidator, assignee, trustee, sequestrator (or similar official) of the Warrant
Agent or of its property or affairs, or any public officer shall take charge or control of the Warrant Agent or of its property or affairs for the purpose of rehabilitation, conservation, winding up or liquidation, a successor Warrant Agent,
qualified as aforesaid, shall be appointed by the Company by an instrument in writing, filed with the successor Warrant Agent. Upon the appointment as aforesaid of a successor Warrant Agent and acceptance by the successor Warrant Agent of such
appointment, the Warrant Agent shall cease to be Warrant Agent hereunder. 
  
 (d) Any successor Warrant Agent appointed hereunder shall execute, acknowledge and deliver to its predecessor and to the Company an instrument accepting such appointment hereunder, and thereupon such successor Warrant
Agent, without any further act, deed or conveyance, shall become vested with all the authority, rights, powers, trusts, immunities, duties and obligations of such predecessor with like effect as if originally named as Warrant Agent hereunder, and
such predecessor, upon payment of its charges and disbursements then unpaid, shall thereupon become obligated to transfer, deliver and pay over, and such successor Warrant Agent shall be entitled to receive, all monies, securities and other property
on deposit with or held by such predecessor, as Warrant Agent hereunder. 
  
 (e) Any corporation into which the Warrant Agent hereunder may be merged or converted or any corporation with which the Warrant Agent may be consolidated, or any corporation resulting from any merger, conversion or
consolidation to which the Warrant Agent shall be a party, or any corporation to which the Warrant Agent shall sell or otherwise transfer all or substantially all the assets and business of the Warrant Agent, provided that it shall be qualified as
aforesaid, shall be 

  

 10 

 
the successor Warrant Agent under this Agreement without the execution or filing of any paper or any further act on the part of any of the parties hereto.

  
 ARTICLE VI 
 MISCELLANEOUS 
  
 SECTION 6.1 AMENDMENT. This Agreement may be amended by the parties hereto, without the consent of the holder of any Warrant Certificate, for the purpose
of curing any ambiguity, or of curing, correcting or supplementing any defective provision contained herein, or making any other provisions with respect to matters or questions arising under this Agreement as the Company and the Warrant Agent may
deem necessary or desirable; provided that such action shall not materially adversely affect the interests of the holders of the Warrant Certificates. 
  
 SECTION 6.2 NOTICES AND DEMANDS TO THE COMPANY AND WARRANT AGENT. If the Warrant Agent shall receive any notice or demand addressed to the Company by the
holder of a Warrant Certificate pursuant to the provisions of the Warrant Certificates, the Warrant Agent shall promptly forward such notice or demand to the Company. 
  
 SECTION 6.3 ADDRESSES. Any communication from the Company to the Warrant Agent with respect to this Agreement shall be
addressed to [                            ], Attention:
[                            ] and any communication from the Warrant Agent to the Company with respect to
this Agreement shall be addressed to Sirna Therapeutics, Inc., 185 Berry Street, Suite 6504, San Francisco, California 94107, Attention: General Counsel (or such other address as shall be specified in writing by the Warrant Agent or by the Company). 
  
 SECTION 6.4 GOVERNING LAW. This Agreement and each Warrant Certificate issued
hereunder shall be governed by and construed in accordance with the laws of the State of New York. 
  
 SECTION 6.5 DELIVERY OF PROSPECTUS. The Company shall furnish to the Warrant Agent sufficient copies of a prospectus meeting the requirements of the
Securities Act of 1933, as amended, relating to the Warrant Securities deliverable upon exercise of the Warrants (the “PROSPECTUS”), and the Warrant Agent agrees that upon the exercise of any Warrant, the Warrant Agent will deliver to the
holder of the Warrant Certificate evidencing such Warrant, prior to or concurrently with the delivery of the Warrant Securities issued upon such exercise, a Prospectus. 
  
 The Warrant Agent shall not, by reason of any such delivery, assume any responsibility for the accuracy or adequacy of such
Prospectus. 
  
 SECTION 6.6 OBTAINING OF GOVERNMENTAL APPROVALS.
The Company will from time to time take all action which may be necessary to obtain and keep effective any and all permits, consents and approvals of governmental agencies and authorities and securities act filings under United States Federal and
state laws (including without limitation a registration statement in respect of the Warrants and Warrant Securities under the Securities Act of 1933, as amended), which may be or become requisite in connection with the issuance, sale, transfer, and
delivery of the Warrant Securities issued upon exercise of the Warrants, the issuance, sale, transfer and delivery of the Warrants or upon the expiration of the period during which the Warrants are exercisable. 
  
 SECTION 6.7 PERSONS HAVING RIGHTS UNDER WARRANT AGREEMENT. Nothing in this
Agreement shall give to any person other than the Company, the Warrant Agent and the holders of the Warrant Certificates any right, remedy or claim under or by reason of this Agreement. 
  
 SECTION 6.8 HEADINGS. The descriptive headings of the several Articles and Sections of this Agreement are inserted for
convenience only and shall not control or affect the meaning or construction of any of the provisions hereof. 
  
 SECTION 6.9 COUNTERPARTS. This Agreement may be executed in any number of counterparts, each of which as so executed shall be deemed to be an original,
but such counterparts shall together constitute but one and the same instrument. 
  

 11 

 SECTION 6.10 INSPECTION OF AGREEMENT. A copy of this Agreement shall be available at all reasonable times
at the principal corporate trust office of the Warrant Agent for inspection by the holder of any Warrant Certificate. The Warrant Agent may require such holder to submit his Warrant Certificate for inspection by it. 
  
 IN WITNESS WHEREOF, the parties hereto have caused this Agreement to be duly
executed, all as of the day and year first above written. 
  
 Sirna Therapeutics, Inc. 
  
  
 By:
                                        
                                        
                
  
 Its:
                                        
                                        
                 
  
 Attest:
                                        
                                        
          
  
                                       
                                        
                          
 Warrant agent 
  
 By:
                                        
                                        
                
  
 Its:
                                        
                                        
                 
  

 12 

 EXHIBIT A 
  
 FORM OF WARRANT CERTIFICATE 
 [FACE OF WARRANT
CERTIFICATE] 
  
 [Form if Warrants are attached to Other
                 Prior to         , this Warrant Securities and are not immediately detachable. Certificate cannot be 

 
 transferred or exchanged 
 unless attached to a 
  
 [Title of Other 
 Securities].] 
  
 [Form of Legend if Warrants are not
                 Prior to         , Warrants
                             
  
 immediately exercisable.
                            evidenced by this Warrant 
  
 Certificate cannot be 
 exercised.] 
  
  
  
 EXERCISABLE ONLY IF COUNTERSIGNED BY THE WARRANT

 AGENT AS PROVIDED HEREIN 
  
 VOID AFTER [                    ] P.M., [CITY] TIME, ON
                            ,
                             
  
  
 SIRNA THERAPEUTICS, INC. 
  
 WARRANT CERTIFICATE REPRESENTING 
 WARRANTS TO PURCHASE 
 COMMON STOCK, PAR VALUE
$0.01 PER SHARE 
  
  

	 	

  
  
 No.
                                        
             Warrants 
  
 This certifies that [                        ] or registered assigns is the registered owner of
the above indicated number of Warrants, each Warrant entitling such owner [if Warrants are attached to Other Securities and are not immediately detachable—, subject to the registered owner qualifying as a “Holder” of this Warrant
Certificate, as hereinafter defined),] to purchase, at any time [after [            ] p.m., [City] time, on
[                            ] and] on or before
[            ] p.m., [City] time, on
                                 ,
             shares of Common Stock, par value $0.01 per share (the “WARRANT SECURITIES”), of Sirna Therapeutics, Inc. (the “COMPANY”) on the following basis: during the period
from [                        ], through and including
[                            ], the exercise price per Warrant Security will be
$[                ] , subject to adjustment as provided in the Warrant Agreement (as hereinafter defined) (the “WARRANT PRICE”). The Holder may exercise the
Warrants evidenced hereby by providing certain information set forth on the back hereof and by paying in full, in lawful money of the United States of America, [in cash or by certified check or official bank check in New York Clearing House funds]
[by bank wire transfer in immediately available funds], the Warrant Price for each Warrant Security with respect to which this Warrant is exercised to the Warrant Agent (as hereinafter defined) and by surrendering this Warrant Certificate, with the
purchase form on the back hereof duly executed, at the corporate trust office of [name of Warrant Agent], or its successor as warrant agent (the “WARRANT AGENT”), which is, on the date hereof, at the address specified on the reverse
hereof, and upon compliance with and subject to the conditions set forth herein and in the Warrant Agreement (as hereinafter defined). 
  
 The term “HOLDER” as used herein shall mean [if Warrants are attached to Other Securities and are not immediately detachable—prior to
[                            , 
             ] (the “DETACHABLE DATE”), the registered owner of the Company’s [title of Other Securities] to which this Warrant Certificate was
initially attached, and after such Detachable Date,] the person in whose name at the 

  

 13 

 
time this Warrant Certificate shall be registered upon the books to be maintained by the Warrant Agent for that purpose pursuant to Section 4 of the
Warrant Agreement. 
  
 The Warrants evidenced by this Warrant
Certificate may be exercised to purchase a whole number of Warrant Securities in registered form. Upon any exercise of fewer than all of the Warrants evidenced by this Warrant Certificate, there shall be issued to the Holder hereof a new Warrant
Certificate evidencing Warrants for the number of Warrant Securities remaining unexercised. 
  
 This Warrant Certificate is issued under and in accordance with the Warrant Agreement dated as of
[                             ,             ]
(the “WARRANT AGREEMENT”), between the Company and the Warrant Agent and is subject to the terms and provisions contained in the Warrant Agreement, to all of which terms and provisions the Holder of this Warrant Certificate consents by
acceptance hereof. Copies of the Warrant Agreement are on file at the above-mentioned office of the Warrant Agent. 
  
 [If Warrants are attached to Other Securities and are not immediately detachable—Prior to the Detachable Date, this Warrant Certificate may be
exchanged or transferred only together with the [Title of Other Securities] (the “OTHER SECURITIES”) to which this Warrant Certificate was initially attached, and only for the purpose of effecting or in conjunction with, an exchange or
transfer of such Other Security. Additionally, on or prior to the Detachable Date, each transfer of such Other Security on the register of the Other Securities shall operate also to transfer this Warrant Certificate. After such date, transfer of
this] [If Warrants are attached to Other Securities and are immediately detachable—Transfer of this] Warrant Certificate may be registered when this Warrant Certificate is surrendered at the corporate trust office of the Warrant Agent by the
registered owner or such owner’s assigns, in the manner and subject to the limitations provided in the Warrant Agreement. 
  
 [If Other Securities with Warrants which are not immediately detachable—Except as provided in the immediately preceding paragraph, after] [If Other
Securities with Warrants which are immediately detachable or Warrants alone—After] countersignature by the Warrant Agent and prior to the expiration of this Warrant Certificate, this Warrant Certificate may be exchanged at the corporate trust
office of the Warrant Agent for Warrant Certificates representing Warrants for the same aggregate number of Warrant Securities. 
  
 This Warrant Certificate shall not entitle the Holder hereof to any of the rights of a holder of the Warrant Securities, including, without limitation,
the right to receive payments of dividends or distributions, if any, on the Warrant Securities (except to the extent set forth in the Warrant Agreement) or to exercise any voting rights. 
  
 Reference is hereby made to the further provisions of this Warrant Certificate set forth on the reverse hereof, which
further provisions shall for all purposes have the same effect as if set forth at this place. 
  
 This Warrant Certificate shall not be valid or obligatory for any purpose until countersigned by the Warrant Agent. 
  
 IN WITNESS WHEREOF, the Company has caused this Warrant to be executed in its name and on its behalf by the facsimile signatures of its duly authorized
officers. 
  
 Sirna Therapeutics, Inc. 
  
 Dated:
                                        
                                        
          
  
 By:
                                        
                                        
                
  
 Its:
                                        
                                        
                 
  
 Attest:
                                        
                                        
          
  
 Countersigned: 
  
                                       
                                        
                          
  

 14 

 As warrant Agent 
  
 By
                                        
                                        
                 
 Warrant agent 
  

 15 

 [REVERSE OF WARRANT CERTIFICATE] 
  
 (Instructions for Exercise of Warrant) 
  
 To exercise any Warrants evidenced hereby for Warrant Securities (as hereinafter defined), the Holder must pay, in lawful
money of the United States of America, [in cash or by certified check or official bank check in New York Clearing House funds] [by bank wire transfer in immediately available funds], the Warrant Price in full for Warrants exercised, to [Warrant
Agent] [address of Warrant Agent], Attn: [                                ], which
payment must specify the name of the Holder and the number of Warrants exercised by such Holder. In addition, the Holder must complete the information required below and present this Warrant Certificate in person or by mail (certified or registered
mail is recommended) to the Warrant Agent at the appropriate address set forth above. This Warrant Certificate, completed and duly executed, must be received by the Warrant Agent within five business days of the payment. 
  
 (To be executed upon exercise of Warrants) 
  
 The undersigned hereby irrevocably elects to exercise
[                        ] Warrants, evidenced by this Warrant Certificate, to purchase
[                ] shares of the Common Stock, par value $0.01 per share (the “WARRANT SECURITIES”), of Sirna Therapeutics, Inc. and represents that he has tendered
payment for such Warrant Securities, in lawful money of the United States of America, [in cash or by certified check or official bank check in New York Clearing House funds] [by bank wire transfer in immediately available funds], to the order of
Sirna Therapeutics, Inc., c/o [insert name and address of Warrant Agent], in the amount of $[            ] in accordance with the terms hereof. The undersigned requests that said Warrant Securities be
in fully registered form in the authorized denominations, registered in such names and delivered all as specified in accordance with the instructions set forth below. 
  
 If the number of Warrants exercised is less than all of the Warrants evidenced hereby, the undersigned requests that a new
Warrant Certificate evidencing the Warrants for the number of Warrant Securities remaining unexercised be issued and delivered to the undersigned unless otherwise specified in the instructions below. 
  
 Dated:
                                        
                 Name
                                        
                                        
         
 (Please Print) 
  
  

					
	 	  	 Address:
                                        
                       
                                       
                                        
   
                                       
                                        
   
 (Insert Social Security or Other
 Indentifying Number of Holder)
	  	 
			
	 Signature Guaranteed
 Signature
	  	                                       
                                        
   
	  	 

  

 16 

					
	 	  	 (Signature must conform in
 all respects to name of
 holder as specified on the
 face of this Warrant
 Certificate and must bear a
 signature guarantee by a
 bank, trust company or
 member broker of the New
 York, Midwest or Pacific
 Stock Exchange)
	  	 
		
	 This Warrant may be exercised at the following addresses:
 By Hand at
                                        
                                        
                             
  
                                       
                                        
                                        
  
	  	 
		
	 By mail at
                                        
                                        
                              
  
                                       
                                        
                                        
             
	  	 

  
 [Instructions as to form and delivery
of Warrant Securities and, if applicable, Warrant Certificates evidencing Warrants for the number of Warrant Securities remaining unexercised—complete as appropriate.] 
  
  
 ASSIGNMENT 
  
 [Form of assignment to be executed if 
 Warrant Holder desires to transfer Warrant) 
  
 FOR VALUE RECEIVED,
                                hereby sells, assigns and transfers unto: 
  

					
	  
                                       
                                        
             
                                       
                                        
             
	 	  
                                       
                                        
             
                                       
                                        
             
	  	 
	 please print name and address
 including zip code)
	 	 Please insert Social Security or other
 Identifying
number
	  	 

  
 the right represented by the within
Warrant to purchase [                    ] shares of [Title of Warrant Securities] of Sirna Therapeutics, Inc. to which the within Warrant relates and appoints
[                    ] attorney to transfer such right on the books of the Warrant Agent with full power of substitution in the premises. 

 
 Dated:
                                        
                        
                                        
                                        
         
 Signature 
  
 (Signature must conform in all respects to 
 name of holder as specified on the face 
 of the Warrant) 
  
  
 Signature Guaranteed 
  
                                       
                                        
                          
  

 17

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