Exhibit 10.22

                            FOURTH AMENDMENT TO LEASE

      This FOURTH AMENDMENT TO LEASE (this "Fourth 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") and a Third
Amendment to Lease (the "Third Amendment"). The lease, First Amendment, Second
Amendment and Third 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 amended by the First
Amendment, the Second Amendment and the Third Amendment, 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. The definition of "Lot" as set forth in Section 1.1 of the Lease is
hereby deleted and the following definition is hereby inserted in place thereof:

            The Land shown on Exhibit A-1 to this Fourth Amendment as the "Fort
            Washington Premises" and more particularly described on Exhibit A-2
            to this Fourth Amendment, both of which are attached hereto and
            incorporated herein by reference thereto.

      3. Anything in the Lease to the contrary notwithstanding, references in
the Lease to "Exhibit A-1" or "Exhibit A-2", including Section 1.2 of the Lease,
shall mean Exhibit A-1 attached hereto and Exhibit A-2 attached hereto,
respectively.

      4. Simultaneously with the execution hereof, Landlord and Tenant shall
execute, acknowledge and deliver an Amended Notice of Lease in the form of
Exhibit B



attached hereto, which Landlord agrees to record with the Registry and file
with the Registry District.

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

      Executed under seal as of the 21 day of February, 2000.

                                        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:

                                        By: /s/ Richard H. Aldrich
                                            ------------------------------------
                                        Name: Richard H. Aldrich
                                        Title: Senior Vice-President, Chief
                                               Business Officer

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