<PAGE>
                                                                   Exhibit 10.20

                                    SUBLEASE

     THIS SUBLEASE (this "Sublease") is dated for reference purposes as of
February 1, 1999, and is made by and between Sequana Therapeutics, Inc. (dba
Axys Pharmaceuticals, Inc.) a California corporation ("Sublessor"), and PPGx,
Inc., a Delaware corporation ("Sublessee"). Sublessor and Sublessee hereby
agree as follows:

     1.   RECITALS: This Sublease is made with reference to the fact that
ARE-11099 North Torrey Pines, LLC, a Delaware Limited Liability Company,
successor in interest to Alexandria Real Estate Equities, Inc., formerly
known as Health Science Properties, Inc., a Maryland corporation as landlord
("Master Lessor"), and Sublessor, as tenant, entered into that certain lease,
dated as of November 20, 1995, as amended by a letter agreement dated
November 20, 1995, a First Amendment to Expansion Lease dated October 1996, a
Second Amendment to Expansion Lease dated May, 1997, and a Third Amendment to
Expansion Lease dated August 24, 1998 (the "Master Lease"), with respect to
premises consisting of approximately 60,056 rentable square feet of space
located at 11099 North Torrey Pines Road, La Jolla, California (the "Demised
Premises"). A copy of the Master Lease is attached hereto as EXHIBIT A.

     2.   PREMISES: Sublessor hereby subleases to Sublessee and Sublessee
hereby subleases from Sublessor, a portion of the Demised Premises consisting
of approximately 4,500 rentable square feet known as Suite 100 (hereinafter,
the "Subleased Premises"). The Subleased Premises are more particularly
described on EXHIBIT B attached hereto.

     3.   TERM: The term (the "Term") of this Sublease shall commence on the
date hereof ("Commencement Date"), and shall terminate on the earliest to
occur of (a) the date which is five (5) days prior to the expiration date of
the Master Lease as provided in Section 2.1.7 (b) of the Master Lease (the
"Expiration Date"), or (b) the date this Sublease is terminated pursuant to
its terms, or (c) the date the Master Lease is terminated pursuant to its
terms.

     4.   RENT:

          A.   BASE RENT. Sublessee shall pay to Sublessor as the base rent
for the Subleased Premises for each month during the Term the amount of Eight
Thousand Six Hundred Twenty One and 55/100 Dollars ($8,621.55) per month
("Base Rent"), subject to the rental adjustment described below. Base Rent
under this Sublease shall be adjusted on January 1 of each year to equal the
Base Rent then payable by Sublessor under the Master Lease with respect to
the Subleased Premises pursuant to Section 2.1.5, as adjusted by Section 6 of
the Master Lease. Notwithstanding the foregoing, in the event Master Lessor
refunds or abates any of the Rent (as defined in Section 4(B) below) with
respect to the Subleased Premises under the Master Lease, Sublessor shall
refund or abate Rent under this Sublease to the extent so refunded or abated
by Master Lessor.

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Base Rent and Additional Rent as defined in Paragraph 4.B below, shall be paid
on or before the first (1st day) of each month. Base Rent and Additional Rent
for any period during the Term hereof which is for less than one (1) month of
the Term shall be a pro rata portion of the monthly installment based on the
actual number of days in such month. Base Rent and Additional Rent shall be
payable without notice or demand and without any deduction, offset, or
abatement, in lawful money of the United States of America. Base Rent and
Additional Rent shall be paid directly to Sublessor at 180 Kimball Way, South
San Francisco, California 94080, Attention: Chief Financial Officer, or such
other address as may be designated in writing by Sublessor.

          B.   ADDITIONAL RENT. All monies other than Base Rent required to
be paid by Sublessee under this Sublease, including, without limitation any
amounts payable by Sublessor to the Master Lessor with respect to the
Subleased Premises as "Operating Expenses" (as defined in Section 7 of the
Master Lease), shall be deemed additional rent ("Additional Rent"). Base Rent
and Additional Rent hereinafter collectively shall be referred to as "Rent".
Sublessee and Sublessor agree, as a material part of the consideration given
by Sublessee to Sublessor for this Sublease, that Sublessee shall pay all
costs, expenses, taxes, insurance, maintenance and other charges of every
kind and nature arising during the Term in connection with the Subleased
Premises such that Sublessor shall receive, as a net consideration for this
Sublease, the Base Rent payable under Paragraph 4.A. hereof.

          C.   TENANT IMPROVEMENTS. Sublessee shall also pay directly to
Master Lessor all amounts (net of Master Lessor's contribution) due with
respect to tenant improvements to the Subleased Premises.

     5.   INTENTIONALLY OMITTED

     6.   HOLDOVER: In the event that Sublessee does not surrender the
Subleased Premises by the Expiration Date in accordance with the terms of
this Sublease. Sublessee shall indemnify, protect, defend and hold harmless
Sublessor from and against all losses, costs, liabilities and damages
resulting from Sublessee's delay in surrendering the Subleased Premises and
pay Sublessor holdover rent as provided in Section 12 of the Master Lease.

     7.   REPAIRS: The parties acknowledge and agree that Sublessee is
subleasing the Subleased Premises on an "as is" basis, and that Sublessor has
made no representations or warranties with respect to the condition of the
Subleased Premises as of the Commencement Date except as set forth in this
paragraph. Sublessor shall have no obligation whatsoever to make or pay the
cost of any alterations, improvements or repairs to the Subleased Premises,
including, without limitation, any improvement or repair required to comply
with any law, regulation, building code or ordinance (including the Americans
with Disabilities Act of 1990). Sublessee shall inform Sublessor of any
repairs that are required to be performed by Master Lessor under the terms of
the Master Lease and Sublessor shall in turn promptly inform Master Lessor of
such repairs which shall remain Master Lessor's sole responsibility.

                                       2

<PAGE>

     8.   ASSIGNMENT AND SUBLETTING: Sublessee may not assign this Sublease,
sublet the Subleased Premises, transfer any interest of Sublessee therein or
permit any use of the Subleased Premises by another party, except in strict
accordance with Section 25 of the Master Lease, as incorporated herein.

     9.   ATTORNMENT: In the event Master Lessor gives Sublessee notice that
Sublessor is in default under the Master Lease, Sublessee shall thereafter
make all payments otherwise due Sublessor under this Sublease directly to
Master Lessor, which payments will be received by Master Lessor without any
liability on Master Lessor except to credit such payment against those due
under the Master Lease. Sublessor hereby agrees that Sublessee may rely on
any such notice by Master Lessor, and Sublessee shall have no duty of inquiry
with respect to the validity of the notice. Sublessee shall have no duty of
inquiry with respect to the validity of the notice. Sublessee shall attorn to
Master Lessor or its successors and assigns should the Master Lease be
terminated for any reason; provided, however, that in no event shall Master
Lessor or its successors or assigns be obligated to accept such attornment.

     10.  USE: Sublessee may use the Subleased Premises only for wet
laboratory biotechnology research, laboratory testing and analysis and office
uses. Sublessee shall not use, store, transport or dispose of any hazardous
material in or about the Subleased Premises except in strict accordance with
the provisions of Section 41 of the Master Lease, as incorporated herein.
Upon execution of this Sublease, Sublessee shall deliver to Sublessor for
Sublessor's approval a list of all hazardous materials Sublessee plans to use
in the Subleased Premises Sublessee shall comply with all reasonable rules
and regulations promulgated from time to time by Sublessor and Master Lessor.
Sublessor shall use reasonable efforts to cause Master Lessor to provide, at
Sublessee's sole cost and expense, signage on the door of the Subleased
Premises and the building directory tablet that is reasonably acceptable to
Sublessee, Sublessor and Master Lessor.

     11.  DELIVERY AND ACCEPTANCE: By taking possession of the Subleased
Premises, Sublessee conclusively shall be deemed to have accepted the
Subleased Premises in their as-is, then-existing condition, without any
warranty whatsoever of Sublessor with respect thereto.

     12.  IMPROVEMENTS. No alteration or improvements shall be made to the
Subleased Premises, except in strict accordance with the provisions of
Section 17 of the Master Lease, as incorporated herein. Any alterations or
improvements to the Subleased Premises shall be at Sublessee's sole cost and
expense. Sublessor shall have no obligation whatsoever to make any
alterations or improvements to the Subleased Premises.

     13.  RELEASE AND WAIVER OF SUBROGATION: Sublessor and Sublessee hereby
release each other to the extent provided in Section 21.7 of the Master
Lease, as incorporated herein. Each party shall use its best efforts to cause
each insurance policy obtained by it to provide that the insurer waives all
right of recovery against the other party and its agents and employees in
connection with any damage or injury covered by


                                       3

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the policy, and each party shall notify the other party if it is unable to
obtain a waiver of subrogation.

     14.  INSURANCE: Sublessee shall obtain and keep in full force and
effect, at Sublessee's sole cost and expense, during the Term, the insurance
required to be carried by Sublessor, as tenant, under Section 21 of the
Master Lease as incorporated herein.

     15.  DEFAULT: Sublessee shall be in material default of its obligations
under this Sublease if any of the following events occur:

          A.   Sublessee fails to pay any Rent when due, when such failure
continues for three (3) days after written notice from Sublessor to Sublessee
that any such sum is due; or

          B.   Sublessee fails to perform any term, covenant or condition of
this Sublease (except those requiring payment of Rent) and fails to cure such
breach within ten (10) days after delivery of a written notice specifying the
nature of the breach; provided, however, that if more than ten (10) days
reasonably are required to remedy the failure, then Sublessee shall not be in
default if Sublessee commences the cure within the ten (10) day period and
thereafter completes the cure within thirty (30) days after the date of the
notice; or

          C.   Sublessee commits any other act or omission which constitutes
an event of default under the Master Lease.

     16.  BROKER: Sublessor and Sublessee each represent to the other that
they have dealt with no real estate brokers, finders, agents or salesmen in
connection with this transaction. Each party agrees to hold the other party
harmless from and against all claims for brokerage commissions, finder's fees
or other compensation made by any other agent, broker, salesman or finder as
a consequence of said party's actions or dealings with such agent, broker,
salesman, or finder.

     17.  NOTICES: Unless at least five (5) days' prior written notice is
given in the manner set forth in this paragraph, the address of each party
for all purposes connected with this Sublease shall be that address set forth
below their signatures at the end of this Sublease. All notices, demands or
communications in connection with this Sublease shall be (a) personally
delivered; or (b) properly addressed and (i) submitted to an overnight
courier service charges prepaid, or (ii) deposited in the mail (registered or
certified, return receipt requested, and postage prepaid). Notices shall be
deemed delivered upon receipt, if personally delivered, one (1) business day
after being submitted to an overnight courier service and three (3) business
days after mailing, if mailed or submitted as set forth above. All notices
given to Master Lessor under the Master Lease shall be considered received
only when delivered in accordance with the Master Lease.

                                       4

<PAGE>

     18.  OTHER SUBLEASE TERMS:

          A.   INCORPORATION BY REFERENCE: Except as set forth below, the
terms and conditions of this Sublease shall include all of the terms of the
Master Lease and such terms are incorporated into this Sublease as if fully
set forth herein, except that: (i) each reference in such incorporated
sections to "Lease" shall be deemed a reference to "Sublease"; (ii) each
reference to the "Demised Premises" shall be deemed a reference to the
"Subleased Premises"; (iii) each reference to "Landlord" and "Tenant" shall
be deemed a reference to "Sublessor" and "Sublessee", respectively, except as
otherwise expressly set forth herein; (iv) with respect to work, services,
repairs, restoration, insurance, indemnities, representations, warranties or
the performance of any other obligation of Master Lessor under the Master
Lease, the sole obligation of Sublessor shall be to request the same in
writing from Master Lessor as and when requested to do so by Sublessee, and
to use Sublessor's reasonable efforts (without requiring Sublessor to spend
more than a nominal sum) to obtain Master Lessor's performance; (v) with
respect to any obligation of Sublessee to be performed under this Sublease,
wherever the Master Lease grants to Sublessor a specified number of days to
perform its obligations under the Lease, except as otherwise provided herein,
Sublessee shall have three (3) fewer days to perform the obligation,
including, without limitation, curing any defaults; (vi) with respect to any
approval required to be obtained from the "Landlord" under the Master Lease,
such consent must be obtained from both Master Lessor and Sublessor, and the
approval of Sublessor shall not be unreasonably withheld or delayed, but may
be withheld if Master Lessor's consent is not obtained; (vii) in any case
where the "Landlord" reserves or is granted the right to manage, supervise,
control, repair or enter the Premises or any areas beneath, above or adjacent
thereto, such reservation or grant of right of entry shall be deemed to be
for the benefit of both Master Lessor and Sublessor; (viii) in any case where
"Tenant" is to indemnify, release or waive claims against "Landlord", such
indemnity, release or waiver shall be deemed to run from Sublessee to both
Master Lessor and Sublessor; and (ix) in any case where "Tenant" is to
execute and deliver certain documents or notices to "Landlord", such
obligation shall be deemed to run from Sublessee to Sublessor.

     Notwithstanding the foregoing: (a) the following provisions are not
incorporated herein: the introductory paragraph, the Recitals, Sections 1-6,
8.6, 9, 10.1 10.7 (except the first sentence), 11, 17.11 (the first sentence
only), 24.11, 25.3, 31, 36.3, 37.3, 41.1 (the fifth through eighth sentences
only), 43.11 and 44-46, and Exhibits A-C, F and H-J; (b) references in the
following provisions to "Landlord" shall mean "Master Lessor": Sections 7.1,
14.1 (except the first two sentences), 15.3, 16.7 (the first sentence only),
16.8, 16.9, 17.6, 17.10, 18.1 (except the last reference), 18.2 (the first
sentence only), 21.1, 21.2, 22, 23, 32 and 36; (c) references in the
following provisions to "Landlord" shall mean "Master Lessor" or "Sublessor":
Sections 10.2, 13, 17.8, 17.9, 18.2 (fifth sentence only) and 19; (d)
references in the following provisions to "Landlord" shall mean "Master
Lessor" and "Sublessor": Sections 21.4, 21.6 and 35, (e) reference to
"Tenant" in Section 7.1 shall mean "Sublessor"; and (f) references to
"Tenant's Pro Rata Share" shall mean 7.5% (as may be adjusted by Master
Lessor).

                                       5

<PAGE>

          B.   SUBORDINATE: This Sublease is and at all times shall be
subject and subordinate to the Master Lease and the rights of Master Lessor
thereunder. In the event of a conflict between the provisions of this
Sublease and the Master Lease, as between Sublessor and Sublessee, the
provisions of this Sublease shall control.

     19.  CONDITIONS PRECEDENT: This Sublease and Sublessor's and Sublessee's
obligations hereunder are conditioned upon the written consent of Master
Lessor. If Sublessor fails to obtain Master Lessor's consent within thirty
(30) days after execution of this Sublease by Sublessor, then Sublessor or
Sublessee may terminate this Sublease by giving the other party written
notice thereof, and Sublessor shall return to Sublessee its payment of the
Security Deposit.

     20.  SUBLESSOR INDEMNITY: Sublessor shall indemnify, defend, protect,
and hold Sublessee harmless from and against all actions, claims, demands,
costs, liabilities, losses, reasonable attorneys' fees, damages, penalties
and expenses which may be brought or made against Sublessee or which
Sublessee may pay or incur to the extent caused by (i) a breach of this
Sublease or the Master Lease by Sublessor, (ii) the gross negligence or
willful misconduct of Sublessor or its agents, officers, directors, invitees
or guests or (iii) obligations of Sublessor which arise prior to the
commencement date of this Sublease.

     21.  EARLY TERMINATION / MODIFICATION OF MASTER LEASE: If the Master
Lease should terminate prior to the expiration of this Sublease for any
reason, Sublessor shall have no liability to Sublessee.


                      [THIS SPACE INTENTIALLY LEFT BLANK]

                                       6
<PAGE>

     22.  STATUS OF LEASE: Sublessor hereby represents and warrants to
Sublessee that (i) the Master Lease attached hereto as EXHIBIT A has been
executed and delivered by Master Lessor and Sublessor and constitutes the
entire agreement of the parties thereto relating to the lease of the
Subleased Premises, (ii) no default or breach by Sublessor exists under the
Master Lease, (iii) no event has occurred that, with the passage of time, the
giving of notice, or both, would constitute a default or breach by Sublessor,
and (iv) subject to receipt of Master Lessor's written consent hereto,
Sublessor has the right and power to execute and deliver this Sublease and to
perform its obligations hereunder.

IN WITNESS WHEREOF, the parties have executed this Sublease as of the day and
year first above written.

SUBLESSOR:                              SUBLESSEE:


AXYS PHARMACEUTICALS, INC.,             PPGX, INC.,
a Delaware corporation                  a Delaware corporation


By: /s/ Frederick Ringseller            By: /s/ Natalie J. Warner
Name: Frederick Ringseller              Name: Natalie J. Warner
Its: Sr. VP & CFO                       Its: Chief Executive Officer & President

Address: 11099 North Torrey Pines Road  Address: 11099 North Torrey Pines Road
La Jolla, California 92037              La Jolla, California 92037
Attn: Chief Financial Officer           Attn:

                                       7
<PAGE>

                                [AXYS LETTERHEAD]


                                                                    July 1, 1999

Attn: Natalie J. Warner, M.D.
Chief Executive Officer and President
PPGx, Inc.
11099 N. Torrey Pines Rd.
Suite 100
La Jolla, CA 92037


Dear Dr. Warner:

         SUBLEASE AGREEMENT BETWEEN SEQUANA THERAPEUTICS, INC. (dba AXYS
   PHARMACEUTICALS, INC.) AND PPGX, INC. DATED FEBRUARY 1, 1999 ("AGREEMENT")

We refer to the Agreement and confirm that it is the mutual understanding of the
parties to amend the Agreement as follows:

1.   Section 2 will be amended to add an additional 1,601.6 rentable square feet
     as described in the attached Exhibit E. The Subleased Premises will be
     amended to consist of approximately 6,101.6 rentable square feet.

2.   Section 4(A), Base Rent will be amended to read $11,683.81 per month.

3.   Section 18(A)(f) "Tenant's Pro Rata Share" shall mean 10%.

4.   A Section 23 will be added as follows:

          SHARED AREAS: Sublessor shall have the non-exclusive right to use,
          in common with Sublessee, the areas shown on Exhibit E attached
          hereto. Sublessor's right to use the Shared Areas shall be limited
          to the right to ingress and egress the premises. Sublessor's use of
          the Shared Areas shall comply with all of Sublessee's reasonable
          rules and regulations and shall not impair Sublessee's operations
          more than reasonably necessary. Sublessor shall indemnify, defend,
          protect and hold harmless Sublessee from all damages, losses,
          costs, liabilities and claims arising from Sublessor's use of the
          Shared Areas, except to the extent due to the negligence or willful
          misconduct of Sublessee.

All other terms of the Agreement remain unchanged.

<PAGE>

We are enclosing duplicate copies of this letter. If the above is acceptable
to PPGx, please sign the copies on behalf of PPGx and return one fully signed
copy to us for our records. This letter will constitute an amendment to the
Agreement effective as of the date first set forth above.


Sincerely,

/s/ Daniel H. Petree

Daniel H. Petree
President, Chief Operating Officer



Accepted this ____ day of __________, 1999.
PPGx, Inc.

/s/ Natalie J. Warner, M.D.
-------------------------------------
Natalie J. Warner, M.D.
Chief Executive Officer and President

<PAGE>

                                  "Exhibit E"

                                     [SUBLEASE FACILITY]
<PAGE>

                                [AXYS LETTERHEAD]

                                                                November 8, 1999

Attn: Joshua Baker
Chief Executive Officer and President
PPGx, Inc.
11099 N. Torrey Pines Rd., Suite 100
La Jolla, CA 92037


Dear Mr. Baker:


SUBLEASE AGREEMENT BETWEEN AXYS PHARMACEUTICALS, INC. (SUCCESSOR IN INTEREST TO
SEQUANA THERAPEUTICS, INC.) AND PPGX, INC. DATED FEBRUARY 1, 1999, AS AMENDED
                                 ("AGREEMENT")

We refer to the Agreement and confirm that it is the mutual understanding of the
parties to amend the Agreement as follows:

1.   Section 2 will be amended to add an additional 7,897 rentable square feet
     as described in the attached Exhibit E. The Subleased Premises will be
     amended to consist of approximately 13,998.6 rentable square feet.

2.   Section 4(A), Base Rent will be amended to read $26,765.32 per month which
     is $1.91 per sq. ft.

3.   Section 18(A)(f) "Tenant's Pro Rata Share" shall mean 23.3%

All other terms of the Agreement remain unchanged. We are enclosing six copies
of this letter. If the above is acceptable to PPGx, please sign the copies on
behalf of PPGx and return five fully signed copies to us for our records. This
letter will constitute an amendment to the Agreement, effective January 1,
1999.


Sincerely,

/s/ Daniel H. Petree

Daniel H. Petree
President, Chief Operating Officer


Accepted this 11 day of November, 1999.
PPGx, Inc.

/s/ Joshua Baker
-------------------------------------
Joshua Baker
Chief Executive Officer and President

<PAGE>

                                  "Exhibit E"

                                     [SUBLEASE FACILITY]