June 16, 2008

         
GlaxoSmithKline
        2301 Renaissance Boulevard

Building #510 RN0420
        King of Prussia, Pennsylvania 19406

Attn:
  Vinod Ramachandran, Ph.D.

Alliance Manager

Re: Third Extension of the Research Term with respect to CENP-E under that
certain Collaboration and License Agreement by and between Glaxo Group Limited,
a GlaxoSmithKline company, (“GSK”) and Cytokinetics, Inc. (“CK”), dated June 20,
2001, as amended (the “Collaboration Agreement”)

Dear Vinod:

Pursuant to this letter amendment to the Collaboration Agreement (the
“Amendment”), GSK and CK desire to further extend the Research Term solely with
respect to the Mitotic Kinesin Target known as CENP-E, and on the terms set
forth herein.

Now therefore, GSK and CK agree, effective as of June 19, 2008, as follows:

  1.   All capitalized terms not defined herein shall have the meaning ascribed
to them in the Collaboration Agreement.

  2.   Notwithstanding GSK’s obligation to notify CK in writing of its exercise
of its option to extend the Research Term under Section 2.8.1 of the
Collaboration Agreement, the Research Term is hereby extended for an additional
one-year period to expire on June 19, 2009 solely with respect to CENP-E to
allow for the conduct of Research Program activities directed to CENP-E, in
accordance with the terms and conditions of the Collaboration Agreement. GSK
shall have no further right to extend the Research Term, unless otherwise
expressly agreed in writing by the Parties.

  3.   GSK shall have no further right to extend its exclusivity rights under
Section 4.2 of the Collaboration Agreement with respect to CENP-E, unless
otherwise expressly agreed in writing by the Parties.

  4.   The Research Plan for the extended Research Term is attached as Exhibit A
hereto.

  5.   Notwithstanding Section 2.8.1 of the Collaboration Agreement, GSK has no
obligation to fund any CK FTEs during this extension of the Research Term for
CENP-E.

  6.   Each Party agrees to execute, acknowledge and deliver such further
instruments, and to do all such other acts, as may be necessary or appropriate
in order to carry out the purposes and intent of this Amendment.

  7.   Except as specifically modified or amended hereby, the Collaboration
Agreement shall remain in full force and effect and, as modified or amended, is
hereby ratified, confirmed and approved. No provision of this Amendment may be
modified or amended except expressly in a writing signed by both Parties, nor
shall any terms be waived except expressly in a writing signed by the Party
charged therewith. This Amendment shall be governed in accordance with the laws
of the State of New York, without regard to principles of conflicts of laws.

If the foregoing is acceptable, please have an authorized representative of GSK
countersign this Amendment where indicated below. Please return one fully
executed original to my attention.

Sincerely,

/s/ Michael S. Rabson
Michael S. Rabson, Ph.D.
Sr. Vice President, Business Development & Legal Affairs, and General Counsel

Cytokinetics, Inc.

Agreed and accepted:

GLAXO GROUP LIMITED

/s/ Paul Williamson
For and on behalf of
Edinburgh Pharmaceutical Industries Limited
Corporate Director

/s/ Paul Blackburn
For and on behalf of
Glaxo Group Limited

Corporate Director

18/6/08

     
cc:
  Adrian Rawcliffe, SVP WW Business Development, GlaxoSmithKline
Lisa A. DeMarco, Esq., VP & Associate General Counsel, GlaxoSmithKline,
R&D Legal Operations

1

Exhibit A

Research Plan

[****]

2

[***]

3