Exhibit 10.23

                            FIFTH AMENDMENT TO LEASE

      This FIFTH AMENDMENT TO LEASE (this "Fifth Amendment") is executed by and
between David E. Clem and David M. Roby, Trustees of Fort Washington Realty
Trust under Declaration of Trust dated June 19, 1995 and recorded with the
Middlesex County (South District) Registry of Deeds (the "Registry") in Book
25422, Page 360 and filed with the Middlesex South Registry District of the Land
Court (the "Registry District") as Document No. 976230 (the "Landlord") and
Vertex Pharmaceuticals Incorporated (the "Tenant").

      Reference is hereby made to that certain lease dated March 3, 1995, by and
between Landlord's predecessor, Fort Washington Limited Partnership, and Tenant
with respect to a portion of the property (the "Premises") located at 40 Erie
Street, Cambridge, Massachusetts (the "Building"), as more particularly
described in the lease, as amended by a First Amendment to Lease (the "First
Amendment"), a Second Amendment to Lease (the "Second Amendment"), a Third
Amendment to Lease (the "Third Amendment") and a Fourth Amendment to Lease (the
"Fourth Amendment"). The lease, First Amendment, Second Amendment, Third
Amendment and Fourth Amendment are herein collectively referred to as the
"Lease".

      WHEREAS, Landlord and Tenant desire to amend and modify the terms of the
Lease and to ratify and confirm the terms of the Lease, as more particularly set
forth below.

      NOW, THEREFORE, in consideration of the mutual promises herein contained,
and other good and valuable consideration, the receipt and legal sufficiency of
which is hereby acknowledged, Landlord and Tenant agree as follows:

      1. Each capitalized term which is used but not defined herein shall have
the respective meaning ascribed thereto in the Lease.

      2. Effective as of May 1, 1999 (the "Amendment Effective Date"), the
definition of Premises set forth in the Lease shall be amended to include the
addition of 400 r.s.f. of space on the second floor of the Building (the "Infill
Space"), which is more particularly shown as "AREA OF INFILL" on Exhibit A
attached hereto and incorporated herein by reference thereto.

      3. From and after the Amendment Effective Date, (A) the term "Additional
Space," as used in the Lease shall mean (i) the Infill Space and (ii) the
Additional Space as defined and described in the Third Amendment, and (B) the
terms and conditions of Section 2 of the Second Amendment, as amended by the
Third Amendment, shall apply to the Infill Space as if originally included
therein as part of the Additional Space.

      4. Effective as of the Amendment Effective Date, Section 5 of the Second
Amendment, as amended by the Third Amendment, is hereby deleted in its entirety
and the following is inserted in place thereof:



      Commencing on the Amendment Effective Date and continuing throughout the
      Additional Space Term (as such term is defined in the Third Amendment),
      Tenant shall pay to Landlord Annual Fixed Rent for the Additional Space in
      the amount of $1,474,186 ($35.50 per r.s.f.), subject to adjustment as set
      forth in the Lease, including Sections 2 and 6 of the Second Amendment.
      The Annual Fixed Rent for the Additional Space shall be payable in equal
      monthly installments in advance on the first day of each month and
      prorated for any portion of a calendar month during the Additional Space
      Term. In accordance with the foregoing, Tenant shall pay $122,865.50 to
      Landlord on the Amendment Effective Date and on the first day of each
      month thereafter until adjustment of Annual Fixed Rent for the Additional
      Space as set forth in the Lease.

      5. The Lease, as amended hereby, is hereby ratified and confirmed in all
respects.

      Executed under seal as of the 1st day of May, 1999.

                                         LANDLORD:

                                         FORT WASHINGTON REALTY TRUST

                                         /s/ David E. Clem, Trustee
                                         ---------------------------------------
                                         David E. Clem, Trustee
                                         and not individually

                                         /s/ David M. Roby
                                         ---------------------------------------
                                         David M. Roby, Trustee
                                         and not individually

                                         TENANT:

                                         VERTEX PHARMACEUTICALS
                                         INCORPORATED

                                         By: /s/ Richard H. Aldrich
                                             -----------------------------------
                                         Name: Richard H. Aldrich
                                         Title: Senior Vice President

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