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                                                                   Exhibit 10.13

                       AMENDMENT TO AGREEMENT AND CONSENT

This Amendment to Agreement and Consent (this "Amendment") is entered into as of
July 31, 2000, by and among CYTOKINETICS, INC., a Delaware corporation
("Cytokinetics"), EXELIXIS, INC., a Delaware corporation formerly known as
Exelixis Pharmaceuticals, Inc. ("Exelixis") and BRITANNIA POINTE GRAND LIMITED
PARTNERSHIP, a Delaware limited partnership ("Britannia")

                                    RECITALS

         A.       Britannia, as "Landlord", and MetaXen, LLC, a Delaware limited
liability company ("MetaXen") as "Tenant", entered into that certain
Build-to-Suit Lease dated May 27, 1997 as amended by the First Amendment to
Lease (the "First Amendment"), dated as of April 13, 1998 and the Second
Amendment to Lease (the "Second Amendment"), dated as of July 20, 1999
(collectively, the "Master Lease") for the following described premises (the
"Premises"):

                  Approximately 50,195 square feet in Building H located at
                  Britannia Pointe Grand Business Park and commonly known as 280
                  East Grand Avenue, South San Francisco, California 94080.

         B.       Pursuant to that certain Sublease Agreement dated May 1, 1998
as amended by the First Amendment to Sublease Agreement dated July 20, 1999 by
and between Cytokinetics, as "Sublessee", and MetaXen, as "Sublessor" (the
"MetaXen-Cytokinetics Sublease"), MetaXen leased to Cytokinetics a portion of
the Premises consisting of approximately 13,750 leaseable square feet as more
particularly described in the MetaXen-Cytokinetics Sublease.

         C.       Pursuant to that certain Sublease Agreement dated as of March
1, 1999 by and between MetaXen and Exelixis (the "MetaXen-Exelixis Sublease"),
MetaXen leased to Exelixis a portion of the Premises as described more
particularly in the MetaXen-Exelixis Sublease.

         D.       Pursuant to that certain Assignment and Assumption Agreement
and Consent dated as of June 11, 1999 among Britannia, MetaXen, Exelixis and
Xenova Group, PLC (the "Assignment and Assumption Agreement"), MetaXen assigned
to Exelixis all of its right, title and interest in and to the Master Lease and
Exelixis assumed all of MetaXen's obligations under the Master Lease.

         E.       Pursuant to that certain Agreement and Consent dated as of
July 20, 1999 by and among Britannia, Cytokinetics and Exelixis (the "Consent
Agreement"), Exelixis agreed to assign its interest as "Tenant" under the Master
Lease to Cytokinetics and Cytokinetics agreed to assume all obligations of
Exelixis as "Tenant" under the Master Lease on the terms and conditions
contained therein. The Consent Agreement provides that Britannia's consent to
the assignment and assumption as described above, is subject to, among other
matters, receipt by Britannia of warrants to purchase Cytokinetics' common
stock; provided, however, that such right is subject to recission in the event
the assignment and assumption fails to occur by July 31, 2000. In addition, the
Consent Agreement provides for the occurrence of certain other events in the
event the assignment and assumption fails to occur by July 31, 2000.

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         F.       The parties hereto desire to amend the Consent Agreement to
extend the time frame for completing the assignment and assumption of the Master
Lease.

         NOW, THEREFORE, for good and valuable consideration, the receipt and
sufficiency of which are hereby acknowledged, the parties hereto hereby agree as
follows:

                                    AGREEMENT

         Section 1.        Amendment to Consent Agreement. The Consent
Agreement is hereby modified as follows:

                  1.1.     The last sentence of Paragraph l(a) of the Consent
Agreement is hereby amended to read in its entirety as follows:

                  Such warrants shall also contain a provision by which they
would be deemed rescinded in the event that Cytokinetics' assumption of the
MetaXen Lease shall have failed to occur by September 30, 2000.

                  1.2      The last sentence of Paragraph 3 of the Consent
Agreement is hereby amended to read in its entirety as follows:

                  In the event that the assignment and assumption of the MetaXen
lease to and by Cytokinetics shall have failed to occur by September 30, 2000
for any reason other than a default by Cytokinetics in any of its obligations
hereunder, Exelixis shall refund all amounts paid by Cytokinetics pursuant to
this Section 3.

                  1.3.     The last sentence of Paragraph 15 of the Consent
Agreement is hereby amended to read in its entirety as follows:

                  Landlord shall hold the signed Memorandum of Termination of
Agreement and is authorized to record it if either (i) this Agreement is
terminated prior to the assignment and assumption of the MetaXen Lease to and by
Cytokinetics becoming effective, or (ii) the assignment and assumption of the
MetaXen Lease to and by Cytokinetics fails to become effective by September 30,
2000.

         Section 2.        Ratification of Consent Agreement. Except as
specifically amended hereby, all of the provisions of the Consent Agreement
shall remain unamended and in full force and effect.

         Section 3.        Governing Law. This Amendment shall be governed by
and construed in accordance with the laws of the State of California.

         Section 4.        Severability. If any term, provision, covenant or
condition of this Amendment or any application thereof should be held by a court
of competent jurisdiction to be

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invalid, void or unenforceable, all terms, provisions, covenants and conditions
hereof and all applications thereof not held invalid, void or unenforceable
shall continue in full force and effect and shall in no way be affected,
impaired or invalidated thereby.

         Section 5.        Successor and Assigns. The provisions of this
Amendment shall be binding upon and inure solely to the benefit of the parties
hereto, and their respective heirs, legal representatives, successors and
assigns.

         Section 6.        Counterparts. This Amendment may be executed in any
number of counterparts each of which shall be an original but all of which
together shall constitute one agreement.

         Section 7.        Incorporation of Recitals. The recitals set forth
above are incorporated herein and made a part hereof.

         IN WITNESS WHEREOF, the undersigned have executed this Amendment as of
the date set forth above.

CYTOKINETICS:

CYTOKINETICS, INC
a Delaware corporation

By: /s/ James Sabry
   -------------------------
Name: James Sabry
Its: CEO

By: /s/ Robert Blum
   -------------------------
Name: Robert Blum
Its: V.P., Business Development

EXELIXIS:

EXELIXIS, INC.
a Delaware corporation

By: /s/ George A. Scangos                    7-31-00
   ---------------------------------
Name: George A. Scangos, Ph.D.
Its: President and CEO

By: /s/ Glen Y. Sato                         7-31-00
   -----------------------------
Name: Glen Y. Sato
Its: CFO and VP, Legal Affairs

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BRITANNIA:

BRITANNIA POINTE GRAND LIMITED
PARTNERSHIP, a Delaware limited partnership

        By:  Britannia Pointe Grand, LLC, a California
             limited liability company, its General partner

             By: /s/ T. J. Bristow
                -------------------------
             Name: T. J. Bristow
             Its:  Manager

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