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                                                                   Exhibit 10.15

                               SUBLEASE AGREEMENT

This Sublease Agreement (this "Sublease,") dated September 28, 2000, for
reference purposes only, is entered into by and between CYTOKINETICS, INC., a
Delaware corporation ("Sublessor") and EXELIXIS, INC., a Delaware corporation
formerly known as Exelixis Pharmaceuticals, Inc. ("Sublessee").

                                    RECITALS

         A.       Britannia Pointe Grand Limited Partnership, a Delaware limited
partnership ("Master Lessor"), 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 11, 1999, copies of which are
attached hereto as Exhibit A (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 Sublessor, as "Sublessee", and MetaXen, as "Sublessor" (the
"MetaXen-Cytokinetics Sublease"), MetaXen leased to Sublessor 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 Sublessee (the "MetaXen-Exelixis Sublease"),
MetaXen leased to Sublessee 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 July 11, 1999 among Master Lessor, MetaXen, Sublessee
and Xenova Group, PLC (the "Assignment and Assumption Agreement"), MetaXen
assigned to Sublessee all of its right, title and interest in and to the Master
Lease and Sublessee assumed all of MetaXen's obligations under the Master Lease.
Concurrently, with its execution and delivery of the Assignment and Assumption
Agreement, by operation of law, Sublessee's estate as sublessee under the
MetaXen-Exelixis Sublease merged into its estate as sublessor under the
MetaXen-Exelixis Sublease thereby rendering the MetaXen-Exelixis Sublease null,
void and of no further force and effect.

         E.       Pursuant to that certain Agreement and Consent dated as of
July 20, 1999 by and among Master Lessor, Sublessor and Sublessee (the "Consent
Agreement"), Sublessee agreed to assign its interest as "Tenant" under the
Master Lease to Sublessor and Sublessor agreed to assume all obligations of
Sublessee as "Tenant" under the Master Lease on the terms and conditions
contained therein. Concurrently herewith, Sublessee, as "Assignor," and
Sublessor, as

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<PAGE>

"Assignee", have executed and delivered that certain Assignment and Assumption
of Lease (the "Assignment and Assumption") pursuant to which Sublessee has
assigned to Sublessor all of Sublessee's interest as "Tenant" under the Master
Lease, and Sublessor has assumed from Sublessee, subject to certain limitations
and exceptions set forth therein, all of Sublessee's obligations as "Tenant"
under the Master Lease on the terms and conditions contained therein. Sublessee
has timely exercised its option to sublease a portion of the Premises from
Sublessor, which portion is defined in Paragraph 1 of this Sublease as the
"Sublease Premises". Sublessor and Sublessee desire to enter into this Sublease
to set forth the terms and conditions pursuant to which Sublessor shall sublease
the Sublease Premises to Sublessee.

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

                                    AGREEMENT

         1.       Premises. Sublessor hereby leases to Sublessee, and Sublessee
hereby hires from Sublessor, on and subject to the terms and conditions
hereinafter set forth, the following premises (the "Sublease Premises"),
situated in the City of South San Francisco, County of San Mateo, State of
California, commonly known as a portion of Building H (the "Building") in the
Britannia Pointe Grand Business Park (the "Center") and consisting of the
following: (i) approximately eight hundred sixty-two (862) leasable square feet
labeled "Exelixis Space" on Exhibit B, attached hereto, and incorporated herein
by this reference (the "Ground Floor Leased Area"); (ii) approximately four
thousand three hundred forty-six (4,346) leasable square feet labeled "Shared
Space" on the above referenced Exhibit B on a non-exclusive basis (the "Ground
Floor Shared Area"); and (iii) approximately twenty-three thousand six hundred
eighty (23,680) leaseable square feet comprising all of the leaseable square
feet of the second floor of the Building as shown on Exhibit B-l, attached
hereto and incorporated herein by this reference (the "Second Floor Leased
Area"). The Sublease Premises include the laboratory benches, water systems,
fume hoods, laminar flow hoods and other laboratory support space as specified
in Exhibit B-2, attached hereto, and incorporated herein by this reference.
Sublessee shall have the right to use the Common Areas as described in the
Master Lease, subject to the rights of Master Lessor, Sublessor and other
tenants of the Center. The right of Sublessee to use the Ground Floor Shared
Area is non-exclusive and shall be subject to reasonable rules and requirements
of Sublessor. Sublessor and Sublessee shall each have equal access to the Ground
Floor Shared Area and shall devise a mutually satisfactory system of sharing the
Ground Floor Shared Area, but in the case of a conflict Sublessor shall have the
primary right to use the Ground Floor Shared Area. Sublessee acknowledges that
it has occupied the Sublease Premises pursuant to the terms of the
MetaXen-Exelixis Sublease and the Master Lease through and including the date
hereof.

         2.       Master Lease.

                  a.       Sublease is Subordinate to Master Lease. This
Sublease is subject and subordinate to the Master Lease. Sublessee shall not
commit or permit to be committed, on the Sublease Premises any act or omission
which shall violate any terms or condition of the Master

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<PAGE>

Lease. If the Master Lease terminates, this Sublease shall terminate. Sublessor
shall have no liability to Sublessee, if the Master Lease terminates without
fault of Sublessor. Sublessor hereby covenants and agrees that, without the
prior written consent of Sublessee, which consent shall not be unreasonably
withheld or delayed, Sublessor shall not (i) terminate the Master Lease, (ii)
amend or otherwise modify the terms of the Master Lease, unless such
modification would apply only to a matter that would take effect only after the
Expiration Date (or earlier termination of this Sublease), or (iii) willfully
breach the terms of the Sublease or the Master Lease. Sublessor hereby agrees to
perform its obligations as tenant under the Master Lease if and to the extent
those obligations are not assumed by Sublessee pursuant to the terms of this
Sublease.

                  b.       Application of Master Lease Provisions. Except as
otherwise expressly provided in this Sublease, Sublessee shall assume and
perform, with respect to the Sublease Premises, the obligations of the Sublessor
as Lessee under the Master Lease arising after the date of this Sublease.
Therefore, except as otherwise provided, for the purpose of this Sublease,
wherever in the Master Lease (as incorporated herein) "Landlord" is used, it
shall be deemed to mean the Sublessor herein, and wherever in the Master Lease
(as incorporated herein) "Tenant" is used, it shall be deemed to mean the
Sublessee herein, and wherever in the Master Lease (as incorporated herein)
"Lease" is used, it shall be deemed to mean this Sublease.

                  c.       Incorporation of Master Lease, Provisions.

                           (1)      All of the terms and conditions in the
Master Lease, as they relate to the Sublease Premises, are incorporated herein
except for: Section 1.1 (Premises); Section 1.2 (Landlord's Reserved Rights);
Sections 1.3 (First Refusal); 2.1 (Term); 2.2 (Early Possession); 2.3 (Delay in
Possession); 2.6 (Option to Extend); 3.1 (Minimum Rent); 4.1 (Stock Warrants);
6.2 (Real Property Taxes); 7 (Operating Expenses); 8.1 (Payment of Utilities);
Tenant's obligation for HVAC repair and entry into a service contract under
10.2(a); 12.1(b); 19.1 (Notices), 19.11 (Financial Information); 19.15
(Brokers); 19.16 (Memorandum of Lease), Articles 4 (Stock Warrants), 5
(Construction) and Article 18 (Security Deposit), the First Amendment to Lease
as it pertains to warrants, and Exhibit C.

                           (2)      Except as otherwise provided herein,
Sublessor is responsible for all financial obligations under the Master Lease.

                  d.       Indemnity. Except to the extent caused by Sublessor,
or Sublessor's agents', employees' or invitees' negligence or willful
misconduct, Sublessee, shall indemnify and hold Sublessor harmless against and
from all liability, judgments, costs, damages, claims or demands, including
reasonable attorney's fees, arising out of Sublessee's failure to comply with or
perform Sublessee's obligations under this Sublease, including, but not limited
to, Sublessee's obligation to immediately disclose any violations of the
warranty as set forth in section 2.f, below. Except to the extent caused by
Sublessee, or Sublessee's agents', employees' or invitees' negligence or willful
misconduct, Sublessor shall indemnify and hold Sublessee harmless against and
from all liability, judgments, costs, damages, claims or demands, including
reasonable attorney's fees, arising out of Sublessor's failure to comply with or
perform Sublessor's obligations under this Sublease.

                                        3

<PAGE>

                  e.       Master Lease in Effect. Sublessee represents to
Sublessor that the Master Lease is in full force and effect and that, to
Sublessee's knowledge, no default exists on the part of any party to the Master
Lease; furthermore Sublessee represents and warrants that it has not received
any notification from Master Lessor of any default under the Master Lease or
notice of any potential default under the Master Lease. Subject to the terms and
provisions of this Sublease, Sublessor agrees to keep the Master Lease in full
force and effect during the term of this Sublease, subject, however, to any
earlier termination of the Master Lease without the default of Sublessor.

                  f.       Warranty. Section 6.2 of the Master Lease creates a
warranty in favor of Sublessor. Sublessee shall provide Sublessor with written
notification of any violation of the warranty in Section 6.2, promptly following
the discovery by Sublessee of such violation.

                  g.       To the extent that any financial terms contained in
this Sublease that are applicable to Sublessor and Sublessee only conflict with
similar provisions in the Master Lease, this Sublease shall prevail.

         3.       Sublease Term; Delivery of Possession.

                  a.       Term. The term of this Sublease shall begin on the
date the Assignment and Assumption shall become effective ("Commencement Date")
and end on April 15, 2001.

                  b.       Delivery of Possession.

                           (1)      If Sublessee desires to make any
improvements performed in either the Ground Floor Leased Area or the Second
Floor Leased Area (Sublessee shall have no right to modify or otherwise alter
the Ground Floor Shared Area) and if Sublessee shall fail to deliver to
Sublessor a schedule of improvements, along with plans and specifications
thereto (including projected costs) reasonably acceptable to Sublessor on or
before sixty (60) days following the execution of this Sublease, Sublessor shall
be deemed to have disapproved such improvements and Sublessee shall not make any
such improvements until such plans and specifications shall have been delivered
to and approved by Sublessor.

                           (2)      Sublessor will deliver the Sublease Premises
subject to all applicable zoning, municipal, county and state laws, ordinances
and regulations governing and regulating the use of the Sublease Premises, and
Sublessee accepts the Sublease Premises subject thereto. Other than as set forth
herein, Sublessee shall accept the Sublease Premises "as is". Sublessor shall
assign its rights or, alternatively, shall itself enforce such rights pursuant
to any manufacturer or other warranty covering the Sublease Premises.

         4.       Rent. Sublessee shall pay to Sublessor no later than the first
day of each calendar month of the Term of this Sublease without deduction, set
off, prior notice or demand, as rent for the Sublease Premises, monthly rent
("Base Rent") as set forth in the rent schedule below, and subject to the terms
and provisions of section 3.2 of the Master Lease. Base Rent shall commence on
the date hereof (the "Rent Commencement Date"); provided, however, for the
period from the

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<PAGE>

date hereof through September 30, 2000, Sublessee shall pay Base Rent, prorated
for the number of days in such period, on the date hereof.

                                  Rent Schedule

<TABLE>
<CAPTION>
Leased Premises                 Base Rent/Month        Rent/Sq.ft./month    Approx. Sq. Feet
---------------                 ---------------        -----------------    ----------------
<S>                             <C>                    <C>                  <C>
Ground Floor Leased Area          $   3,361.80               $3.90                  862
Ground Floor Shared Area          $   8,474.70               $1.95                4,346
Second Floor Leased Area          $  92,352.00               $3.90               23,680
       Total                      $ 104,188.50
</TABLE>

Sublessor and Sublessee each hereby acknowledge and agree that the parties
hereto intend this Sublease to be on a "full-service" basis and Sublessee shall
only be responsible to pay. Base Rent and no other operating costs or service
expenses.

         5.       Security Deposit. Concurrently herewith, Sublessee shall
deposit One Hundred Four Thousand One Hundred Eighty-eight Dollars and Fifty
Cents ($104,188.50) with Sublessor as security for Sublessee's performance of
its obligations under the terms of this Sublease. Sublessor shall not be
required to keep any cash portion of the Security Deposit separate from its
general funds, or to pay any interest thereon. The Security Deposit shall be
held by Sublessor and may be applied by Sublessor in accordance with the
provisions of Article 18 of the Master Lease. If Sublessee defaults with respect
to any provision of this Sublease, including, without limitation, the provisions
relating to the payment of rental and other sums due hereunder, Sublessor shall
have the right, but shall not be required, to use, apply or retain all or any
part of the Security Deposit for the payment of rental or any other amount which
Sublessor may spend or become obligated to spend by reason of Sublessee's
default or to compensate Sublessor for any other loss or damage which Sublessor
may suffer by reason of Sublessee's default. If any portion of the Security
Deposit is so used or applied, Sublessee shall, within ten (10) days after
written demand therefor, deposit cash with Sublessor in an amount sufficient to
restore the Security Deposit to the full amount required hereunder and
Sublessee's failure to do so shall be a material breach of this Sublease. If
Sublessee fully and faithfully performs every provision of this Sublease to be
performed by it, the Security Deposit, or any balance thereof, shall be returned
to Sublessee or, at Sublessor's option, to the last assignee of Sublessee's
interest hereunder, at the expiration of the term of this Sublease and after
Sublessee has vacated the Premises. In the event of termination of Sublessor's
interest in this Sublease, Sublessor shall transfer all deposits then held by
Sublessor under this Section to Sublessor's successor in interest, whereupon
Sublessee agrees to release Sublessor from all liability for the return of such
deposit or the accounting thereof.

         6.       Use.

                  a.       Permitted Use. The Sublease Premises shall be used
and occupied only for office, biotechnology/pharmaceutical research and
development, manufacturing, warehousing related to such uses, and other
permitted uses under the Master Lease. Sublessee shall use and

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<PAGE>

occupy the Sublease Premises in accordance with the Master Lease, including, but
not limited to Section 11.6, thereof.

                  b.       No Representations or Warranties, Sublessee
acknowledges that neither Sublessor nor Sublessor's agents have made any
representation or warranty as to the suitability of the Sublease Premises for
the conduct of Sublessee's business.

         7.       Operating Expenses; HVAC Repair. At no time prior to the
Expiration Date is Sublessee required to pay as additional rent the amounts for
which Sublessor is liable to Master Lessor pursuant to Article 7 (Operating
Expenses) of the Master Lease. In addition, until the Expiration Date,
Sublessee, shall have no liability for any utilities it consumes in the
Premises. In addition, until the Expiration Date, Sublessee, shall have no
obligation to maintain or repair the HVAC equipment and related mechanical
systems. The parties hereto hereby acknowledge that this Sublease is on a "full
service" basis and Sublessee shall not be responsible for any operating costs or
expenses, insurance or taxes.

         8.       Alterations. Notwithstanding the provisions of Article 9 of
the Master Lease, any alteration, which requires Master Lessor's approval
pursuant to the Master Lease, shall not be commenced by Sublessee unless and
until such consent is obtained. Any such alteration shall be at Sublessee's sole
cost and expense. At the time Sublessor and Master Lessor consent to any
alteration, additions or improvements, Sublessor and Master Lessor shall inform
Sublessee in writing whether Sublessee is responsible for the removal of such
alterations and improvements at the expiration or earlier termination of the
term of this Sublease, provided that Sublessee, in its request for consent to
the alteration, addition, or improvement, has expressly requested that Sublessor
and Master Lessor specify the nature and extent of any such removal obligation.
If such notification is not made, Sublessee shall have no responsibility to
remove any such alteration or improvement at the expiration or earlier
termination of this Sublease. Any alteration made by Sublessee shall become
a part of the Sublease Premises, and at Sublessor's election (and to the extent
required, the consent of the Master Lessor), shall be surrendered to Sublessor
at the end of the Sublease term. Any alteration made by Sublessee shall, at
Sublessor's election, become Sublessor's property throughout the Sublease term
except for any specialized improvements installed by Sublessee (which
improvements shall be part of Sublessee's Equipment and Alterations, as defined
in Exhibit B-2), which improvements shall remain the property of Sublessee and
which improvements shall be removed by Sublessee at the expiration or earlier
termination hereof. In the event Sublessor is (or becomes) obligated under the
Master Lease to remove any of Sublessee's alterations, Sublessee shall be
obligated to remove same at Sublessee's sole cost and expense and to restore the
Sublease Premises to its condition prior to the alteration but only to the
extent required by Sublessor or Master Lessor in their written consent to any
such alteration. In the event that Sublessee removes any items it is permitted
to remove under Exhibit B-2, Sublessee, subject to the provisions of the second
sentence of this Section 8, at its sole cost and expense, shall restore the
Sublease Premises to its condition prior to alteration. Sublessee's obligation
to remove any alteration made to any portion of the Sublease Premises prior to
the Commencement Date as Tenant under the Master Lease shall be governed by the
terms of the Master Lease.

                                       6
<PAGE>

         9.       Repairs. Pursuant to the Master Lease, Master Lessor is
responsible to repair and maintain the roof (structural portions only), exterior
walls and other structural portions of the Building and the Common Areas. As to
such matters, subject to the terms of the following sentence, Sublessor's sole
obligation to Sublessee shall be to request performance of such obligations by
Master Lessor. In the event Master Lessor breaches its obligations, Sublessor
will assign to Sublessee its right to enforce such obligation and shall
otherwise cooperate with Sublessee in connection therewith, provided, however,
Sublessee, at its sole cost and expense, shall be responsible for enforcement
thereof without reimbursement from Sublessor; provided, further, that if the
nature of Sublessor's repair and maintenance obligation is such that both
Sublessor and Sublessee shall be benefited by the enforcement thereof, Sublessor
and Sublessee shall jointly participate in the enforcement thereof and Sublessor
and Sublessee shall share in the cost and expense incurred therewith in the same
proportion that their respective premises bears to the total premises.
Sublessee, not Master Lessor, shall be responsible for the repair of the roof
and structural portions of the Building to the extent the need for maintenance
or repair is caused by the gross negligence or willful misconduct of Sublessee,
in which case Sublessee shall pay to Sublessor the cost of (including reasonable
overhead expense of Sublessor) the maintenance and repairs caused by Sublessee
(except (i) to the extent the damage is covered by any insurance maintained by
Master Lessor or Sublessor, or, (ii) if Master Lessor fails to maintain the
insurance required to be maintained by Master Lessor pursuant to the terms of
the Master Lease, to the extent the damage would have been covered by insurance,
if Master Lessor had maintained the required insurance). There shall be no
abatement of Base Rent and no liability of Master Lessor or Sublessor by reason
of any injury to or interference with Sublessee's business arising from the
making of any repairs, alterations or improvements in or to the fixtures,
appurtenances and equipment therein, provided that Sublessor shall request
Master Lessor to use reasonable efforts to minimize the interruption of
Sublessee's use and occupancy of the Sublease Premises in connection with its
performance of the repairs and maintenance (although nothing contained herein
shall be deemed to obligate Master Lessor to pay any overtime costs in order to
minimize such interference, or otherwise to perform the repairs or maintenance
during hours other than normal business hours). As to all matters that neither
Master Lessor nor Sublessee is required to maintain or repair under the Master
Lease or this Sublease, as the case may be, Sublessor shall be responsible
therefor, and shall promptly and regularly maintain and repair the Sublease
Premises. Notwithstanding the foregoing, any damage caused by the negligence or
willful misconduct of Sublessee, shall promptly be repaired by Sublessee, at
Sublessee's own cost and expense, and in a manner reasonably acceptable to
Master Lessor and Sublessor.

         10.      Insurance.

         a.       Sublessee shall maintain commercial general liability
insurance coverage as required by section 12.1 (a) of the Master Lease which has
been incorporated into this Sublease by reference. Each policy of insurance,
which Sublessee is required to maintain pursuant to this Lease, shall name both
Sublessor and Master Lessor (as well as Master Lessor's general partners and
Managing Agent) as additional insureds (including cross-liability endorsements).
Sublessee's insurance coverage shall be primary and non-contributory as respects
any insurance maintained by Sublessor and/or Master Lessor. Sublessee shall
deliver evidence of the coverage

                                       7
<PAGE>
required hereunder (i) on the date hereof and (ii) within ten (10) days of the
renewal date for each policy of insurance required hereunder.

                  b.       Pursuant to the terms of the Master Lease as provided
in Section 12.1(b) thereof, Master Lessor is obligated to maintain certain
insurance coverage with respect to certain perils. Subject to the terms of the
following sentence, Sublessor's sole obligation to Sublessee with respect to
Master Lessor's obligations pursuant to said Section 12.1(b) shall be to request
performance of such obligations by Master Lessor. In the event Master Lessor
breaches its obligations, Sublessor will assign to Sublessee its right to
enforce such obligation and shall otherwise cooperate with Sublessee in
connection therewith, provided, however, Sublessee, at its sole cost and
expense, shall be responsible for enforcement thereof without reimbursement from
Sublessor; provided, further, that if the nature of Sublessor's insurance
obligation is such that both Sublessor and Sublessee shall be benefited by the
enforcement thereof, Sublessor and Sublessee shall jointly participate in the
enforcement thereof and Sublessor and Sublessee shall share in the cost and
expense incurred therewith in the same proportion that their respective premises
bears to the total premises. In the event Sublessor cannot assign such rights,
Sublessor shall diligently enforce its rights as Tenant under the Master Lease.

                  c.       Sublessor shall maintain insurance in the amounts and
of the types required pursuant to Section 12 of the Master Lease.

         11.      Damage or Destruction.

                  a.       Master Lessor Has Obligation to Restore. If the
Sublease Premises are damaged or destroyed, Master Lessor has the obligation
pursuant to Article 15 of the Master Lease to repair the Sublease Premises
unless Master Lessor has the right to terminate. If Master Lessor fails to
perform its obligations pursuant to Article 15 of the Master Lease, Sublessor's
sole obligation to Sublessee shall be to request performance of such obligations
by Master Lessor. In the event Master Lessor breaches its obligations, Sublessor
will assign to Sublessee, its right to enforce such obligation, provided,
however, Sublessee, at its sole cost and expense, shall be responsible for
enforcement thereof without reimbursement from Sublessor. In the event Sublessor
cannot assign its rights, Sublessor shall diligently enforce its rights under
the Master Lease.

                  b.       Termination of Master Lease. If the Master Lease
terminates pursuant to Article 15 of the Master Lease, this Sublease shall
terminate concurrently with the termination of the Master Lease.

                  c.       Sublessee Notice; Right to Terminate. Within twenty
(20) days following written request from Sublessor, Sublessee shall give notice
to Sublessor in writing whether Sublessee agrees to continue this Sublease in
effect if Master Lessor reasonably determines that the repair of the Sublease
Premises or the Building cannot be completed within three hundred sixty five
(365) days after the casualty. If Sublessee does not so agree to continue this
Sublease in effect then this Sublease shall terminate. If Sublessee agrees to
continue this Sublease in effect as aforesaid, then Sublessor shall have no
right to exercise its right (if any) to terminate the

                                       8
<PAGE>

Master Lease or this Sublease. If (i) Master Lessor reasonably determines that
the repair of the Sublease Premises or the Building cannot be-completed within
three hundred sixty five (365) days after the casualty, (ii) neither Master
Lessor nor Sublessor has elected to terminate the Master Lease, and (iii)
Sublessee agrees to continue this Sublease in effect notwithstanding the time to
reconstruct, then this Sublease shall continue in effect, and Sublessee shall
fulfill all of the obligations of Sublessor pursuant to the provisions of
Article 15 of the Master Lease, as it pertains to the Sublease Premises.

                  d.       Limited Obligation to Repair. Master Lessor's
obligation, should Master Lessor elect or be obligated to repair or rebuild,
shall be limited to the terms and conditions of the Master Lease. Master Lessor
shall have no obligation to replace or restore the Sublessee Equipment and
Alterations (as described in Exhibit B-2) or any other alterations installed by
Sublessor or Sublessee, unless specifically required by the Master Lease.

                  e.       Abatement of Rent. Rent under this Sublease shall
abate to the same extent as the Rent owing by Sublessor under the Master Lease
abates during any casualty repair period.

                  f.       Damage Near End of Term. In addition to the rights to
terminate specified in subsection 1l.c of this Sublease, either Sublessor or
Sublessee shall have the right to cancel and terminate this Sublease as of the
date of the occurrence of destruction or damage if the Sublease Premises or the
Building is substantially destroyed or damaged (i.e., there is damage or
destruction which Sublessor reasonably determines would require more than sixty
(60) days to repair) and made untenantable during the last twelve (12) months of
the term of the Master Lease. Sublessor or Sublessee, as applicable, shall give
written notice of its election to terminate this Sublease under this subsection
f. within thirty (30) days after Master Lessor or Sublessor determines that the
damage or destruction would require more than sixty (60) days to repair. If
either Master Lessor or Sublessor elects to terminate the Master Lease pursuant
to Article 15 of the Master Lease, this Sublease shall terminate concurrently
with the termination of the Master Lease. If neither Master Lessor nor Sublessor
terminates the Master Lease and if neither Sublessor nor Sublessee elects to
terminate this Sublease, the repair of the damage shall be governed by Article
15 of the Master Lease.

                  g.       Insurance Proceeds. If this Sublease is terminated,
Master Lessor and Sublessor may each keep all their respective insurance
proceeds resulting from the damage except for those proceeds, if any, which
specifically insured Sublessee's personal property and trade fixtures which
Sublessee has a right or obligation to remove upon the expiration of the
Sublease term. Sublessor shall be entitled to receive from Sublessee the
proceeds of insurance carried by Sublessee with respect to Sublessee
Improvements or other alterations installed in the Sublease Premises by
Sublessor or at Sublessor's expense. To the extent that Sublessee has paid for
any alterations regardless of whether the alterations may become the property of
Sublessor upon termination of this Sublease, Sublessee shall receive any portion
of the insurance proceeds payable with respect to the then unamortized cost
(based on a 2 year life of the alteration on a straight line amortization
schedule) for the applicable alterations, reduced by the amounts necessary to
pay off any equipment lease or other lien against the applicable alteration, and
the balance of the proceeds, if any, will be payable to Sublessor. With respect
to those Alterations,

                                       9
<PAGE>

which Sublessee is obligated to remove at the end of the Sublease term which are
the property of Sublessee, all proceeds of any insurance, carried by Sublessor
or Sublessee shall be paid to Sublessee.

                  h.       Uninsured Casualty. If the Master Lease terminates
pursuant to the provisions of Article 15 of the Master Lease, this Sublease
shall terminate.

         12.      Eminent Domain. If all or any part of the Sublease Premises is
taken for public or quasi-public use by a governmental authority under the power
of eminent domain or is conveyed to a governmental authority in lieu of such
taking, and if the taking or conveyance causes the remaining part of the
Sublease Premises to be untenantable and inadequate for use by Sublessee for the
purpose for which they were leased, then Sublessee, at its option and by giving
notice within fifteen (15) days after the taking, may terminate this Sublease as
of the date Sublessee is required to surrender possession of the Sublease
Premises. If a part of the Sublease Premises is taken or conveyed but the
remaining part is tenantable and adequate for Sublessee's use in Sublessee's
reasonable determination, then this Sublease shall be terminated as to the part
taken or conveyed as of the date Sublessee surrenders possession. All
compensation awarded for the taking or conveyance shall be the property of
Master Lessor and Sublessor, as their interests may appear, and Sublessee
hereby assigns to Sublessor all its right, title and interest in and to the
award, unless the governmental authority makes only one (1) award, and the award
contains compensation for the value of moving expenses, Sublessee's personal
property, trade fixtures and alterations (including the Sublessee Improvements),
Sublessee's Equipment and Alterations, in which case, subject to the rights of
any mortgagee or beneficiary of a deed of trust holding a lien on the Property
and to Master Lessor's rights under the Master Lease, Sublessee shall be
entitled to the compensation paid for Sublessee's moving expenses, trade
fixtures, personal property, Sublessee's Equipment and Alterations, and the
portion of the award attributable to the then unamortized cost of alterations
and improvements constructed at Sublessee's expense (which are to be amortized
on a straight line basis over the initial term of this Sublease). Sublessee
shall have the right, however, to recover from the governmental authority, but
not from Sublessor or Master Lessor, except as provided in the preceding
sentence, such compensation as may be awarded to Sublessee on account of the
interruption of Sublessee's business, moving and relocation expenses and removal
of Sublessee's trade fixtures and personal property.

         13.      Assignment and Subletting. Notwithstanding any provision of
this Sublease to the contrary, if Sublessor consents to a sublet, Sublessee
shall pay to Sublessor on a monthly basis as additional Rent, on the date Base
Rent is due, an amount equal to fifty percent (50%) of the amount by which the
rent payable to Sublessee ("Subrent") under the sublease exceeds the rent due
for the applicable portion of the Sublease Premises after deducting from the
Subrent (A) the actual out-of-pocket costs incurred by Sublessee for brokerage
commissions and tenant concessions (which concessions are not reflected in the
reduced Subrent) and (B) the costs of any additional improvements constructed by
Sublessee in connection with the sublease (amortized on a straight line basis
over the term of the sublease). Notwithstanding the foregoing, Sublessee may
assign this Sublease or sublet any portion of the Sublease Premises without
Sublessor's or Master Lessor's consent (but with prior or concurrent notice
thereof to Master Lessor and Sublessor) to any of the following (i) any
corporation which controls, is controlled by or under

                                       10
<PAGE>

common control with Sublessee; (ii) any corporation resulting from the merger or
consolidation of Sublessee; and (iii) any person or entity which acquires all of
the assets of Sublessee as a going concern (collectively, "Sublessee
Affiliate"), provided that such assignee assumes in full the obligations of
Sublessee under the Sublease. Any right of Sublessor or Master Lessor to
terminate the Sublease or the Master Lease in response to a requested assignment
or subletting shall not apply to an assignment of the Sublease or a subletting
of the Sublease Premises to a Sublessee Affiliate. Sublessee shall have the same
assignment and sublease rights and limitations as provided in section 13.1 of
the Master Lease.

         14.      Access to Premises. Master Lessor shall have the same right of
access to the Sublease Premises as Sublessor which right of access is described
in Section 14 of the Master Lease.

         15.      Surrender at End of Term. Upon expiration or termination of
this Sublease, Sublessee shall surrender the Sublease Premises to Sublessor in
good and sanitary order, except for any alterations Sublessee is not required to
remove, normal wear and tear, acts of God, damage, destruction (except to the
extent Sublessee is obligated to restore the same under this Sublease) and
eminent domain covered by the provisions of this Sublease. Sublessee shall
remove from the Sublease Premises all of Sublessee's personal property and trade
fixtures, Sublessee's Equipment and Alterations, and any alterations and
improvements Sublessee is required to remove pursuant to Sublessor's or Master
Lessor's written consent to such alterations and improvements, and shall repair
all damage caused by the removal. Except to the extent caused by Sublessor's or
Master Lessor's or their agents', employees' or invitees', negligence or willful
misconduct, Sublessee shall indemnify Sublessor against all loss or liability
resulting from delay by Sublessee in so surrendering the Sublease Premises,
including without limitation, any claims made by any succeeding tenant, losses
to Sublessor due to lost opportunities to lease to a succeeding tenant, and
reasonable attorneys' fees and costs.

         16.      Sublessor Indemnity re: Hazardous Materials.

                  a.       [INTENTIONALLY OMITTED.]

                  b.       Sublessee shall indemnify, defend and hold Sublessor
harmless from and against any claim, damage, loss, liability, cost or expense
(including reasonable attorneys' fees) arising out of any spill or release of
any Hazardous Substance (as defined in Section 11.6 of the Master Lease which
has been incorporated by reference into this Sublease) on or about the Sublease
Premises or any other portion of the Premises occupied by Sublessee pursuant to
the MetaXen-Exelixis Sublease or the Master Lease by Sublessee, its employees,
agents or contractors during the period of time Sublessee has occupied the
Sublease Premises or such other portion of the Premises. Sublessor shall
indemnify, defend and hold Sublessee harmless from and against any claim,
damage, loss, liability, cost or expense (including reasonable attorneys' fees)
arising out of any spill or release of any Hazardous Substance (as defined in
Section 11.6 of the Master Lease which has been incorporated by reference into
this Sublease) on or about the Sublease Premises by Sublessor, its employees,
agents or contractors during the period of time Sublessee occupies the Sublease
Premises.

                                       11
<PAGE>

                  c.       Sublessor is entitled to indemnification from the
Master Lessor under certain circumstances as provided in Section 11.6 of the
Master Lease. To the extent such indemnification may apply to the benefit of
Sublessee, Sublessor agrees to cooperate with Sublessee to enforce such
indemnity obligation against Master Lessor; provided, however, Sublessee shall
pay any and all costs incurred by Sublessor or Sublessee in connection with the
enforcement thereof for the benefit of Sublessee.

         17.      Signs. Master Lessor shall have the same approval rights with
respect to signs as Sublessor; Sublessor shall use its best efforts to obtain
Master Lessor's approval of signage rights reasonably satisfactory to Sublessee,
so long as such rights do not unreasonably interfere with the rights of
Sublessor under the Master Lease.

         18.      Holding Over. This Sublease shall terminate without further
notice at the expiration of the Sublease term. Any holding over by Sublessee
after expiration or sooner termination of this Sublease without the consent of
Sublessor shall be construed to be a tenancy at sufferance. Base Rent for the
Sublease Premises during any tenancy at sufferance, or if Sublessor shall have
consented to Sublessee's holding over, shall be at a rate equal to 150% of the
Base Rent for the last month of the term, and shall otherwise be on the terms
and conditions herein specified insofar as applicable.

         19.      Brokers. Sublessor and Sublessee each warrants and represents
for the benefit of the other that it has had no dealings with any real estate
broker or agent in connection with the negotiation of this Sublease, and that it
knows of no real estate broker or agent who is or might be entitled to a real
estate brokerage, commission or finder's fee in connection with this Sublease.
Sublessor and Sublessee warrant and represent that they have dealt with no real
estate broker in connection with this Sublease.

                  a.       Broker Disclaimer. Sublessor and Sublessee agree and
accept that, except as otherwise expressly stated herein, no broker or agent has
made or conducted any investigation, determination, warranty or representation
with respect to any of the following: (a) the legality of the present or any
possible future use of the Sublease Premises under any federal, state or local
law; (b) the physical condition or square footage of the Sublease Premises; (c)
the terms of the Master Lease or any other relevant legal document or agreement;
or (d) the presence or location of any hazardous materials on or about the
property in which the Sublease Premises are located (including, but not limited
to, asbestos, PCB's, other toxic, hazardous or contaminated substances, and
underground storage tanks).

                  b.       Acknowledgement. The parties acknowledge that they
are not relying on information from any real estate licensee relating to the
field of toxic materials, hazardous waste, underground tanks, and asbestos
contamination, location of the property within or outside a specific flood zone
or special studies seismic area, nor the property's compliance with the
guidelines as set forth in the Americans with Disabilities Act (ADA) regarding
the determination of the condition of the subject Premises, but rather from
their own independently initiated investigations.

                                       12
<PAGE>

         20.      Notices. All notices or demands of any kind required to be
given by Sublessor or Sublessee hereunder shall be in writing and shall be
deemed delivered forty-eight (48) hours after depositing the notice or demand in
the United States Mail, certified or registered, postage prepaid, or on the next
business day after delivering the same with a reputable overnight courier
service, in each case addressed to the Sublessor or Sublessee respectively at
the addresses set forth after their signatures at the end of this Sublease.
Either party may change its address by written notice to the other party in
accordance with this Section 20. All Base Rent shall be paid by Sublessee to
Sublessor at the same address.

         21.      Condition To Effectiveness of This Sublease. This Sublease is
contingent upon Sublessor obtaining the written consent of the Master Lessor to
this Sublease concurrently with the execution of this Sublease, pursuant to the
terms of the Consent Agreement. Sublessor and Sublessee acknowledge and agree,
that in granting such consent, notwithstanding any other provisions contained in
or implied in this Sublease, Master Lessor shall not be deemed or construed (a)
to have released Sublessor from any responsibility for the full and timely
performance of all obligations of Sublessor as Tenant under the Master Lease
accruing from and after the date hereof, nor (b) to have authorized Sublessor to
act on Master Lessor's behalf in exercising or waiving any rights, remedies or
privileges of Master Lessor as Landlord under the Master Lease as it pertains to
the Sublease Premises, nor (c) to have assumed, incurred or undertaken any
obligations or liabilities running directly to Sublessee with respect to the
Sublease Premises, it being the explicit intention and understanding of the
parties that notwithstanding the incorporation by reference of the Master Lease
into this Sublease (except as specifically excluded by Section 2 above, and as
otherwise specifically excluded in this Sublease), Master Lessor and Sublessor
shall look solely to one another for the performance of their respective
obligations with respect to the Sublease Premises as Landlord and Tenant under
the Master Lease (except to the extent that such obligation arose prior to the
date hereof pursuant to the Master Lease, as to which Master Lessor and
Sublessee shall look solely to one another for the performance of their
respective obligations with respect to the Sublease Premises as Landlord and
Tenant under the Master Lease), and that Sublessor and Sublessee shall look
solely to one another for the performance of their respective obligations with
respect to the Sublease Premises under this Sublease. Nothing in this Section 21
is intended, however, to preclude Sublessee from enforcing, by direct action
against Master Lessor, any rights of Sublessor under the Master Lease to the
extent such rights are expressly assigned by Sublessor to Sublessee pursuant to
this Sublease.

         22.      Authority. Each person executing this Sublease on behalf of a
party hereto represents and warrants that he or she is authorized and empowered
to do so and to thereby bind the party on whose behalf he or she is signing.

         23.      Attorneys Fees. In the event either party shall bring any
action or proceeding for damages or for an alleged breach of any provision of
this Sublease to recover rents, or to enforce, protect or establish any right or
remedy hereunder, the prevailing party shall be entitled to recover reasonable
attorneys' fees and court costs as part of such action or proceeding.

                                       13
<PAGE>

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

         25.      Entire Agreement. This written Sublease, together with the
exhibits hereto, contains all the representations and the entire understanding
between the parties hereto with respect to the subject matter hereof. Any prior
correspondence, memoranda or agreements are replaced in total by this Sublease
and the exhibits hereto. This Sublease may be modified only by an agreement in
writing signed by each of the parties.

                                       14
<PAGE>

         IN WITNESS WHEREOF, the undersigned have executed this Sublease as of
the dates set forth below.

SUBLESSOR:

CYTOKINETICS, INC
a Delaware corporation

By:   /s/ James Sabiy
      --------------------
Name: JAMES SABIY                                       Date September 28, 2000
Its:  CEO

By:   /s/ Robert I Blum
      --------------------
Name: ROBERT I BLUM                                      Date September 28, 2000
Its:  VP, BUSINESS DEVELOPMENT

280 East Grand Avenue
South San Francisco, CA 94080

SUBLESSEE:

EXELIXIS, INC.
a Delaware corporation
formerly known as Exelixis Pharmaceuticals, Inc.

By:   --------------------
Name: --------------------                              Date: September --, 2000
Its:  --------------------

By:   --------------------
Name: --------------------                              Date: September --, 2000
Its:  --------------------

170 Harbor Way
South San Francisco, CA 94083

<PAGE>

         IN WITNESS WHEREOF, the undersigned have executed this Sublease as of
the dates set forth below.

SUBLESSOR:

CYTOKINETICS, INC
a Delaware corporation

By:   --------------------
Name: --------------------                              Date: September --, 2000
Its:  --------------------

By:   --------------------
Name: --------------------                              Date: September --, 2000
Its:  --------------------

280 East Grand Avenue
South San Francisco, CA 94080

SUBLESSEE;

EXELIXIS, INC.
a Delaware corporation
formerly known as Exelixis Pharmaceuticals, Inc.

By:   /s/ Colen Sato
      --------------------
Name: COLEN SATO                                         Date September 28, 2000
Its:  CFO

By:   /s/ [ILLEGIBLE]
      --------------------
Name: [ILLEGIBLE]                                       Date September 28, 2000
Its:  [ILLEGIBLE]

170 Harbor Way
South San Francisco, CA 94083

<PAGE>

                            CONSENT BY MASTER LESSOR

         Master Lessor hereby consents to the sublease of the Sublease Premises
by Sublessor to Sublessee pursuant to the terms of this Sublease. Master Lessor
acknowledges and agrees that the obligation of Sublessor to deliver warrants to
Master Lessor is governed solely by the terms of Paragraph l(a) of the Consent
Agreement and that Sublessor shall have no additional obligation to deliver
additional warrants to Master Lessor whether pursuant to the Master Lease, the
First Amendment or otherwise. Master Lessor further acknowledges and agrees that
Sublessor has fully discharged its obligation to Master Lessor with respect to
such warrants prior to the date hereof and has fulfilled all other conditions
precedent to Master Lessor's consent to the assignment and assumption of the
Master Lease as set forth in Paragraph l(a) or elsewhere in the Consent
Agreement. Master Lessor represents and warrants to Sublessor that (i) Master
Lessor knows of no claims or defenses or circumstances, which with the passage
of time, would lead to claims or defenses by Master Lessor against Sublessor as
tenant under the Master Lease; (ii) this Sublease does not violate any provision
of the Master Lease; (iii) no provision of this Sublease is in violation of the
terms of the Master Lease; and (iv) all rent and other charges due under the
Master Lease have been paid through and including September 30, 2000.

MASTER LESSOR:

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
       Title: Manager

September 29, 2000

<PAGE>

                                    EXHIBIT A

[attach copies of master Lease and First and Second Amendments to Master Lease]