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                                                                    Exhibit 4.12

         THIS SECURITY HAS NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933,
AS AMENDED (THE "ACT") AND MAY NOT BE OFFERED. SOLD OR OTHERWISE TRANSFERRED,
PLEDGED OR HYPOTHECATED UNLESS AND UNTIL REGISTERED UNDER THE ACT OR AN
EXEMPTION FROM THE REGISTRATION REQUIREMENTS OF THE ACT IS AVAILABLE FOR SUCH
OFFER, SALE, OR TRANSFER, PLEDGE OR HYPOTHECATION IN THE OPINION OF LEGAL
COUNSEL REASONABLY SATISFACTORY TO THE COMPANY.

                                     WARRANT

                      To Purchase Shares of Common Stock of
                               CYTOKINETICS, INC.

         THIS CERTIFIES that, for value received Laurence S. Shuhsan and
Magdalena Shushan, Trustees of the Laurence and Magdalena Shushan Family Trust,
under agreement dated October 8, 1997 (the "Holder"), is entitled, upon the
terms and subject to the conditions hereinafter set forth, at any time after the
date hereof and prior to the Commencement Date (as defined below), to subscribe
for and purchase from Cytokinetics, Inc., a Delaware corporation (the
"Company"), 4,000 shares of the Company's Common Stock at an exercise price
("Exercise Price") of $0.29 per share, subject to adjustment as set forth below.

         1. Title of Warrant. Prior to the expiration hereof and subject to
compliance with applicable laws, this Warrant and all rights hereunder are
transferable, in whole or in part, at the office or agency of the Company,
referred to in Section 2 hereof, by the holder hereof in person or by duly
authorized attorney, upon surrender of this Warrant together with the Assignment
Form annexed hereto properly endorsed.

         2. Exercise of Warrant. The purchase rights represented by this Warrant
are exercisable by the registered holder hereof, in whole or in part, at any
time after the effective date of the assignment and assumption to and by the
Company of that certain Lease, dated April 13, 1998 (as amended) by and between
MetaXen, LLC and Britannia Pointe Grand Limited Partnership ("BPGLP") and prior
to the date that is five (5) years after the closing date of-an underwritten
initial public offering ("IPO") of the Company's Common Stock pursuant to a
registration statement filed with the Securities and Exchange Commission (the
"SEC") under the Act, subject to adjustment as hereinafter provided, by the
surrender of this Warrant and the Notice of Exercise Form annexed hereto duly
executed at the office of the Company, in South San Francisco, California (or
such other office or agency of the Company as it may designate by notice in
writing to the registered holder hereof at the address of such holder appearing
on the books of the Company), and upon payment of the Exercise Price for the
shares thereby purchased (i) by cash or check or bank draft payable to the order
of the Company, (ii) by cancellation of indebtedness of the Company payable to
the holder hereof at the time of exercise, or (iii) by delivery of an election
in writing to receive a number of shares of Common Stock equal to the aggregate
number of shares of Common Stock subject to this Warrant (or the portion thereof
being cancelled upon such exercise), less that number of shares of Common Stock
having a

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fair market value as of such date equal to the aggregate Exercise Price of the
Warrant (or such portion thereof); whereupon the holder of this Warrant shall be
entitled to receive a certificate for the number of shares so purchased. The
Company agrees that if at the time of the surrender of this Warrant and purchase
the holder hereof shall be entitled to exercise this Warrant, the shares so
purchased shall be and be deemed to be issued to such holder as the record owner
of such shares as of the close of business on the date on which this Warrant
shall have been exercised as aforesaid. For purposes of clause (iii) of the
preceding sentence, the fair market value of one share of the Company's Common
Stock shall be determined as follows: (1) if the Company's Common Stock is
listed on a national stock exchange, on the NASDAQ National Market System or on
any other over-the-counter market, then such fair market value shall be the
closing price per share reported for such class on such national stock exchange
or on the NASDAQ National Market System, or the average of the final "bid" and
"asked" prices reported on such over-the-counter market, as applicable, at the
close of business on the date of calculation, as reported in the Wall Street
Journal; and (2) if the Company's Common Stock is not listed on any national
stock exchange, on the NASDAQ National Market System or on any other
over-the-counter market, then the Board of Directors of the Company shall
determine such fair market value as of the date of calculation in its reasonable
good faith judgment, and shall (upon written request by the holder hereof)
advise the holder hereof of such determination prior to any decision by such
holder to exercise its purchase rights under this Warrant.

         Certificates for shares purchased hereunder shall be delivered to the
holder hereof within a reasonable time, but not later than ten (10) days, after
the date on which this Warrant shall have been exercised as aforesaid.

         If this Warrant is exercised with respect to less than all of the
shares covered hereby, the holder hereof shall be entitled to receive a new
Warrant, in this form, covering the number of shares with respect to which this
Warrant shall not have been exercised.

         The Company covenants that all shares of stock which may be issued upon
the exercise of rights represented by this Warrant will, upon exercise of the
rights represented by this Warrant, be duly authorized, validly issued, fully
paid and nonassessable and free from all taxes, liens and charges in respect of
the issue thereof (other than taxes in respect of any transfer occurring
contemporaneously with such issue).

         3. No Fractional Shares or Scrip. No fractional shares or scrip
representing fractional shares shall be issued upon the exercise of this
Warrant.

         4. Charges, Taxes and Expenses. Issuance of certificates for shares of
Common Stock upon the exercise of this Warrant shall be made without charge to
the holder hereof for any issue or transfer tax or other incidental expense in
respect of the issuance of such certificate, all of which taxes and expenses
shall be paid by the Company, and such certificates shall be issued in the name
of the holder of this Warrant or in such name or names as may be directed by the
holder of this Warrant; provided, however, that in the event certificates for
shares of Common Stock are to be issued in a name other than the name of the
holder of this Warrant, this Warrant when surrendered for exercise shall be
accompanied by the Assignment Form attached hereto duly executed by the holder
hereof; and provided further, that upon any transfer involved in the issuance or
delivery of any

                                      -2-
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certificates for shares of Common Stock, the Company may require, as a condition
thereto, the payment of a sum sufficient to reimburse it for any transfer tax
incidental thereto.

         5. No Rights as Shareholders. This Warrant does not entitle the holder
hereof to any voting rights or other rights as a shareholder of the Company
prior to the exercise hereof. Notwithstanding the foregoing, the Company shall,
upon written request by the holder hereof to the chief financial officer of the
Company from time to time (but not more often than twice in any 12-month period)
provide to such holder copies of the following documents within a reasonable
time after such request (but in all events only to the extent that, and no
sooner than the time that, such documents have been made available to the
Company's shareholders): (i) the Company's most recent audited annual financial
statements or, if audited statements are not available, then the Company's
unaudited annual financial statements as of the end of the Company's most
recently ended fiscal year and (ii) unaudited quarterly financial statements for
each quarter of the Company's fiscal year since the date of the annual financial
statements delivered pursuant to clause (i) above. Notwithstanding the preceding
sentence, during any period in which the Company has outstanding a class of
publicly-traded securities or is for any other reason reporting company under
the Securities Exchange Act of 1934, it shall be sufficient compliance with any
information request from the holder hereof pursuant to such sentence for the
Company to provide copies of its most recent Form 10-K and annual report, any
Form 10-Qs and/or Form 8-Ks filed by the Company with the SEC since the date of
such Form 10-K, and any proxy statements.

         6. Exchange and Registry of Warrant. This Warrant is exchangeable, upon
the surrender hereof by the registered holder at the above-mentioned office or
agency of the Company, for a new Warrant of like tenor and dated as of such
exchange.

         The Company shall maintain at the above-mentioned office or agency a
registry showing the name and address of the registered holder of this Warrant.
This Warrant may be surrendered for exchange, transfer or exercise, in
accordance with its terms, at such office or agency of the Company, and the
Company shall be entitled to rely in all respects, prior to written notice to
the contrary, upon such registry.

         7. Loss, Theft, Destruction or Mutilation of Warrant. Upon receipt by
the Company of evidence reasonably satisfactory to it (such as an affidavit of
the holder hereof) of the loss, theft, destruction or mutilation of this
Warrant, and in case of loss, theft or destruction, of indemnity or security
reasonably satisfactory to it, and upon reimbursement to the Company of all
reasonable expenses incidental thereto, and upon surrender and cancellation of
this Warrant, if mutilated, the Company will make and deliver a new Warrant of
like tenor and dated as of such cancellation, in lieu of this Warrant.

         8. Saturdays, Sundays, Holidays, etc. If the last or appointed day for
the taking of any action or the expiration of any right required or granted
herein shall be a Saturday or a Sunday or shall be a legal holiday, then such
action may be taken or such right may be exercised on the next succeeding day
not a legal holiday.

                                      -3-

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         9. Adjustment. The number of shares for which this Warrant is
exercisable and the time period for exercise are subject to adjustment from time
to time as follows:

                  (a) Rectification, etc. If the Company at any time shall, by
subdivision, combination or reclassification of securities or otherwise, change
any of the securities to which purchase rights under this Warrant exist into the
same or a different number of securities of any class or classes, this Warrant
shall thereafter be to acquire such number and kind of securities as would have
been issuable as the result of such change with respect to the securities which
were subject to the purchase rights under this Warrant immediately prior to such
subdivision, combination, reclassification or other change and the Exercise
Price shall be proportionately adjusted.

                  (b) Cash Distributions. No adjustment on account of cash
dividends or interest on the Company's Common Stock or other securities
purchasable hereunder will be made to the Exercise Price.

                  (c) This warrant shall be deemed rescinded in the event that
the Company's assumption of that certain Lease, dated April 13, 1998 (as
amended) by and between BPGLP and MetaXen, LLC shall not have occurred by
September 30, 2000.

         10. Miscellaneous.

                  (a) Termination Upon Merger, Sale of Assets, etc. If at any
time after the date hereof the Company proposes to merge with or into any other
corporation, effect a consolidation or reorganization with or into any other
entity, or sell or convey all or substantially all of its assets to any other
entity (collectively, a "Merger"), the Company shall give the Holder written
notice ("Merger Notice") of such impending transaction not later than thirty
(30) days prior to the closing of such transaction. The Merger Notice shall
describe the material terms and conditions of the impending transaction,
including the aggregate value of consideration to be received by the Holder for
the shares underlying this Warrant on an as exercised basis, and the Company
shall thereafter give the Holder prompt notice of any material changes to such
terms and conditions.

                           (i)      If, pursuant to such Merger, the
shareholders of the Company receive solely cash and/or publicly traded
securities in exchange for their shares of stock in the Company, as stated in
the Merger Notice, and this Warrant has not been exercised prior to the closing
of such transaction, this Warrant shall terminate.

                           (ii)     Notwithstanding anything to the contrary,
if, pursuant to such Merger, the shareholders of the Company receive
non-publicly traded securities in exchange for their shares of stock in the
Company, or if the aggregate value of the consideration consisting of cash
and/or publicly traded securities to be received by the Holder for the
securities underlying this Warrant, as stated in the Merger Notice, does not
equal or exceed the aggregate Exercise Price of such underlying securities, then
this Warrant shall not terminate pursuant to the provisions of Section 10(a)(i)
above, and the Company shall, as a condition precedent to such transaction,
cause effective provisions to be made so that the holder hereof shall have the
right thereafter, by exercising this Warrant (in lieu of the shares of the
common stock of the Company immediately theretofore

                                      -4-

<PAGE>

purchasable and receivable upon exercise of this Warrant) to purchase the kind
and amount of shares of stock and other securities and property (including cash)
receivable upon such transaction. Any such provision shall include provisions
for adjustments in respect of such shares of stock and other securities and
property that shall be as nearly equivalent as may be practicable to the
adjustments provided for in this Warrant. The foregoing provisions of this
Section 10(a)(ii) shall similarly apply to successive transactions, unless this
Warrant is first terminated pusuant to the provisions of Section 10(a)(i) above.

                  (b) Issue Date. The provisions of this Warrant shall be
construed and shall be given effect in all respect as if it had been issued and
delivered by the Company on the date hereof. This Warrant shall be binding upon
any successors or assigns of the Company. This Warrant shall constitute a
contract under the laws of the State of California and for all purposes shall be
construed in accordance with and governed by the laws of said state.

                  (c) Restrictions. The holder hereof acknowledges that the
Common Stock acquired upon the exercise of this Warrant shall have restrictions
upon its resale imposed by state and federal securities laws.

                  (d) Authorized Shares. The Company covenants that during the
period the Warrant is exercisable, it will reserve from its authorized and
Unicode Common Stock a sufficient number of shares to provide for the issuance
of Common Stock upon the exercise of any purchase rights under this Warrant. The
Company further covenants that its issuance of this Warrant shall constitute
full authority to its officers who are charged with the duty of executing stock
certificates to execute and issue the necessary certificates for shares of the
Company's Common Stock upon the exercise of the purchase rights under this
Warrant.

                  (e) No Impairment. The Company will not, by amendment of its
Articles of Incorporation or any other voluntary action, avoid or seek to avoid
the observance or performance of any of the terms of this Warrant, but will at
all times in good faith assist in the carrying out of all such terms and in the
taking of all such action as may be necessary or appropriate in order to protect
the rights of the holder hereof against impairment.

                  (f) Notices of Record Date. In case:

                           (i)      the Company shall take a record of the
holders of its Common Stock for the purposes of entitling them to receive any
dividend (other than a cash dividend) or other distribution, or any right to
subscribe for, purchase or otherwise acquire any shares or stock of any class or
any other securities or property, or to receive any other right; or

                           (ii)     of any capital reorganization of the
Company, any reclassification of the capital stock of the Company, any
consolidation or merger of the Company with or into another corporation, or any
conveyance of all or substantially all of the assets of the Company to another
corporation; or

                                      -5-

<PAGE>

                           (iii)    of the voluntary or involuntary dissolution,
liquidation or winding-up of the Company;

         then, and in each such case, the Company will mail or cause to be
mailed to the holder of this Warrant a notice specifying, as the case may be,
(i) the date on which a record is to be taken for the purpose of such dividend,
distribution or right, and stating the amount and character of such dividend,
distribution or right, or (ii) the date on which such reorganization,
reclassification, consolidation, merger, conveyance, dissolution, liquidation or
winding-up is to take place, and the time, if any is to be fixed, as of which
the holders of record of Common Stock shall be entitled to exchange their shares
of Common Stock for securities or other property deliverable upon such
reorganization, reclassification, consolidation, merger, conveyance,
dissolution, liquidation or winding-up. Such notice shall be mailed at least
thirty (30) days prior to the date therein specified.

                  (g) Attorneys' Fees. In any litigation, arbitration or other
legal proceeding between the Company and the holder hereto relating to or
arising out of this Warrant, the prevailing party shall be entitled to recover
all its fees, costs and expenses incurred in connection with such proceeding,
including (but not limited to) reasonable fees and expenses of attorneys and
accountants and including (but not limited to) all such fees, costs and expenses
incurred in connection with any appeals and/or in connection with the
enforcement of any judgment or award rendered in such proceeding.

                                      -6-

<PAGE>

         IN WITNESS WHEREOF, Cytokinetics, Inc. has caused this Warrant to be
executed by its officers thereunto duly authorized.

Dated: July 20, 1999

CYTOKINETICS, INC.

By: /s/ James Sabry
    -----------------------

Title: CEO & President

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<PAGE>

                               NOTICE OF EXERCISE

         To: CYTOKINETICS, INC.

         (1) ___________, the undersigned, hereby elects to purchase ___________
shares of Common Stock (the "Shares") at an exercise price of $0.29 per share of
Cytokinetics, Inc. pursuant to the terms of the attached Warrant, and tenders
herewith payment of the aggregate purchase price of $______ in full (if
exercising pursuant to net exercise provisions, enter $-0-), together with all
applicable transfer taxes, if any:

                  (choose one)

                  [ ]      By cash, check or sale draft payable to Cytokinetics,
                           Inc.; or

                  [ ]      By cancellation of indebtedness of Cytokinetics,
                           Inc., payable to the undersigned as of the date
                           hereof; or

                  [ ]      By net exercise pursuant to the provisions of Section
                           2(iii) of the attached warrant (no tender of payment
                           for the Shares needed).

         (2) Please issue a certificate or certificates representing the Shares
(or the number of shares of Common Stock remaining after application of the net
exercise provisions of Section 2 (iii) of the attached warrant) in the name of
the undersigned or in such other name as is specified below:

         ___________________________
         (Name)

         ___________________________

         ___________________________
          (Address)

         (3) The undersigned represents that the aforesaid shares of Common
Stock are being acquired for the account of the undersigned for investment and
not with a view to, or for resale in connection with, the distribution thereof
and that the undersigned has no present intention of distributing or reselling
such shares.

         ________________________________________
         (Date) (Signature)

                                      -8-

<PAGE>

                                 ASSIGNMENT FORM

(To assign the foregoing warrant, execute this form and supply required
information. Do not use this form to purchase shares.)

         FOR VALUE RECEIVED, the foregoing Warrant and all rights evidenced
thereby are hereby assigned to

         _________________________________________
         (Please Print)

         whose address is  _______________________

                           _______________________
                           (Please Print)

         Dated: _____________________, ______.

         Holder's Signature: _____________________

         Holder's Address: _______________________

                           _______________________

         NOTE: The signature to this Assignment Form must correspond with the
name as it appears on the face of the Warrant, without alteration or enlargement
or any change whatever, and must be guaranteed by a bank or trust company.
Officers of corporations and those acting in a fiduciary or other representative
capacity should file proper evidence of authority to assign the foregoing
Warrant.

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