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                                                                   EXHIBIT 10.20
                                  SUB-SUBLEASE


1.    PARTIES. This Sub-Sublease is made and effective this 11th day of
      September, 1997 ("Effective Date") by and among Quickturn Design Systems,
      Inc., a Delaware corporation ("Sublessor"), Microcide Pharmaceuticals,
      Inc., a Delaware corporation ("Sub-Sublessor"), Alpha Blox Corporation
      ("Sub-Sublessee"), and Portola Land Company, a California limited
      partnership ("Master Lessor").

2.    PREMISES. Sub-Sublessor hereby leases to Sub-Sublessee and Sub-Sublessee
      hereby sub-subleases from Sub-Sublessor for the Term (as defined below in
      Section 3), at the Rent set forth in Section 4 below, and upon all of the
      conditions set forth herein, that certain property situated in the County
      of Santa Clara, State of California, commonly known as 800 Maude Avenue
      and 830 Maude Avenue, Mountain View, CA 94040 and described as one (1)
      freestanding building consisting of approximately eighteen thousand forty
      (18,040) rentable square feet. (See attached Exhibit A for further
      description.)

3.    TERM:

3.1   TERM. The term of this Sub-Sublease shall be for approximately one (1)
      year (the "Term") commencing on or about November 1, 1997 ("Commencement
      Date") and ending on October 31, 1998, unless sooner terminated pursuant
      to any provision herein ("Termination Date"). Sub-Sublessee shall, at its
      option, have the right to extend the term of the Sub-Sublease for two (2)
      three (3) month extension periods. Sub-Sublessee must notify Sub-Sublessor
      in writing of its intention to exercise the first three (3) month
      extension no later than October 1, 1998. Sub-Sublessee must also notify
      Sublessor in writing of its intention to exercise the second three (3)
      month extension no later than December 1, 1998.

3.2   DELAY IN COMMENCEMENT. If for any reason Sub-Sublessor cannot deliver
      possession of the Premises to Sub-Sublessee on November 1, 1997,
      Sub-Sublessor shall not be subject to any liability therefore, nor shall
      such failure affect the validity of this Lease or the obligations of
      Sub-Sublessee hereunder or extend the term of this Sub-Sublease.
      Notwithstanding the foregoing, Sub-Sublessee shall not be obligated to pay
      rent until Sub-Sublessor tenders possession of the Premises to
      Sub-Sublessee. If Sub-Sublessor has not delivered possession of the
      Premises by December 1, 1997, Sub-Sublessee may, at its option, notify
      Sub-Sublessor in writing on or before December 10, 1997, that
      Sub-Sublessee intends to cancel this Sub-Sublease. In the event of such
      cancellation, the parties shall be discharged from all obligations
      hereunder.

3.3   EARLY COMMENCEMENT. If Sub-Sublessee occupies the Premises prior to
      November 1, 1997, such occupancy shall be subject to all provisions of
      this Sub-Sublease, such occupancy shall not advance the Termination Date
      and Sub-Sublessee shall pay rent for such period at the initial monthly
      rates set forth below.

4.    RENT; OPERATING EXPENSES AND REAL ESTATE TAXES.

4.1   RENT FOR PREMISES. The Rent is calculated for the Premises as follows:

      800 Maude Avenue: 11/1/97 through 10/31/98          $1.95/sq.ft./month NNN

      Sub-Sublessee shall pay to Sub-Sublessor as rent for the Premises equal
      monthly payments of Thirty Five Thousand One Hundred Seventy Eight Dollars
      $35,178.00), in advance, on the first day of each month for months one (1)
      through twelve (12) of the Term (collectively, 


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      the "Rent"). Sub-Sublessee shall pay to Sub-Sublessor on the Effective
      Date the sum of Thirty Five Thousand One Hundred Seventy Eight Dollars
      ($35,178.00) as payment for the first month's Rent. Sub-Sublessor shall
      apply any partial Rent payments made by Sub-Sublessee on a pro-rata basis
      toward the monthly Rent then due. Rent shall be payable in lawful money of
      the United States to Sub-Sublessor at the address stated herein or to such
      other persons or at such other places as Sub-Sublessor may designate in
      writing.

4.2   OPERATING EXPENSES; REAL ESTATE TAXES. This Sub-Sublease is a "Triple Net"
      or "NNN" Sub-Sublease and Sub-Sublessee shall be responsible for paying
      directly for services such as utilities, janitorial costs, all operating
      expenses and annual increases (including without limitation building and
      common area maintenance such as parking, landscaping and lighting), costs
      of insurance, repairs, operations and real property taxes. Sub-Sublessee
      shall have the right to verify all operating expenses.

5.    SECURITY DEPOSIT. Sub-Sublessee shall deposit with Sub-Sublessor on the
      effective Date the sum of Thirty Five Thousand One Hundred Seventy Eight
      Dollars ($35,178.00), as security for Sub-Sublessee's faithful performance
      of its obligations hereunder (the "Security Deposit"). If Sub-Sublessee
      fails to pay Rent or other charges due hereunder, or otherwise defaults
      with respect to any provision of this Sub-Sublease, Sub-Sublessor may use,
      apply or retain all or any portion of the Security Deposit for the payment
      of any Rent or other charge in default or for the payment of any other sum
      to which Sub-Sublessor may become obligated by reason of Sub-Sublessee's
      default, or to compensate Sub-Sublessor for any loss or damage which
      Sub-Sublessor may suffer thereby. If Sub-Sublessor so uses or applies all
      or any portion of the Security Deposit, Sub-Sublessee shall, within ten
      (10) days after its receipt of Sub-Sublessor's written demand, deposit
      cash with Sub-Sublessor in an amount sufficient to restore the Security
      Deposit to the full amount set forth above. Sub-Sublessee's failure to do
      so shall be a material breach of this Sub-Sublease. Sub-Sublessor shall
      not be required to keep the Security Deposit separate from its general
      accounts. If Sub-Sublessee performs all of its obligations hereunder,
      following the Termination Date and after Sub-Sublessee has vacated the
      Premises, Sub-Sublessor shall return the Security Deposit (or the portion
      thereof which has not been applied by Sub-Sublessor) to Sub-Sublessee (or
      at Sub-Sublessor's option, to the last assignee, if any, of
      Sub-Sublessee's interest in the Sub-Sublease). No trust relationship is
      created herein between Sub-Sublessor and Sub-Sublessee with respect to the
      Security Deposit. No payment of interest or other incremental charge shall
      be payable to Sub-Sublessee for Sub-Sublessor's use of the Security
      Deposit.

6.    USE.

6.1   USE. The Premises shall be used and occupied by Sub-Sublessee only for
      research and development, office and administration, storage and other
      legal uses approved by the City of Mountain View.

6.2   COMPLIANCE WITH LAW.
      (a) SUB-SUBLESSOR'S WARRANTY RE: COMPLIANCE. As of the Commencement Date,
      Sub-Sublessor warrants to Sub-Sublessee that, to the best of
      Sub-Sublessor's knowledge and without independent investigation, the
      Premises, in their existing state, do not violate any applicable building
      code regulation or ordinance; provided, however, Sub-Sublessor's warranty
      is given without regard to the use for which Sub-Sublessee intends to use
      the Premises. Sub-Sublessor shall rectify promptly, at its sole cost and
      expense, any violation of such warranty, after written notice from
      Sub-Sublessee. Notwithstanding the foregoing, the Premises may not meet
      all requirements of the Americans with Disabilities Act ("ADA") and
      Sub-Sublessor makes no warranty nor assumes any liability with respect to
      ADA compliance. It shall be conclusively deeded that no violation of
      Sub-Sublessor's warranty 


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      existed unless written notice to the contrary is received by Sub-Sublessor
      prior to the first anniversary of the Commencement Date.

      (b) SUB-SUBLESSEE'S COMPLIANCE. Except as provided in Section 6.2(a),
      Sub-Sublessee shall, at its sole expense, comply promptly with all
      applicable statutes, ordinances, rules, regulations, orders, restrictions
      of record, and requirements in effect during the Term regulating
      Sub-Sublessee's use of the Premises. Sub-Sublessee shall not use or permit
      the use of the Premises in any manner that will tend to (i) create waste
      or a nuisance; or (ii) to disturb other tenants of the Premises, if there
      are multiple tenants in the building containing the Premises.

6.3   CONDITION OF PREMISES; IMPROVEMENTS.
      (a) "AS IS" CONDITION OF PREMISES. Sub-Sublessor shall provide
      Sub-Sublessee with sufficient opportunity to conduct its own review of all
      operating systems prior to the Commencement date. Operating systems will
      be in good condition on commencement of Sub-Sublease. Except as expressly
      provided in Section 6.2(a) and this Section 6.3, Sub-Sublessee hereby
      accepts (a) the Premises "AS IS' in their condition existing as of the
      Commencement Date, subject to all applicable zoning, municipal, county and
      state laws, ordinances, and regulations governing and regulating the use
      of the Premises; (b) this Sub-Sublease subject thereto; and (c) all
      matters disclosed thereby and in any exhibits attached to this
      Sub-Sublease. Notwithstanding the foregoing, prior to the Commencement
      Date, Sub-Sublessor shall clean the carpets and floors at the Premises and
      replace defective ceiling tiles and light bulbs. All other improvements
      shall be at Sub-Sublessee's sole expense. Sub-Sublessee acknowledges that
      neither Sub-Sublessor nor Sub-Sublessor's agents have made any
      representation or warranty as to the suitability of the Premises for the
      conduct of Sub-Sublessee's business.

      (b) IMPROVEMENTS BY SUB-SUBLESSEE: Sub-Sublessee shall have the right to
      modify the building interiors of the Premises with Sub-Sublessor's,
      Sublessor's and Master Lessor's prior written consent, subject to the
      appropriate provisions of the Master Lease. Sub-Sublessor makes no
      representation as to the condition of these systems. To the best of
      Sub-Sublessor's knowledge, there are no permit violations of existing
      building improvements. Sub-Sublessee shall sublease the premises on an "as
      is" basis. Sub-Sublessee shall be allowed to make improvements to the
      premises at its sole cost and expense.

6.4   SUB-SUBLESSEE'S INDEMNIFICATION RE: HAZARDOUS SUBSTANCES. Sub-Sublessee
      shall indemnify, defend and hold Sub-Sublessor, Sublessor and Master
      Lessor, and each of their agents, employees and lenders, harmless from and
      against any and all losses, costs, claims, damages, liabilities and causes
      of action (including attorney's fees and costs and consultants' fees)
      arising out of or in any way connected with any hazardous substance
      located on the Premises immediately subsequent to the Commencement Date or
      any subsequent presence of hazardous substances on or about the Premises,
      including the soils and ground waters thereof, caused or permitted by
      Sub-Sublessee. Sub-Sublessee's obligations under this provision shall
      survive and expiration or termination of the Sub-Sublease.

7.    MASTER LEASE.

7.1   SUBLEASE, MASTER LEASE; CONFLICTS; DEFINITIONS. Sub-Sublessor is the
      Sublessee of the Premises by virtue of a sublease by and between
      Sub-Sublessor and Sublessor dated as of July 27, 1997 (the "Sublease"), a
      copy of which is attached hereto as Exhibit B-1 and incorporated herein by
      reference. Sublessor is the lessee of the Premises by virtue of a lease by
      and between Sublessor and Master Lessor dated April 26, 1994, as amended
      by that certain Addendum to Master Lease dated April 26, 1994
      (collectively the "Master Lease"), a copy of which is attached hereto as
      Exhibit B-2 and incorporated herein by reference. This 


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      Sub-Sublease is and shall be at all times subject and subordinate to the
      Sublease and the Master Lease. The terms, conditions and respective
      obligation of Sub-Sublessor and Sublessee to each other under this
      Sub-Sublease shall be the terms and conditions of the Sublease and the
      Master Lease except for those provisions of the Sublease and / or Master
      Lease which are expressly changed by this Sub-Sublease. In the event of
      any conflict between the terms of this Sub-Sublease and the terms of the
      Sublease and / or Master Lease, the terms of this Sub-Sublease shall
      control over the Sublease, and the terms of the Sublease shall control
      over the Master Lease. The term "Sublessor" in the Sublease shall be
      deemed in this Sub-Sublease to refer to the " Sub-Sublessor". The term
      "Sublessee" in the Sublease shall be deemed in this Sub-Sublease to refer
      to the " Sub- Sublessee ". The term "Lessor" in the Master Lease shall be
      deemed in this Sub-Sublease to refer to the "Sublessor". The term "Lessee"
      in the Master Lease shall be deemed in this Sub-Sublease to refer to the
      "Sublessee". Any capitalized terms not defined in this Sub-Sublease shall
      have the meaning ascribed to them by the Sublease and / or the Master
      Lease.

7.2   SUB-SUBLESSEE'S ASSUMPTION OF OBLIGATIONS. During the term and for all
      periods subsequent with respect to obligations which have arisen prior to
      the Termination Date, Sub-Sublessee does hereby expressly assume and agree
      to perform and comply with, for the benefit of Sub-Sublessor, Sublessor,
      and Master Lessor, each and every obligation of Sub-Sublessor under the
      Sublease ("Sub-Sublessee's Assumed Obligations"). Sub-Sublessee shall hold
      Sub-Sublessor free and harmless of and from all liability, judgments,
      costs, damages, claims or demands, including reasonable attorney's fees,
      arising out of Sub-Sublessee's failure to comply with or perform
      Sub-Sublessee's Assumed Obligations.

7.3   SUB-SUBLESSOR'S COMPLIANCE WITH MASTER LEASE. Sub-Sublessor represents to
      Sub-Sublessee that as of the effective date the Sublease and Master Lease
      is in full force and effect and that no default exists on the part of any
      party to the Sublease or the Master Lease.

8.    ASSIGNMENT OF SUB-SUBLEASE AND DEFAULT.

8.1   ASSIGNMENT TO MASTER LESSOR. Sub-Sublessor hereby assigns and transfers to
      Sublessor its interest in this Sub-Sublease and all Rents (with the
      exception of fifty percent (50%) of the "Excess Rent"), subject however to
      terms of Paragraph 8.2 hereof. Sub-Sublessor retains all right, title and
      interest in and to fifty percent (50%) of the Excess Rent. "Excess Rent"
      is defined in Section 23 of the Addendum to the Master Lease as "any
      consideration payable to Lessee, after deducting therefrom all reasonable
      costs necessary to effect the assignment or sublet, including, without
      limitation, brokerage and attorneys' fees, advertising costs, redecorating
      costs, and the cost of Lessee of the installation of the Lessee
      Improvements in the Premises."

8.2   DEFAULTS BY SUB-SUBLESSOR. Sublessor and Master Lessor agree that unless
      and until Sub-Sublessor materially defaults on the Sublease, Sub-Sublessor
      may receive, collect and enjoy the rents accruing under this Sublease.
      However, if Sub-Sublessor defaults in the performance of its obligations
      to Sublessor, then Sublessor may, at its option, receive and collect,
      directly from Sub-Sublessee, all rent owing and to be owed under this
      Sub-Sublease; provided, however, Sublessor agrees that it shall only be
      entitled to fifty percent (50%) of the Excess Rent. Sublessor shall not,
      by reason of this assignment of the Sub-Sublease, nor by reason of the
      collection of the rents from the Sub-Sublessee, be deemed liable to
      Sub-Sublessee for any failure of the Sub-Sublessor to perform and comply
      with its obligations hereunder.

8.3   SUB-SUBLESSEE'S PAYMENT TO SUB-SUBLESSOR. Sub-Sublessor hereby irrevocably
      authorizes and directs Sub-Sublessee, upon receipt of any written notice
      from the Sublessor stating that a default exists in the performance of
      Sub-Sublessor under the Sublease to pay to 

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      Sublessor the rents due and to become due under the Sub-Sublease.
      Sub-Sublessor agrees that Sub-Sublessee shall have the right to rely upon
      any such statement and request from Sublessor, and that Sub-Sublessee
      shall pay such rents to Sublessor without any obligation or right to
      inquire as to whether such default exists and notwithstanding any notice
      from or claim from Sub-Sublessor to the contrary. Sub-Sublessor agrees
      that it shall have no right or claim against Sub-Sublessee for any such
      rents so paid by Sub-Sublessee.

8.4   NO MODIFICATIONS WITHOUT SUBLESSOR'S AND MASTER LESSOR'S CONSENT. No
      changes or modifications shall be made to this Sub-Sublease without the
      consent of Sublessor and Master Lessor.

9.    CONSENT OF MASTER LESSOR, SUBLESSOR TO SUB-SUBLEASE.

9.1   CONSENT. By executing this Sub-Sublease, Master Lessor and Sublessor
      hereby consent to the terms and conditions of this Sub-Sublease. Such
      consent will not release Sub-Sublessor from its obligation to pay rent and
      perform and comply with all of its obligations under the Sublease.

9.2   NO WAIVER. The acceptance of rent by Sublessor from Sub-Sublessee or any
      one else liable under the Sub-Sublease shall not be deemed a waiver by
      Sublessor of any provisions of the Sublease. The consent to this
      Sub-Sublease shall not constitute a consent to any subsequent subletting
      or assignment. In the event of any default of Sub-Sublessor under the
      Sublease, Sublessor may proceed directly against Sub-Sublessor or any one
      else liable under the Sublease or this Sub-Sublease without first
      exhausting Master Lessor's remedies against any other person or entity
      liable thereon to Sublessor.

9.3   CONSENTS TO ADDITIONAL SUBLETTING. Master Lessor and Sublessor may consent
      to subsequent sublettings and assignments of the Master Lease, Sublease or
      this Sub-Sublease or any amendments or modifications thereto. Master
      Lessor and Sublessor shall promptly notify Sub-Sublessor of such proposed
      sublettings or assignments in writing.

9.4   ATTORNMENT. In the event that Sub-Sublessor defaults in its obligations
      under the Sublease, then Sublessor and / or Master Lessor, at its option
      and without being obligated to do so, may require Sub-Sublessee to attorn
      to Sublessor and / or Master Lessor. In such event, Sublessor and / or
      Master Lessor shall undertake the obligations of Sub-Sublessor under this
      Sub-Sublease from the time of the exercise of said option until the
      termination date. Notwithstanding the foregoing, Master Lessor nor
      Sublessor shall not be liable for any prepaid rents nor any security
      deposit paid by Sub-Sublessee, nor shall it be liable for any other
      defaults of the Sub-Sublessor under the Sub-Sublease.

9.5   MASTER LEASE IN GOOD STANDING. Master Lessor acknowledges that, to the
      best of Master Lessor's knowledge, no default presently exists under the
      Master Lease of obligations to be performed by Sublessor and that the
      Master Lease is in full force and effect. Sublessor acknowledges that, to
      the best of Sublessor's knowledge, no default presently exists under the
      Sublease of obligations to be performed by Sub-Sublessor and that the
      Sublease is in full force and effect.

9.6   COPIES OF DEFAULT NOTICES TO SUB-SUBLESSEE. In the event that
      Sub-Sublessor defaults under its obligations to be performed under the
      Sublease, Sublessor agrees to deliver to Sub-Sublessee a copy of any such
      notice of default. Sub-Sublessee shall have the right to cure any default
      of Sub-Sublessor described in any notice of default within ten (10) days
      after service of such notice of default on Sub-Sublessee. If such default
      is cured by Sub-Sublessee then Sub-Sublessee shall have the right of
      reimbursement and offset from and against Sub-Sublessor.

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IN WITNESS WHEREOF, the authorized representatives of each of the parties duly
execute this Sub-Sublease in the county of Santa Clara, California, as of the
date first written above.


SUBLESSOR: QUICKTURN DESIGN SYSTEMS, INC.

By: /s/ R. K. OSTBY
- ------------------------------------------------
Title:  Vice-President


SUB-SUBLESSOR: MICROCIDE PHARMACEUTICALS, INC.

By: /s/ MATTHEW J. HOGAN
- ------------------------------------------------
Title:  Chief Financial Officer


SUB-SUBLESSEE: ALPHA BLOX CORPORATION

By: /s/ JEREMY BROWN
- ------------------------------------------------
Title:  Vice-President / Chief Financial Officer


MASTER LESSOR: PORTOLA LAND COMPANY, A CALIFORNIA LIMITED PARTNERSHIP

By: /s/ WILLIAM HURWICK
- ------------------------------------------------
Title:  Managing Partner

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                                    EXHIBIT A

This exhibit is of a drawing that depicts the Site Plan for the Premises 800-850
Maude Avenue, Mountain View, California.





                                   EXHIBIT B-1

See Microcide's exhibit 10.19 as filed with the Securities and Exchange
Commission.





                                   EXHIBIT B-2

See Microcide's exhibit 10.19 as filed with the Securities and Exchange
Commission.



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