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                                                                  Exhibit 10.1.3

                                   FOREST CITY
                                   DEVELOPMENT

December 30, 1999

Mr. Joel Cummings
Genzyme Tissue Repair
64 Sidney Street
Cambridge, MA 02139

RE:  Lease between Genzyme Corporation, successor to BioSurface Technology, Inc.
     ("Genzyme") as Tenant, and FC 64 Sidney Street, Inc. ("Forest City") as
     Landlord, dated as of April 30, 1990 as amended from time to time (the
     "Lease")

Dear Joel:

This letter is intended to confirm our discussions regarding the mutual release
by Genzyme and Forest City of Genzyme's rights and responsibilities for two
spaces currently occupied by Genzyme Tissue Repair on the first floor of 64
Sidney Street.

Effective December 31, 1999 Genzyme's rights and obligations with respect to the
following two components of space shall terminate, except as otherwise set forth
below:

     1.   Suite 120, comprising 3,421 square feet. Genzyme has vacated this
          space, and Forest City has accepted it in As-Is condition.

     2.   A portion of Suite 150, currently comprising 4,827 square feet, with
          2,686 square feet to be vacated (hereinafter, "Suite 170"), leaving
          2,141 square feet remaining in Suite 150, as indicated on the plan
          attached hereto as Exhibit A.

Upon termination, Forest City shall be free to re-lease both Suite 120 and Suite
170 to other parties. However, in order to complete the separation of the
current Suite 150 into two independent suites, a new electrical service will
need to be installed for the new suite 170, and certain other minor
modifications will also be required. Genzyme and Forest City will cooperate to
ensure that these changes can be implemented in a cost-effective manner, and
will split the cost of this work 50/50, subject to a cap on Genzyme's share of
the cost equal to $5,000. Genzyme has left a quantity of wall-mounted and other
furnishings in the area that will become Suite 170, and in negotiating with
potential replacement tenants, Forest City will attempt to sell the furnishings
as an offset to Genzyme's share of the cost of modifications.

[UNIVERSITY PARK AT MIT LOGO]

Forest City Development, 38 Sidney Street, Cambridge, MA 02139-4169,
617/225-0310. FAX: 617/225-0311

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Mr. Joel Cummings
December 30, 1999
Page 2

Joel, if you are in agreement with these terms for release, please indicate
below your acknowledgement on behalf of Genzyme Tissue Repair. Our Lease
Administration Department will draft any additional confirmatory documentation
necessary to put this change into effect. We are pleased to have been able to
reach a mutually beneficial agreement regarding the disposition of these spaces,
and look forward to working closely with you and your associates at Genzyme
Tissue Repair to ensure that your facilities at University Park continue to meet
your needs in the most effective ways possible.

Sincerely,

/s/ Peter Calkins

Peter Calkins

Vice President, Planning & Project Development

ACKNOWLEDGED AND AGREED FOR
GENZYME TISSUE REPAIR

By: /s/ Joel Cummings
   ------------------------------

Title: VICE PRESIDENT, OPERATIONS
      ---------------------------

Date:   1/6/00
     ----------------------------

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                                    EXHIBIT A

[GRAPHIC OF FIRST FLOOR PLAN]<Page>

                                                                  Exhibit 10.1.4

                            FOURTH AMENDMENT TO LEASE

     THIS FOURTH AMENDMENT TO LEASE (hereinafter referred to as the "Amendment")
is dated as of this 23rd day of March, 2001 by and between FOREST CITY 64 SIDNEY
STREET, INC., an Ohio corporation ("Landlord") and GENZYME CORPORATION, a
Massachusetts corporation ("Tenant"). Capitalized terms used herein and not
otherwise defined shall have the meaning ascribed to such term in the Lease.

                               W I T N E S S E T H

     WHEREAS, Landlord and Tenant's predecessor-in-interest, BioSurface
Technology, Inc., entered into that certain Lease dated as of April 30, 1990, as
amended by that certain Amendment to Lease dated as of September 11, 1995, that
certain Second Amendment to Lease dated as of March 1, 1996, and that certain
letter agreement dated December 30, 1999 (collectively, the "Lease"), with
respect to certain premises located at 64 Sidney Street, Cambridge,
Massachusetts; and

     WHEREAS, Landlord and Tenant desire to amend the Lease to confirm the
parking rights and obligations under the Lease, as set forth in this Amendment.

     NOW, THEREFORE, in consideration of the premises and mutual covenants
hereinafter contained and other valuable consideration, the receipt and
sufficiency of which are hereby acknowledged, Landlord and Tenant hereby agree
as follows:

     1.   Notwithstanding any prior agreement to the contrary, commencing on the
          date hereof, Landlord shall be obligated to provide and Tenant shall
          be obligated to pay for as Additional Rent, at market rates in effect
          from time to time, one and one half parking passes per 1,000 rentable
          square feet of floor space of the Premises (currently 45,161 rentable
          square feet).

     2.   This Amendment may be executed in any number of counterparts, each of
          which shall be deemed an original and all of which together shall
          constitute one and the same instrument.

     3.   The Lease, as amended hereby, is in full force and effect, and is
          ratified and confirmed, and there are no other amendments or
          modifications thereto.

     4.   This Amendment will be governed by and construed in accordance with
          the laws of the Commonwealth of Massachusetts.

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          IN WITNESS WHEREOF, the parties hereto have executed this Amendment
under seal, as of the day, month and year first above written.

                                             FOREST CITY 64 SIDNEY STREET, INC.
                                             an Ohio corporation

                                               By:   /s/ Michael Farley
                                                   ----------------------------
                                             Name: Michael Farley
                                            Title: Vice President
                                                   Asset Management

                                             GENZYME CORPORATION
                                             a Massachusetts corporation

                                               By:   /s/ Evan M. Lebson
                                                   ----------------------------
                                             Name: EVAN M. LEBSON
                                            Title: VICE-PRESIDENT AND
                                                       TREASURER<Page>

                                                                  Exhibit 10.3.1

                            FIRST AMENDMENT TO LEASE

     This FIRST AMENDMENT TO LEASE AGREEMENT (this "FIRST AMENDMENT") is entered
into as of the 31st day of March, 1999, by and between FIRST SECURITY BANK,
N.A., not in its individual capacity, but solely as Trustee under the Owner
Trust Agreement, dated as of October 21, 1998, as lessor ("LESSOR"), and GELTEX
PHARMACEUTICALS, INC., a Delaware corporation, as lessee ("LESSEE").

                                    RECITALS:

     A.   Lessor and Lessee entered into that certain Lease Agreement, dated as
of October 21, 1998 (the "LEASE"), pursuant to which Lessee leased from Lessor
that certain property located in Waltham, Massachusetts, described in Schedule A
annexed to the Lease, together with all Improvements thereon (the "PROPERTY").
Capitalized terms used herein and not specifically defined herein shall have the
respective meanings set forth in the Lease.

     B.   Lessor and Lessee desire to modify certain terms and conditions of the
Lease as set forth herein.

     NOW, THEREFORE, in consideration of the mutual agreements and covenants
contained herein, the parties hereto agree as follows:

     1.   The last sentence of the second paragraph of Section (1) of Schedule C
annexed to the Lease, which reads as follows, shall be deleted in its entirety:

     "For purposes of this Lease, including without limitation Schedule G,
Lessor and Lessee hereby agree that such land acquisition costs are equal to
$9,000,000."

     2.   NO OTHER MODIFICATIONS. Lessor and Lessee hereby acknowledge and agree
that, except as provided in this First Amendment, the Lease has not been
modified, amended, canceled, terminated, released, superseded or otherwise
rendered of no force or effect. All prior negotiations and agreements by Lessor
and Lessee with respect to the subject matter hereof are fully merged herein and
superseded by the terms of this First Amendment.

     3.   CONFIRMATION AND RATIFICATION. The Lease as hereby amended is hereby
ratified and confirmed by Lessor and Lessee, and every provision, covenant,
condition, obligation, right, term and power contained in and under the Lease,
as hereinabove amended, shall continue in full force and effect.

     4.   COUNTERPARTS. This First Amendment may be executed in counterparts.

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                                       -2-

     5.   GOVERNING LAW. This First Amendment shall be governed by the laws of
the Commonwealth of Massachusetts.

     6.   SUCCESSORS AND ASSIGNS. This First Amendment shall be binding on the
successors and assigns of each of Lessor and Lessee.

                            [SIGNATURE PAGE FOLLOWS]

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     IN WITNESS WHEREOF, the undersigned have executed this First Amendment as
of the day and year first above written.

                                LESSOR:

                                FIRST SECURITY BANK, N.A., not in its
                                individual capacity except as expressly set
                                forth herein, but solely as Trustee under the
                                Owner Trust Agreement dated as of October 21,
                                1998

                                By: /s/ Val T. Orton
                                   --------------------------------------
                                   Name:  Val T. Orton
                                   Title: Vice President

                                LESSEE:

                                GELTEX PHARMACEUTICALS, INC.

                                By: /s/ Paul J. Mellett, Jr.
                                   --------------------------------------
                                   Name:  Paul J. Mellett, Jr.
                                   Title: Vice President, Finance and
                                          Administration; Chief Financial
                                          Officer

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