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                                                                    Exhibit 10.7
                               BDG PISCATAWAY, LLC
                              6800 JERICHO TURNPIKE
                          SYOSSET, NEW YORK 11791-4498

                                                             As of March 1, 2000

Praecis Pharmaceuticals Incorporated
1 Hampshire Street
Cambridge, Massachusetts  02139-1572

Attention:        Kevin McLaughlin

                             Re: Lease dated as of August 19, 1998 (as amended,
                             the "Lease") by and between BDG Piscataway, LLC
                             ("Lessor") and Praecis Pharmaceuticals Incorporated
                             ("Lessee") for certain space, as more particularly
                             described in the Lease (the "Demised Premises"), in
                             the building (the "Building") located at the
                             property commonly known as 10 Knightsbridge Road,
                             Piscataway, New Jersey (the "Office Building Area")

  Dear Mr. McLaughlin:

                   Lessor and Lessee have entered into the above-referenced
  Lease with respect to the Demised Premises. Capitalized terms used herein and
  not otherwise defined shall have the meanings given to them in the Lease.
  Lessor and Lessee have agreed to enter into this side letter regarding certain
  provisions of the Lease. Notwithstanding anything to the contrary contained in
  the Lease, Lessor and Lessee, intending to be legally bound, agree that
  commencing on the date set forth above:

                        1.   Lessor and Lessee hereby acknowledge and confirm
                             the following: (a) the Demised Premises is serviced
                             currently by two (2) HVAC systems; (b) one (1) such
                             system was installed by Lessee and the other system
                             was installed by Lessor's predecessor in interest,
                             prior to the date that the Lease was executed; and
                             (c) all utilities servicing the Demised Premises
                             are currently sub-metered or metered and invoiced
                             in Landlord's name.

                        2.   Lessee hereby agrees to pay when due, the cost
                             of all utilities consumed by Lessee at the Demised
                             Premises (as reasonably determined by Lessor's
                             utility usage consultant), including, but not
                             limited to, electrical power consumed by Lessee at
                             the Demised Premises, including, but not limited
                             to, electrical power used for the HVAC servicing
                             the Demised Premises, as well as the overhead
                             lighting and electrical outlets located within the
                             Demised Premises. In connection with the foregoing,
                             Lessee further



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                             agrees: (a) that paragraph 3 and SCHEDULE 1 of
                             that certain letter agreement, dated August 19,
                             1998, executed by the parties hereto are deleted
                             in their entirety; (b) to pay to Lessor the
                             reasonable cost incurred by Lessor to have the
                             utility sub-meters for the Demised Premises read
                             and analyzed by a utility usage consultant hired by
                             Lessor; and (c) that all such amounts to be paid by
                             Lessee pursuant to this paragraph shall be
                             considered as Additional Rent.

                        3.   The Monthly Basic Rent due under the Lease shall
                             be reduced to: (a) $18,973.75 through the
                             expiration of the 60th month following the Rent
                             Commencement Date; and (b) $22,009.55 commencing on
                             the fifth anniversary of the Rent Commencement Date
                             through the expiration of the 120th month following
                             the Rent Commencement Date. The amount of the
                             Annual Basic Rent shall be adjusted accordingly.

                        4.   In the event that (a) Lessor requests Lessee to
                             establish an account in Lessee's name for any
                             utility service which is supplied to the Demised
                             Premises through a meter serving only the Demised
                             Premises, or (b) for any sub-metered utility
                             service supplied to the Demised Premises, Lessee
                             and Lessor determine that it is reasonably feasible
                             to have the Demised Premises separately metered for
                             such utility service, Lessee shall (at Lessee's
                             sole cost and expense) promptly cause the utility
                             company providing such service to the Demised
                             Premises to establish an account in Lessee's name
                             and/or install a meter to measure the applicable
                             utility consumption at the Demised Premises. Upon
                             such installation and/or establishment of such
                             account in Lessee's name, Lessor shall no longer be
                             obligated under the Lease to furnish such utility
                             service to the Demised Premises. All such amounts
                             to be paid by Lessee pursuant to this paragraph
                             shall be considered as Additional Rent.

                   Unless explicitly set forth above, all other terms and
conditions of the Lease remain unchanged and in full force and effect. Kindly
execute this letter below to acknowledge your acceptance of the above
provisions. This letter may be executed in counterparts.

                       [signatures commence on next page]



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  AGREED TO AND ACCEPTED BY:

  Praecis Pharmaceuticals Incorporated

  By:     /s/ Kevin F. McLaughlin
          --------------------------------
          Name:
          Title:   Sr. V.P. and C.F.O..


  BDG Piscataway, LLC

  BY:  BDG Piscataway, Inc.

  By:     /s/ David Blumenfeld
          --------------------------------
          Name:    David Blumenfeld
          Title:   Vice President