<PAGE>

                                                                    Exhibit 99.3


                       FIRST AMENDMENT OF LEASE AGREEMENT


         THIS FIRST AMENDMENT OF LEASE AGREEMENT (hereinafter referred to as
this "Amendment"), made as of the 16th day of January, 2003, by and between
TOWNSEND PROPERTY TRUST LIMITED PARTNERSHIP, a limited partnership organized and
existing under the law of Maryland doing business in New Jersey as TPT Limited
Partnership (hereinafter referred to as the "Landlord") and LEXICON
PHARMACEUTICALS (NEW JERSEY), INC., a corporation organized and existing under
the law of Delaware (hereinafter referred as the "Tenant"),

         WITNESSETH, THAT WHEREAS by a Lease Agreement dated May 23, 2002, by
and between the Landlord and the Tenant, (hereinafter referred to as the
"Lease"), the Landlord has leased to the Tenant all of that real property in
Hopewell Township, Mercer County, New Jersey, which is described therein; and

         WHEREAS the parties hereto desire by this Amendment to amend the
provisions of the Lease.

         NOW, THEREFORE, FOR AND IN CONSIDERATION of the mutual entry into this
Amendment by the parties hereto, and for other good and valuable consideration,
the receipt and adequacy of which are hereby acknowledged by each party hereto,
the parties hereto hereby agree as follows:

         Section 1. Amendment of Lease. The provisions of the Lease are hereby
amended in the following manner:

                  A. The parties agree that anything in Sections 1 (e) (i) or
         (ii) to the contrary notwithstanding, Landlord shall not be obligated
         to proceed with the subdivision of the Land from the balance of the
         east side of the Campus, it being understood and agreed by the parties
         that, as of the date hereof, there are no plans to provide for such
         subdivision. Landlord hereby authorizes Tenant to file (in Tenant's
         sole discretion) its own separate subdivision plan solely for the
         subdivision of the Land from the balance of the east side of the Campus
         (unless otherwise mutually agreed by Landlord and Tenant), at Tenant's
         sole cost and expense, as of the date hereof through any time prior to
         the commencement of the Third Lease Year.

                  B. The parties hereby confirm that the Commencement Date of
         the Term, pursuant to Section 2 (a) of the Lease, shall be May 24, 2002
         and the Expiration Date shall be May 31, 2012, subject to extension or
         early termination in accordance with the terms of the Lease.

                  C. The Base Rental schedule set forth in Section 3 (a) of the
         Lease is hereby deleted in its entirety and replaced with the
         following, however the balance of Section 3(a) shall remain unchanged:

<PAGE>

<Table>
<Caption>
-------------------------------------------------------------------------------------------------------------
   Lease Month         Annual Base Rental      Rent P.S.F.      Monthly Base Rental      Sq. Ft. Rent Paid
-------------------------------------------------------------------------------------------------------------
<S>                   <C>                      <C>              <C>                      <C>
      *1 - 2                   $0                   $0                    $0                      0
-------------------------------------------------------------------------------------------------------------
        **3               $  577,621.32           $11.32              $48,135.11               51,032
-------------------------------------------------------------------------------------------------------------
     ***4 - 7             $1,155,242.55           $22.64              $96,270.21               51,032
-------------------------------------------------------------------------------------------------------------
      8 - 36              $1,720,458.41           $22.64             $143,371.53               76,000
-------------------------------------------------------------------------------------------------------------
      37 - 72             $1,834,458.41           $24.14             $152,871.53               76,000
-------------------------------------------------------------------------------------------------------------
     73 - 108             $1,956,058.41           $25.74             $163,004.87               76,000
-------------------------------------------------------------------------------------------------------------
     109 - 120            $2,085,258.41           $27.44             $173,771.53               76,000
-------------------------------------------------------------------------------------------------------------
</Table>

         * The parties agree that for purposes of the Base Rental Schedule month
         number 1 shall mean June, 2002. The parties further agree that for the
         partial month beginning on the Commencement Date through May 31, 2002
         as well as for months 1 and 2 of the Term, Base Rental only shall be
         abated. Notwithstanding such abatement of Base Rental, Additional Rent
         shall be due and payable by Tenant beginning on the Commencement Date.

         ** The Monthly Base Rental for Lease Month 3 is $48,135.11 as a result
         of the abatement of rent until August 16, 2002 pursuant to Section 4(g)
         of the Lease.

         *** Tenant paid $91,432.33 to Landlord pursuant to the second paragraph
         of Section 3(a) of the Lease, which was applied by Landlord to the
         Monthly Base Rental for Lease Month 4.

                  D. The parties agree that (i) a valid Certificate of Occupancy
         for the Building was delivered to Tenant on August 16, 2002 in
         accordance with Section 4 (g) of the Lease, (ii) the Base Rental
         abatement for months 1 and 2 of the Term and the reduced Base Rental
         for month 3 of the Term, set forth in Subsection 1(c) above, is
         Tenant's sole remedy for Landlord's failure to deliver the Certificate
         of Occupancy by June 15, 2002, and (iii) Tenant is not entitled to any
         further abatement or reduction of Rent in connection with Landlord's
         delivery of the Certificate of Occupancy, beyond that set forth herein.

         Section 2. Effect of this Amendment. Except as is hereinabove set
forth, the provisions of the Lease shall hereafter remain in full force and
effect. Capitalized terms used but not defined herein shall have the meaning
given them in the Lease.

         Section 3. Representations. The Landlord and the Tenant hereby
represent and warrant to each other that, as of the date hereof, it (a) is the
sole legal and beneficial owner of all of the right, title and interest reserved
by it (in the case of the Landlord) or granted to it (in the case of the Tenant)
by the provisions of the Lease, (b) except with respect to Landlord's lenders,
has not sold, transferred or encumbered any or all of such right, title or
interest, and (c) has the full and sufficient right at law and in equity to
execute and deliver this Amendment as the owner of such right, title and
interest, without the necessity of obtaining any other person's consent thereto
or joinder therein.


                                       2
<PAGE>


         IN WITNESS WHEREOF, each party hereto has executed and ensealed this
Amendment or caused it to be executed and ensealed on its behalf by its duly
authorized representatives, as of the day and year first above written.

WITNESS:                                  TOWNSEND PROPOERTY TRUST LIMITED
                                          PARTNERSHIP, doing business in
                                          New Jersey as TPT Limited Partnership

                                          By: DWT A II, LLC, its general partner


    /s/ Maria S. Rubi                         By: /s/ David Townsend      (SEAL)
--------------------------------                 -------------------------------
                                               Name:  David Townsend
                                                    ----------------------------
                                               Title: Vice President
                                                     ---------------------------




                                          LEXICON PHARMACEUTICALS (NEW
                                          JERSEY), INC.


    /s/ Jeffrey L. Wade                   By: /s/ Julia P. Gregory        (SEAL)
--------------------------------             -----------------------------------
                                          Name:   Julia P. Gregory
                                               ---------------------------------
                                          Title:  Vice President and Treasurer
                                                --------------------------------


                                       3
<PAGE>

                       ASSIGNMENT AND AMENDMENT AGREEMENT

                  This ASSIGNMENT AND AMENDMENT AGREEMENT (this "Agreement") is
made as of the 20th day of May, 2003, by and among Townsend Property Trust
Limited Partnership, a Maryland limited partnership, doing business in New
Jersey as TPT Limited Partnership ("Assignor"); Hopewell Property, LLC, a
Delaware limited liability company ("Assignee"); Lexicon Pharmaceuticals (New
Jersey), Inc., a Delaware corporation ("Tenant"); the Princeton Bio-Technology
Center Condominium Association, Inc., a New Jersey nonprofit corporation (the
"Association"); and Lexicon Genetics Incorporated, a Delaware corporation
("Guarantor").

                                    RECITALS:

                  WHEREAS, Assignor and Tenant entered into that certain Lease
Agreement dated as of May 23, 2002 (the "Original Lease") pursuant to which
Assignor leased to Tenant, and Tenant leased from Assignor, approximately
nineteen (19) acres of land with the building containing approximately 76,000
rentable square feet of space (the "Building") and other improvements located
thereon known as 350 Carter Road, Hopewell, New Jersey (the "Original
Premises"), which is part of an approximately 192 acre tract of land being Block
40, Lot 14 of Hopewell Township, Mercer County, New Jersey (the "Business
Campus"); and

                  WHEREAS, the Original Lease was amended by that certain First
Amendment of Lease Agreement (the "Lease Amendment") between Tenant and Assignor
dated January 16, 2003 (the Original Lease, as amended by the Lease Amendment,
is referred to herein as the "Lease");

                  WHEREAS, Guarantor has guaranteed Tenant's obligations under
the Lease pursuant to a Guaranty dated May 23, 2002 (the "Guaranty"); and

                  WHEREAS, Assignor has submitted the Business Campus to the
provisions of the New Jersey Condominium Act pursuant to that certain Master
Deed of even date herewith (the "Master Deed") creating the condominium to be
known as Princeton Bio-Technology Center Condominium (the "Condominium"), a copy
of which Master Deed is attached hereto as Exhibit C; and

                  WHEREAS, the Association is the organization by and through
which the affairs of the Condominium are administered; and

                  WHEREAS, the "Units," the "Common Elements" and the "Limited
Common Elements" (as each such term is defined in the Master Deed) of the
Condominium are shown on the Condominium Plat prepared by Langan Engineering
dated February 27, 2003, attached hereto as Exhibit A (the "Condo Plat"); and

                  WHEREAS, Unit No. 1 of the Condominium ("Unit 1") includes (i)
the Building and such other structures and appurtenances within Unit 1 which are
not Common Elements or Limited Common Elements; (ii) an "Undivided Interest" (as
defined in the Master Deed) in the Common Elements of the Condominium, as more
particularly described in the Master Deed; and

                                       1

<PAGE>

(iii) a "Limited Common Elements Interest" (as defined in the Master Deed) in
the Limited Common Elements designated for use by Unit 1 in the Master Deed; and

                  WHEREAS, Assignor has conveyed Unit 1 to Assignee; and

                  WHEREAS, in connection with the conveyance of Unit 1 to
Assignee, Assignor desires to assign to Assignee, and Assignee desires to
assume, the rights and obligations of Assignor under (i) the Lease, (ii) the
Guaranty, (iii) that certain Confidentiality Agreement (the "Confidentiality
Agreement") between Assignor, Tenant and Guarantor, and (iv) that certain
Irrevocable Standby Letter of Credit No. 3050053 issued by Bank of America in
favor of Assignor on July 9, 2002 in an amount not to exceed $430,114.59 (the
"Letter of Credit"); and

                  WHEREAS, Guarantor desires to join in this Agreement for the
purpose of consenting to the assignment of the Lease, Guaranty, Confidentiality
Agreement and Letter of Credit by Assignor to Assignee; and

                  WHEREAS, the Association desires to join in this Agreement for
the purposes of evidencing its consent to the terms and conditions of Sections 5
(u), 5 (h) and 6 (a) of this Agreement; and

                  WHEREAS, Assignee and Tenant desire to amend the Lease to
reflect the creation of the Condominium, to substitute Unit 1 for the Original
Premises as the "Premises" thereunder, and in certain other respects, all as
more particularly set forth in this Agreement.

                  NOW, THEREFORE, for and in consideration of the sum of Ten
Dollars ($10.00) and other good and valuable consideration, the receipt and
sufficiency of which are hereby acknowledged, and intending to legally bind
themselves and their respective successors, transferees, and assigns, Assignor,
Assignee, Tenant, the Association and Guarantor hereby agree as follows:

                  1.       Defined Terms. Capitalized terms used, but not
defined, herein, shall have the meaning given to such terms in the Lease.

                  2.       Assignment. Effective as of the date of this
Agreement, Assignor hereby assigns, and Assignee hereby assumes, all of
Assignor's right, title and interest in the Lease, Guaranty, Confidentiality
Agreement and Letter of Credit. Tenant and Guarantor hereby consent to the
foregoing assignment and assumption and agree to perform their respective
obligations under the Lease, Guaranty, Confidentiality Agreement and Letter of
Credit for the benefit of Assignee. Assignor hereby expressly agrees to continue
to be bound by the terms of the Confidentiality Agreement, notwithstanding such
assignment.

                  3.       Assignor Estoppel. Assignor hereby represents and
warrants to Assignee and Tenant that (a) the Lease as amended by this Agreement
is in full force and effect; (b) the Lease has not been amended prior to this
Agreement, and, except as otherwise contemplated herein, Assignor has not
assigned the Lease; (c) there are no defaults existing on the part of the Tenant
under the Lease; (d) Assignor has received no notices of default from Tenant
and, to the best of Assignor's knowledge, no condition exists which, with the
giving of notice, the passage

                                       2

<PAGE>

of time, or both, would constitute an Event of Default under the Lease; and (e)
the Commencement Date under the Lease was May 24, 2002.

                  4.       Tenant Estoppel. Tenant hereby represents and
warrants to Assignee that (a) the Lease as amended by this Agreement is in full
force and effect; (b) the Lease has not been amended prior to this Agreement and
Tenant has not assigned the Lease nor sublet any portion of the Premises; (c)
there are no defaults existing on the part of the Assignor under the Lease; (d)
Tenant has received no notices of default from Assignor and, to the best of
Tenant's knowledge, no condition exists which, with the giving of notice, the
passage of time, or both, would constitute an Event of Default under the Lease;
and (e) the Commencement Date under the Lease was May 24, 2002.

                  5.       Amendments to Lease. Tenant and Assignor hereby amend
the Lease in the following respects:

                           (a) Hopewell Property, LLC, a Delaware limited
liability company with an address at 210 West Pennsylvania Avenue, Suite 700,
Towson, Maryland 21204, is hereby substituted for Townsend Property Trust
Limited Partnership as the "Landlord" under the Lease.

                           (b) The two WHEREAS paragraphs under the heading
"Witnesseth:" in the introductory section of the Lease are hereby deleted and
replaced with the following:

                           "WHEREAS, Landlord is the owner of Unit 1 ("Unit 1")
         of the Princeton Bio-Technology Center Condominium (the "Condominium")
         created under Master Deed dated May ___, 2003 (the "Master Deed") by
         Townsend Property Trust Limited Partnership, a Maryland limited
         partnership doing business in New Jersey as TPT Limited Partnership
         (the "Declarant") which Condominium covers that certain land located in
         Hopewell Township, Mercer County, New Jersey as more particularly
         described in Exhibit A attached hereto, with the buildings and other
         improvements thereon and shown on the Condominium Plat prepared by
         Langan Engineering dated February 27, 2003, attached hereto as Exhibit
         B (the "Condo Plat"); and

                           WHEREAS, Landlord desires to lease to Tenant and
         Tenant desire to lease from Landlord, Unit 1, all as more particularly
         set forth below."

                           (c) Paragraph 1(a) of the Lease is hereby deleted and
replaced by the following:

                           "Landlord hereby rents and leases to Tenant, and
         Tenant hereby rents and leases from Landlord, Unit 1 (the "Premises").
         The Premises consists of: (i) the Building containing 76,000 rentable
         square feet ("RSF") known as 350 Carter Road, Hopewell, New Jersey (the
         "Building"); (ii) the land within Unit 1 containing approximately
         21.196 acres and shown as Unit No. 1 on the Condo Plat (the "Land");
         (iii) any utility facilities or lines located on the Land and serving
         exclusively the Building; (iv) the driveways, walkways, parking areas,
         and related improvements now located on the Land; and (v) the right to
         use any appurtenances, rights, privileges and easements, if any,
         benefiting the Land or Building, except to

                                       3

<PAGE>

         the extent limited by the terms of this Lease. Landlord and Tenant
         hereby agree that the Building contains 76,000 RSF. The phrase "east
         side of the campus" and the word "Campus" as used in this Lease shall
         refer to the portion of the Condominium consisting of, in the
         aggregate, approximately 191.719 acres of land as described on Exhibit
         B, comprised of 21.196 acres in Unit 1 of the Condominium, 63.411 acres
         in Unit 2 of the Condominium, 31.317 acres in Unit 3 of the
         Condominium, 9.740 acres in Unit 4 of the Condominium, 29.234 acres in
         Unit 5 of the Condominium, and 36.821 acres designated on the Condo
         Plat as Common Element."

                           (d) The first sentence of Paragraph 1(b) of the Lease
is hereby deleted in its entirety.

                           (e) Paragraph 1(d)(i) of the Lease is hereby amended
by deleting the first sentence thereof and replacing it with the following:

                           "Landlord specifically reserves the right to sell or
         ground lease the Premises and any other portion of or Unit in the
         Condominium owned by Landlord without the consent of Tenant."

                           (f) Paragraph 1(e)(ii) of the Lease is hereby deleted
and replaced with the following:

                           "(ii) Solely in conjunction with the exercise of
         Tenant's purchase option pursuant to Paragraph 39 of this Lease, Tenant
         shall have the right, at any time prior to the commencement of the
         Third Lease Year, to file a subdivision plan for the subdivision of the
         Premises from the balance of the Condominium ("Tenant's Subdivision
         Plan"). For the purpose of this Paragraph, "Final Approval" shall mean
         that all required governmental approvals of the Tenant's Subdivision
         Plan have been granted and all appeal periods associated therewith have
         expired without appeal being filed (or, if an appeal is filed, upon the
         full and final determination of such appeal).

                           (g) Paragraph 3(f)(i) of the Lease is hereby amended
by replacing the word "Landlord" in the second sentence with the word
"Association"

                           (h) Paragraph 3(f)(ii) of the Lease is hereby amended
by deleting the first sentence and replacing it by the following:

                           "Until such time as such utilities are provided
                           directly to Tenant by the local utility or
                           municipality or a private utility company, Landlord
                           agrees to cause the Association to operate, maintain,
                           repair, and replace the Utility Facilities in
                           compliance with applicable Laws and to cause to be
                           provided to the Building (in compliance with all
                           applicable Laws):"

                           (i) Paragraph 3(h)(i) of the Lease is hereby deleted
and replaced by the following:

                                       4

<PAGE>

                           "Until such time as the Premises are separately
         assessed, Tenant shall pay to Landlord as Additional Rent Tenant's Tax
         Share for each Tax Year (defined below)."

                           (j) Paragraph 3(h)(iii)(D) of the Lease is hereby
deleted and replaced by the following:

                           "(D) "Tenant's Land Tax Proportionate Share" shall be
         9.98% for the period from the Commencement Date to the date the Condo
         Plat is filed in the office of the Clerk of Mercer County, New Jersey;
         and 11.06% for the period from and after such filing date."

                           (k) Paragraph 3(h)(v) of the Lease is hereby amended
by deleting the phrase "subdivided from the balance of the east side of the
Campus and the Premises becomes a separate tax parcel" in the first two lines
thereof, and inserting in lieu thereof the words "separately assessed."

                           (l) Paragraph 3(i)(i) of the Lease is hereby amended
by deleting the following at the beginning thereof:

                           "At such time as the Premises are subdivided from the
         balance of the east side of the Campus and real estate taxes are
         separately assessed for the Land and Building,"

and replacing it with the following:

                           "At such time as the Premises are separately assessed
         for real estate taxes,".

                           (m) Paragraph 3(i)(iv) of the Lease is hereby amended
by deleting the first two lines thereof and replacing them with the following":

                           "After such time as the Premises are separately
         assessed for real estate taxes, Tenant".

                           (n) Paragraph 8(f) of the Lease is hereby amended by
replacing the phrase "services or utilities provided by Landlord" in two places
in the first sentence with the phase "services or utilities provided by the
Association" and by replacing the phrase "utilities or services provided by
Landlord" in the third sentence with the phrase "services or utilities provided
by the Association".

                           (o) Paragraph 12(c) of the Lease is hereby deleted
and replaced with the following:

                           "(c) Landlord's right to any award made in any
         condemnation proceeding, action or ruling relating to the Premises
         shall be subject to the right of the holder of any mortgage or deed of
         trust encumbering the Premises to apply such award toward the reduction
         of any indebtedness secured by the Premises."

                                       5

<PAGE>

                           (p) Paragraph 16(b) of the Lease is hereby amended by
deleting the word "Campus" in the fifth sentence thereof and replacing it with
"Premises."

                           (q) Paragraph 30 of the Lease is hereby amended by
deleting the first sentence thereof and replacing it with the following:

                           "Landlord's liability to Tenant with respect to this
         Lease shall be limited solely to Landlord's interest in the Premises
         and any other Units in the Condominium owned by Landlord."

                           (r) Paragraph 33(a) of the Lease is hereby amended by
deleting the third sentence thereof and replacing it with the following:

                           "Notwithstanding the foregoing, in the event Tenant
         purchases the Premises pursuant to the Purchase Option or the Right of
         First Refusal, the Notice of Lease shall continue to be effective with
         respect to (i) the Expansion Right (to the extent it remains in effect
         at the time Tenant purchases the Premises), and (ii) the Parking
         Easement, to the extent that the Parking Lot (or the Replacement Lot,
         as the case may be) is not located on the Premises."

Landlord and Tenant shall execute, contemporaneously with this Agreement, an
amended and restated notice of lease amending and restating the original Notice
of Lease recorded in connection with the Lease, which shall be recorded in the
Office of the Clerk of Mercer County.

                           (t)Paragraph 34(d) is hereby amended by deleting in
                  the first sentence thereof the phrase "after Final Approval."

                           (u)Paragraphs 39(a)(iv), 39(a)(v), and 39(a)(vi)of
                  the Lease are hereby deleted and replaced with the following:

                           "(iv) such utility easements and service agreements
         as may be entered into between Tenant and the Association for the
         provision, use, operation, maintenance, and repair of the Utility
         Facilities owned by the Association; (v) the Parking Easement and any
         parking agreements as may be entered into between Tenant and the owner
         of Unit 2 of the Condominium for the use, maintenance, repair, and
         replacement of the Parking Lot; (vi) such facts as an accurate survey
         will disclose; and (vii) real estate taxes assessed but not yet due and
         payable. Notwithstanding the foregoing, the Lease shall not be subject
         to the Master Deed. If the Tenant exercises the purchase option
         pursuant to this Paragraph 39, the Association and Tenant shall execute
         and record an easement agreement reasonably acceptable to the parties
         providing for, inter alia, to the extent the Premises are not served by
         public utilities, the provision of utility service to the Premises from
         the Utility Facilities, the operation, repair and maintenance of the
         Utility Facilities and the allocation of the costs associated
         therewith. The provisions of said easement agreement shall bind the
         Premises and the Association."

                           (v)Paragraph 39(c) of the Lease is hereby deleted and
                  replaced with the following:

                                       6

<PAGE>

                           "(c) If Landlord sells the Premises, Landlord agrees
         to obtain from such transferee for the benefit of Tenant a written
         acknowledgement by such transferee of both the Option to Purchase and
         Tenant's right to elect to file a subdivision plan as set forth in this
         Lease."

                           (w)The first sentence of Paragraph 42(a) of the Lease
                  is hereby deleted and replaced with the following:

                           "During Lease Years 1 through 5 of the Term,
         Landlord, in its capacity as owner of Unit 2 of the Condominium, will,
         if requested in writing by Tenant, negotiate in good faith the terms
         and conditions of the development, and leasing by Tenant, of one or
         more additional building(s) (the "Expansion Building(s)") to be
         constructed on Unit 2 of the Condominium (the "Expansion Parcel").".

                           (x)Exhibits A and B of the Lease are hereby deleted
                  and replaced by Exhibits A and B attached to this Agreement.

                  6.       Miscellaneous.

                           (a)      Tenant, by its execution of this Agreement,
shall not be deemed to have agreed or consented to be subject to the Master Deed
or any other Condominium document and in the case of any inconsistencies or
conflicts between the terms and conditions contained in the Lease (as amended
hereby) and the terms and conditions contained in the Master Deed or any other
Condominium document (including, but not limited to, the bylaws of the
Association), the terms and conditions contained in the Lease (as amended
hereby) shall control.

                           (b)      This Agreement shall be governed and
construed under the laws the State of New Jersey.

                           (c)      This Agreement may be executed in
counterparts and all counterparts together shall constitute a single Agreement.

                           (d)      The Lease, as amended hereby, and the
obligations of Landlord and Tenant thereunder, remain in full force and effect.

                                       7

<PAGE>

                  IN WITNESS WHEREOF, the parties hereto have executed this
Agreement as of the day and year first written above.

                                    ASSIGNOR:

                                    Townsend Property Trust Limited Partnership

                                    By: DWT A II, LLC, a Delaware limited
                                        liability company, its sole general
                                        partner

                                        By: /s/ DAVID TOWNSEND
                                           -------------------------------------
                                            Name: David Townsend
                                            Title: Vice President

                                    ASSIGNEE:

                                    Hopewell Property, LLC

                                    By: /s/ DAVID TOWNSEND
                                       -----------------------------------------
                                        Name: David Townsend
                                        Title: Vice President

                                    TENANT:

                                    Lexicon Pharmaceuticals (New Jersey), Inc.

                                     By: /s/ JULIA P. GREGORY
                                        ----------------------------------------
                                         Name: Julia P. Gregory
                                         Title: Vice President and Treasurer

                                    ATTEST:

                                    /s/ JEFFREY L. WADE
                                    --------------------------------------------
                                    Authorized Signature

                                    its: Vice President and Secretary
                                    --------------------------------------------

                                    Jeffrey L. Wade
                                    --------------------------------------------
                                    (type name and title of signatory)

                                       8

<PAGE>
                                     GUARANTOR:

                                     Lexicon Genetics Incorporated

                                     By: /s/ JULIA P. GREGORY
                                        ----------------------------------------
                                         Name: Julia P. Gregory
                                         Title: Executive Vice President and CFO

                                     ATTEST:

                                     /s/ JEFFREY L. WADE
                                     -------------------------------------------
                                     Authorized Signature

                                     its: Executive Vice President and General
                                     -------------------------------------------
                                     Counsel
                                     -------------------------------------------

                                     Jeffrey L. Wade
                                     -------------------------------------------
                                     (type name and title of signatory)

                                     THE ASSOCIATION:

                                     Princeton Bio-Technology Center Condominium
                                     Association, Inc.

                                     By: /s/ DAVID TOWNSEND
                                        ----------------------------------------
                                         Name: David Townsend
                                         Title: Executive Board

                                     ATTEST:

                                     /s/ MARIA S. RUBI
                                     -------------------------------------------
                                     Authorized Signature

                                     its: ______________________________________

                                     Maria S. Rubi
                                     -------------------------------------------
                                     (type name and title of signatory)

                                       9

<PAGE>

                                    EXHIBIT A

                     Legal Description of Land Subjected To
                                 The Condominium

                                       10

<PAGE>

                                    EXHIBIT B

                                Subdivision Plat

                                       11

<PAGE>

                                    EXHIBIT C

                                   Master Deed

                                       12
<PAGE>
                    AMENDED AND RESTATED MEMORANDUM OF LEASE

                  THIS AMENDED AND RESTATED MEMORANDUM OF LEASE (this
"Memorandum of Lease") is made and entered into by and between the parties
hereto, to evidence their execution of a certain lease dated as of May 23, 2002,
as amended by First Amendment of Lease Agreement dated as of January 16, 2003,
and as further amended by a certain Assignment and Amendment Agreement dated as
of the date hereof (collectively, the "Lease").

                                   WITNESSETH:

                  1.       TERMINATION OF PRIOR MEMORANDUM OF LEASE. The
Memorandum of Lease (the "Prior Memorandum of Lease") between Lexicon
Pharmaceuticals (New Jersey), Inc. and Townsend Property Trust Limited
Partnership, doing business in New Jersey as TPT Limited Partnership (the "Prior
Landlord"), dated as of May 23, 2002 and recorded with the Office of the
Clerk/Register in Mercer County, is hereby amended and restated in its entirety
as provided herein and the parties hereto desire to remove and replace of record
the Prior Memorandum of Lease with this Memorandum of Lease.

                  2.       LANDLORD. The name of the Landlord is HOPEWELL
PROPERTY, LLC, a Delaware limited liability company ("Landlord"), whose address
is c/o Townsend Capital, LLC, 210 West Pennsylvania Avenue, Suite 700, Towson,
Maryland 21204.

                  3.       TENANT. The name of the Tenant is LEXICON
PHARMACEUTICALS (NEW JERSEY), INC., a Delaware corporation ("Tenant"), whose
address is 350 Carter Road, Princeton, New Jersey 08540.

                  4.       PREMISES. The "Premises" consist of approximately
21.196 acres of land shown as Unit No. 1 on the Condominium Plan attached hereto
as Exhibit B (the "Land"), which is a unit in that certain condominium known as
the Princeton Bio-Technology Center Condominium, which consists of approximately
191.719 acres of land as described by metes and bounds in Exhibit A hereto (the
"Condominium") and being Block 40, Lot 14 of Hopewell Township, Mercer County,
New Jersey, along with the approximately 76,000 square foot office building
located on the Land and known as the "Education Building," 350 Carter Road,
Hopewell, New Jersey. The affairs of the Condominium are administered by the
Princeton Bio-Technology Center Condominium Association, a New Jersey nonprofit
corporation (the "Association").

                  5.       LEASE TERM. The term of this Lease (the "Lease Term")
commenced on May 24, 2002 (the "Commencement Date"), and expires on May 31,
2012, provided that if Tenant elects to receive the Additional T. I. Allowance
(as defined in the Lease), the Lease Term

<PAGE>

shall be extended through the date that is the tenth anniversary of the date
Landlord funds the Additional T. I. Allowance, subject to sooner termination as
provided in the Lease or extension as provided in the Lease as described below.

                  6.       EXTENSION OPTIONS. Tenant has two (2) consecutive
options to extend the Lease Term for five (5) years each.

                  7.       OPTION TO PURCHASE.

                           (a)      Tenant has the option to purchase (the
"Purchase Option") the Premises during the period commencing on the Commencement
Date and ending on the last day of the calendar month in which the third
anniversary of the Commencement Date falls (the "Purchase Option Period"), but
Tenant may not exercise the Purchase Option unless Final Approval (as defined in
the Lease) has been obtained for the subdivision of the Premises from the
balance of the Condominium. The Purchase Option Period will be extended for a
period commencing on the first day of the calendar month immediately following
the calendar month in which the third anniversary of the Commencement Date falls
and ending on the date that is one hundred twenty (120) days following such date
if Tenant elects to file a subdivision plan for the subdivision of the Premises
from the balance of the Condominium as described in Subparagraph 1(e)(ii) of the
Lease. Furthermore, prior to Landlord's mortgagee commencing foreclosure
proceedings or accepting a deed in lieu of foreclosure for the Premises and/or
the Expansion Parcel (as hereinafter defined), Tenant has the option to exercise
the Purchase Option to purchase both the Premises and the Expansion Parcel.

                           (b)      Conclusive evidence that the Purchase Option
has teminated shall include an instrument (the "Option/Right Termination") in
the form attached hereto as Exhibit D signed by Tenant and recorded in the
Mercer County Recorded of Deeds Office (the "Recorder's Office") stating that
the Purchase Option has been terminated. Tenant hereby unconditionally
constitutes and appoints Landlord (and any transferee of Landlord's interest in
the Premises) as its true and lawful attorney and agent for the sole purposes,
except as otherwise set forth herein, of executing and recording the
Option/Right Termination on Tenant's behalf upon the termination of the Purchase
Option. Tenant acknowledges that the foregoing power granted to Landlord (and
any transferee of Landlord's interest in the Premises) is coupled with an
interest and shall not be revocable by Tenant in any manner or for any reason,
including Tenant's dissolution, and any individual or entity may rely on this
appointment.

                  8.       RIGHT OF FIRST REFUSAL.

                           (a)      Tenant has the right of first refusal
("Right of First Refusal") during the period (the "ROFR Period") commencing on
the first day of the calendar month immediately following the calendar month in
which the third anniversary of the Commencement Date falls and ending on the
last day of the calendar month in which the tenth anniversary of the
Commencement Date falls, with respect to any third party offer for the purchase
of the Premises, or of property including the Premises, which Landlord intends
to accept (not including a deed in lieu of foreclosure, a foreclosure sale, or
sale by operation of law).

                                       2

<PAGE>

                           (b)      A sale or ground lease by Landlord of the
Premises (or of a larger portion of the Condominium including the Premises)
which is consummated prior to the ROFR Period shall be expressly subject to
Tenant's Right of First Refusal.

                           (c)      If Tenant fails pursuant to the terms and
conditions of the Lease to purchase the Premises or a larger parcel including
the Premises that is offered to Tenant pursuant to the terms of the Lease during
the ROFR Period, then the Right of First Refusal shall terminate; provided,
however, that in the event the property in question is a larger parcel including
the Premises, the Right of First Refusal with respect to the Premises shall
survive the sale of such larger parcel and be binding on the purchaser of such
larger parcel for the balance of the ROFR Period.

                           (d)      Conclusive evidence that the Right of First
Refusal has terminated shall include an Option/Right Termination in the form
attached hereto as Exhibit D signed by Tenant and recorded in the Recorder's
Office stating that the Right of First Refusal has been terminated. Tenant
hereby unconditionally constitutes and appoints Landlord (and any transferee of
Landlord's interest in the Premises) as its true and lawful attorney and agent
for the sole purpose, except as otherwise set forth herein, of executing and
recording the Option/Right Termination on Tenant's behalf upon the termination
of the Right of First Refusal. Tenant acknowledges that the foregoing power
granted to Landlord (and any transferee of Landlord's interest in the Premises)
is coupled with an interest and shall not be revocable by Tenant in any manner
or for any reason, including Tenant's dissolution, and any individual or entity
may rely on this appointment.

                  9.       EXPANSION RIGHT.

                           (a)      Tenant has the following rights under the
Lease (collectively, the "Expansion Right"):

                                    (i)      During the period commencing on the
Commencement Date and ending on the last day of the calendar month in which the
fifth anniversary of the Commencement Date falls, Landlord agrees to reserve
sufficient "floor area ratio" (as such term is used in Hopewell Township's land
use and development ordinances) ("FAR") on the portion of the Condominium shown
on Exhibit B hereto as Unit No. 2 (the "Expansion Parcel") in order to develop
one or more buildings (the "Expansion Building(s)") for Tenant, and Landlord
agrees during such time period, upon written notice from Tenant, to negotiate in
good faith, the development, and leasing by Tenant, of the Expansion
Building(s). During the portion of such period commencing on the Commencement
Date and ending on the last day of the calendar month in which the third
anniversary of the Commencement Date falls, Landlord's obligation to reserve FAR
and to negotiate in good faith the development, and leasing by Tenant, of the
Expansion Building(s) on the Expansion Parcel shall be limited to 150,000
rentable square feet in the aggregate. During the portion of such period
commencing on the third anniversary of the Commencement Date and ending on the
last day of the calendar month in which the fifth anniversary of the
Commencement Date falls, Landlord's obligation to reserve FAR and to negotiate
in good faith the development, and leasing by Tenant, of the Expansion
Building(s) on the Expansion Parcel shall be limited to 100,000 rentable square
feet in the aggregate.

                                       3

<PAGE>

                                    (ii)     If Landlord and Tenant conduct a
negotiation with respect to the development of Expansion Building(s) during the
period commencing on the Commencement Date and ending on the last day of the
calendar month in which the fifth anniversary of the Commencement Date falls
and, notwithstanding the good faith attempts of the parties to reach agreement,
(1) no agreement is reached and negotiations are terminated by either party (the
date of the termination being referred to herein as the "Negotiation Termination
Date"), or (2) no agreement is reached by the last day of such period; or (3)
Tenant does not request in writing to Landlord to negotiate with respect to the
development, and leasing by Tenant, of an Expansion Building prior to the end of
the last day of such period, then, effective upon the earlier of the Negotiation
Termination Date or the last day of such period, Tenant shall have no further
right to require Landlord to negotiate for the development, and leasing by
Tenant, of an Expansion Building; provided, however, that for the balance of the
Lease Term, Landlord shall notify Tenant prior to developing the last available
100,000 RSF of FAR for the Expansion Parcel and Tenant shall have the right,
within ten (10) business days of receiving said notice, to notify Landlord in
writing ("Tenant's Negotiation Notice") that Tenant desires to negotiate in good
faith with Landlord for the development, and lease by Tenant, of an Expansion
Building not to exceed 100,000 RSF, and if Tenant so notifies Landlord, Landlord
and Tenant shall promptly commence good faith negotiations regarding the terms
of such development and leasing; and if, notwithstanding the good faith efforts
of Landlord and Tenant, (A) no such agreement is reached and negotiations are
terminated by either party, or (B) no such agreement is reached on or before the
date which is forty five (45) days from the date Landlord receives Tenant's
Negotiation Notice, then Tenant's Expansion Right shall expire and be null and
void and Tenant shall have no further Expansion Right under the Lease.

                           (b)      A sale or ground lease by Landlord of the
Expansion Parcel (or of a larger portion of the Condominium including the
Expansion Parcel) which is consummated during the Lease Term while the Expansion
Right is still effective shall be expressly subject to Tenant's Expansion Right.

                           (c)      Conclusive evidence that Tenant's Expansion
Right has terminated shall include an Option/Right Termination in the form
attached hereto as Exhibit D signed by Tenant and recorded in the Recorder's
Office stating that Tenant's Expansion Right has been terminated. Tenant hereby
unconditionally constitutes and appoints Landlord (and any transferee of
Landlord's interest in the Expansion Parcel) as its true and lawful attorney and
agent for the sole purpose, except as otherwise set forth herein, of executing
and recording the Option/Right Termination on Tenant's behalf upon the
termination of Tenant's Expansion Right. Tenant acknowledges that the foregoing
power granted to Landlord (and any transferee of Landlord's interest in the
Expansion Parcel) is coupled with an interest and shall not be revocable by
Tenant in any manner or for any reason, including Tenant's dissolution, and any
individual or entity may rely on this appointment.

                  10.      UTILITY SERVICE.

                           (a)      Landlord is obligated under the terms of the
Lease, and the Association, has agreed, to operate, maintain, repair and replace
certain Utility Facilities (as defined in the Lease) in order to supply the
Premises with utility services until such time as, with respect to each
particular type of such utility service, such utility service is provided
directly to

                                       4

<PAGE>

the Premises by the local utility or municipality or a private utility company,
and subject to Tenant's obligation to pay its pro rata cost of such utility
service as set forth in the Lease (collectively, "Tenant's Utility
Rights/Obligations").

                           (b)      Certain of the Utility Facilities are
located on the Premises and certain of the Utility Facilities are located on
other portions of the Condominium off the Premises. A sale or ground lease by
Landlord of the Land or any other portion of the Condominium (including a sale
or ground lease by Prior Landlord of any portions not owned by Landlord) that
includes any Utility Facilities serving the Premises shall be expressly subject
to Tenant's Utility Rights/Obligations, and the parties to any such transaction
shall execute and record the Utility Easement (as defined in the Lease). A sale
of the Premises to Tenant pursuant to the exercise of Tenant's Purchase Option
shall be expressly subject to Landlord, the Association and Tenant executing and
recording an easement agreement reasonably acceptable to the parties and Tenant
providing for, inter alia, to the extent the Premises are not served by public
utilities, the provision of utility service to the Premises from the Utility
Facilities, the operation, repair and maintenance of the Utility Facilities and
the allocation of the costs associated therewith. The provisions of said
easement agreement shall (i) be binding on the Premises and the Association, and
(ii) be consistent with Paragraph 3(f) of the Lease so that Tenant has the same
rights and benefits and obligations provided under the Lease (and no additional
obligations or expenses) related thereto.

                  11.      PARKING RIGHTS.

                           (a)      During the Lease Term, including any
extensions thereof, Tenant has the right to use the parking lot located on Unit
2 of the Condominium (but not on the Premises) as identified on Exhibit B
attached hereto (the "Parking Lot"), until such time (if any) that the Parking
Lot is replaced by the Replacement Lot (as defined in the Lease), at which time
Tenant shall have the exclusive right to use the Replacement Lot and no further
right to use the Parking Lot. Tenant's rights and obligations under the Lease
with respect to the Parking Lot and Replacement Lot shall be referred to herein
as "Tenant's Parking Rights/Obligations."

                           (b)      A sale or ground lease by Landlord of all or
any portion of the Condominium (including a sale or ground lease by Prior
Landlord of any portions not owned by Landlord) that includes the Parking Lot or
Replacement Lot shall expressly be subject to Tenant's Parking
Rights/Obligations with respect thereto including, but not limited to, the
obligations of the parties to such transaction to execute and record the Parking
Easement (as defined in the Lease).

                  12.      TERMINATION. This Memorandum of Lease shall terminate
and cease to be effective upon the termination of the Lease. Conclusive evidence
that the Lease has terminated shall include an instrument in the form attached
hereto as Exhibit C (the "Termination Notice") signed by Tenant and recorded
with the Recorder's Office stating that the Lease has been terminated. Upon
termination of this Lease, the Purchase Option, Right of First Refusal and
Tenant's Expansion Right shall each automatically terminate. Notwithstanding the
foregoing, a termination of the Lease in connection with the purchase of the
Premises pursuant to the Purchase Option or Right of First Refusal by Lexicon
Pharmaceuticals (New Jersey), Inc., or an Affiliated Entity (as defined in the
Lease) or a Permitted Assignee (as defined in the Lease) of

                                       5

<PAGE>

Lexicon Pharmaceuticals (New Jersey), Inc. shall not extinguish the Expansion
Right (to the extent still effective under the terms of the Lease at the time of
such purchase of the Premises) and the Expansion Right (to the extent still
effective under the terms of the Lease at the time of such purchase of the
Premises) shall continue to bind the Expansion Parcel and such purchaser
pursuant to the terms of the Lease following such purchase and termination of
the Lease for so long as the Premises is owned by Lexicon Pharmaceuticals (New
Jersey), Inc., or an Affiliated Entity (as defined in the Lease) or a Permitted
Assignee (as defined in the Lease) of Lexicon Pharmaceuticals (New Jersey), Inc.
Tenant hereby unconditionally constitutes and appoints Landlord (and any
transferee of Landlord's interest in the Lease) as its true and lawful attorney
and agent for the sole purpose, except as otherwise set forth herein, of
executing and recording the Termination Notice on Tenant's behalf upon the
termination of the Lease. Tenant acknowledges that the foregoing power granted
to Landlord (and any transferee of Landlord's interest in the Lease) is coupled
with an interest and shall not be revocable by Tenant in any manner or for any
reason, including Tenant's dissolution, and any individual or entity may rely on
this appointment.

                  13.      INCORPORATION OF LEASE. Reference is made to the
Lease for the remaining terms and provisions thereof, all of which are
incorporated herein by reference.

                  14.      COUNTERPARTS. This Memorandum of Lease may be signed
in one or more counterparts, all of which when taken together shall constitute
one and the same instrument.

                                       6

<PAGE>

                  IN WITNESS WHEREOF, the parties have executed this Amended and
Restated Memorandum of Lease as of the 20th day of May, 2003.

                                        LANDLORD:

                                        HOPEWELL PROPERTY, LLC

                                        By: /s/ DAVID TOWNSEND
                                            ----------------------------------
                                            Name: David Townsend
                                            Title: Vice President


                                        TENANT:

                                        LEXICON PHARMACEUTICALS
                                        (NEW JERSEY), INC.

                                        By: /s/ JULIA P. GREGORY
                                            ----------------------------------
                                            Name: Julia P. Gregory
                                            Title: Vice President and Treasurer


                                        PRIOR LANDLORD:

                                        TOWNSEND PROPERTY TRUST
                                        LIMITED PARTNERSHIP, DOING
                                        BUSINESS IN NEW JERSEY AS TPT LIMITED
                                        PARTNERSHIP

                                        By: DWT A II, LLC, ITS GENERAL PARTNER

                                        By: /s/ DAVID TOWNSEND
                                            ----------------------------------
                                            Name: David Townsend
                                            Title: Vice President


                                        PRINCETON BIO-TECHNOLOGY
                                        CENTER CONDOMINIUM
                                        ASSOCIATION

                                        By: /s/ DAVID TOWNSEND
                                            ----------------------------------
                                            Name: David Townsend
                                            Title: Vice President


                                       7

<PAGE>

STATE OF MARYLAND

COUNTY OF ______________

                  I, the undersigned, a notary public in and for the county and
state aforesaid, hereby certify that ___________________, the _______________ of
DWT A II, LLC, general partner of Townsend Property Trust Limited Partnership,
personally appeared before me, and that such member, being authorized to do so,
executed the foregoing instrument for the purposes therein contained by signing
the name of said company as general partner of said partnership as such member.

                  Given under my hand and official seal on _____________, 2003.

                                          ______________________________________
                                          Notary Public

My commission expires: __________

STATE OF MARYLAND

COUNTY OF ______________

                  I, the undersigned, a notary public in and for the county and
state aforesaid, hereby certify that ___________________, the _______________ of
Hopewell Property, LLC, personally appeared before me, and that such __________,
being authorized to do so, executed the foregoing instrument for the purposes
therein contained by signing the name of said limited liability company as such
____________.

                  Given under my hand and official seal on _____________, 2003.

                                          ______________________________________
                                          Notary Public

My commission expires: __________

                                       8

<PAGE>

STATE OF MARYLAND

COUNTY OF ______________

                  I, the undersigned, a notary public in and for the county and
state aforesaid, hereby certify that ___________________, the _______________ of
the Princeton Bio-Technology Center Condominium, personally appeared before me,
and that such officer, being authorized to do so, executed the foregoing
instrument for the purposes therein contained by signing the name of said
corporation as such officer.

                  Given under my hand and official seal on _____________, 2003.

                                          ______________________________________
                                          Notary Public

My commission expires: __________

STATE OF NEW JERSEY

COUNTY OF ______________

                  I, the undersigned, a notary public in and for the county and
state aforesaid, hereby certify that ________________, the ___________ of
Lexicon Pharmaceuticals (New Jersey), Inc., personally appeared before me and
that such officer, being authorized to do so, executed the foregoing instrument
for purposes therein contained by signing the name of the corporation as such
officer.

                  Given under my hand and official seal on _____________, 2003.

                                          ______________________________________
                                          Notary Public

My commission expires: __________

Mail to:          Mark S. DePillis, Esquire
                  Ballard Spahr Andrews & Ingersoll, LLP
                  1735 Market Street, 51st Floor
                  Philadelphia, Pennsylvania 19103

                                       9

<PAGE>

                                   EXHIBIT A

             Legal Description of Real Estate Comprising Condominium
<PAGE>

                                   EXHIBIT B

         Condominium Plan Showing Land, Expansion Parcel and Condominium

<PAGE>

                                   EXHIBIT C

                        FORM OF LEASE TERMINATION NOTICE

                  THIS TERMINATION OF LEASE ("Termination") is made as of this
______ day of ______________, 20___, by and between ______________ ("Landlord")
and _________________ ("Tenant").

                  WHEREAS, [Hopewell Property, LLC (as successor to Townsend
Property Limited Partnership)] [Landlord], as landlord, and [Lexicon
Pharmaceuticals (New Jersey), Inc.] [Tenant], as tenant, entered into that
certain Lease Agreement dated May __, 2002, as amended by First Amendment of
Lease Agreement dated as of January 16, 2003, and as further amended by a
certain Assignment and Amendment Agreement dated as of _________, 2003
(collectively, the "Lease") with respect to the Land described in Exhibit A
hereto and the Building and other improvements thereon located at 350 Carter
Road, Hopewell, New Jersey [explain any lease assignments or amendments here],
which Lease is evidenced by an Amended and Restated Memorandum of Lease dated
____________, 2003 and recorded in the Office of the Clerk of Mercer County, New
Jersey (the "Recorder's Office") in Book ___, Page ___ (the "Memorandum").

                  [See Article 33 regarding survival of certain rights]

                  WHEREAS, the Lease (including, but not limited to, the
Purchase Option, Right of First Refusal and Expansion Right, as each such term
is defined in the Lease) has terminated.

                  WHEREAS, this Termination is entered into for the purpose of
setting forth upon the public record that the Lease (including, but not limited
to, the Purchase Option, Right of First Refusal and Expansion Right, as each
such term is defined in the Lease) is terminated and null and void and the
Memorandum is of no further force and effect.

                  NOW THEREFORE, for good and valuable consideration, each to
the other in hand paid, the parties hereto, intending to be legally bound
hereby, agree as follows:

                  1.       The Lease (including, but not limited to, the
Purchase Option, Right of First Refusal and Expansion Right, as each such term
is defined in the Lease) is hereby terminated and released and the Memorandum is
of no further force or effect.

                  2.       The parties hereto acknowledge that this Termination
is intended to be recorded in the Recorder's Office.

                                      C-1

<PAGE>

                  IN WITNESS WHEREOF, the parties hereto have executed this
Termination as of the day and year first above written.

                                          LANDLORD:

                                          [NAME OF LANDLORD]

                                          By: __________________________________
                                          Name:
                                          Title:

                                          TENANT:
                                              [NAME OF TENANT]

                                          By: __________________________________
                                          Name:
                                          Title:

                                      C-2

<PAGE>

STATE OF _______________

COUNTY OF ______________

                  I, the undersigned, a notary public in and for the county and
state aforesaid, hereby certify that ___________________, the _______________ of
_____________________, personally appeared before me, and that such officer,
being authorized to do so, executed the foregoing instrument for the purposes
therein contained by signing the name of said company as such officer.

                  Given under my hand and official seal on _____________, 20___.

                                          ______________________________________
                                          Notary Public
                                          My commission expires: __________

STATE OF _______________

COUNTY OF ______________

                  I, the undersigned, a notary public in and for the county and
state aforesaid, hereby certify that ________________, the ___________ of
______________________, personally appeared before me and that such officer,
being authorized to do so, executed the foregoing instrument for purposes
therein contained by signing the name of the corporation as such officer.

                  Given under my hand and official seal on _____________, 20___.

                                          ______________________________________
                                          Notary Public
                                          My commission expires: __________

                                      C-3

<PAGE>

                                   EXHIBIT "A"

                                LEGAL DESCRIPTION

                                      C-4

<PAGE>

                                   EXHIBIT D

                        FORM OF OPTION/RIGHT TERMINATION

                  THIS OPTION TERMINATION ("Termination") is made as of this
______ day of ______________, 20___, by and between ______________ ("Landlord")
and _________________ ("Tenant").

                  WHEREAS, [Hopewell Property, LLC (as successor to Townsend
Property Limited Partnership)] [Landlord], as landlord, and [Lexicon
Pharmaceuticals (New Jersey), Inc.] [Tenant], as tenant, entered into that
certain Lease Agreement dated May __, 2002, as amended by First Amendment of
Lease Agreement dated as of January 16, 2003, and as further amended by a
certain Assignment and Amendment Agreement dated as of _________, 2003
(collectively, the "Lease") with respect to the Land described in Exhibit A
hereto and the Building and other improvements thereon located at 350 Carter
Road, Hopewell, New Jersey [explain any lease assignments or amendments here],
which Lease is evidenced by an Amended and Restated Memorandum of Lease dated
____________, 2003 and recorded in the Office of the Clerk of Mercer County, New
Jersey (the "Recorder's Office") in Book ___, Page ___ (the "Memorandum").

                  WHEREAS, the Lease includes a [Purchase Option] [Right of
First Refusal] [Expansion Right] (as such term is defined in the Lease) in favor
of Tenant (the "Right"), which right is referenced in the Memorandum.

                  WHEREAS, the Right has terminated.

                  WHEREAS, this Termination is entered into for the purpose of
setting forth upon the public record that the Right is terminated and null and
void.

                  NOW THEREFORE, for good and valuable consideration, each to
the other in hand paid, the parties hereto, intending to be legally bound
hereby, agree as follows:

                  1.       The Right is hereby terminated and released and of no
further force or effect.

                  2.       The parties hereto acknowledge that this Termination
is intended to be recorded in the Recorder's Office.

                                      D-1

<PAGE>

                  IN WITNESS WHEREOF, the parties hereto have executed this
Termination as of the day and year first above written.

                                          LANDLORD:

                                          [NAME OF LANDLORD]

                                          By:    _______________________________
                                          Name:
                                          Title:

                                          TENANT:
                                              [NAME OF TENANT]

                                          By:    _______________________________
                                          Name:
                                          Title:

                                      D-2

<PAGE>

STATE OF _______________

COUNTY OF ______________

                  I, the undersigned, a notary public in and for the county and
state aforesaid, hereby certify that ___________________, the _______________ of
_____________________, personally appeared before me, and that such officer,
being authorized to do so, executed the foregoing instrument for the purposes
therein contained by signing the name of said company as such officer.

                  Given under my hand and official seal on _____________, 20___.

                                          ______________________________________
                                          Notary Public
                                          My commission expires: __________

STATE OF _______________

COUNTY OF ______________

                  I, the undersigned, a notary public in and for the county and
state aforesaid, hereby certify that ________________, the ___________ of
______________________, personally appeared before me and that such officer,
being authorized to do so, executed the foregoing instrument for purposes
therein contained by signing the name of the corporation as such officer.

                  Given under my hand and official seal on _____________, 20___.

                                          ______________________________________
                                          Notary Public
                                          My commission expires: __________

                                      D-3

<PAGE>

                                   EXHIBIT "A"

                                LEGAL DESCRIPTION

                                      D-4
<PAGE>
                       [HOPEWELL PROPERTY LLC LETTERHEAD]


                                  July 2, 2003

BY FACSIMILE (609) 448-8299
and FEDERAL EXPRESS

Mr. Mike Messinger
Lexicon Pharmaceuticals
350 Carter Road
Princeton, NJ 08540

                                            Re: 350 Carter Road, Hopewell, NJ

Dear Mike:

     This letter shall confirm that the $2,000,000 Additional T.I. Allowance was
wired to Lexicon Pharmaceuticals (New Jersey), Inc. ("Lexicon"), on the morning
of June 30, 2003. Pursuant to Section 5(h) of the Lease by and between Lexicon
and Hopewell Property, LLC dated May 23, 2002 (as amended, the "Lease"),
effective as of June 30, 2003 (a) the Expiration Date for the initial Term of
the Lease is extended to June 30, 2013 and (b) the Annual Base Rental and
Monthly Base Rental amounts for the balance of the initial Term of the lease are
as follows:

<Table>
<Caption>
----------------------------------------------------------------------------------------------------
                                    Annual Base                         Monthly Base        Sq. Ft.
Lease Year                             Rent           Rent P.S.F.           Rent           Rent Paid
----------------------------------------------------------------------------------------------------
<S>                                <C>                  <C>             <C>                 <C>
June 30, 2003-May 31, 2005
(balance of Lease Year 2 after
rent change - Lease Year 3)        $2,064,738.41        $27.17          $172,061.53         76,000
----------------------------------------------------------------------------------------------------
June 1, 2005-May 31, 2008
(Lease Years 4-6)                  $2,178,738.41        $28.67          $181,561.53         76,000
----------------------------------------------------------------------------------------------------
June 1, 2008-May 31, 2011
(Lease Years 7-9)                  $2,300,338.41        $30.27          $191,694.87         76,000
----------------------------------------------------------------------------------------------------
June 1, 2011-June 30, 2013
(Lease Year 10-Expiration Date)    $2,429,538.41        $31.97          $202,461.53         76,000
----------------------------------------------------------------------------------------------------
</Table>

     Please begin paying in the new Monthly Base Rent amounts beginning with
your rent payment for the month of July, 2003. In addition, please remit
payment in the amount of $956.33, which is the prorated Monthly Base Rent
increase amount for June 30, 2003, the effective date of such increases.
<PAGE>
Mr. Mike Messinger
July 2, 2003
Page Two


     For your convenience, with respect to the Option Purchase Price I have
noted below the corresponding actual dates for the Lease Years set forth in
Exhibit F to the Lease:

-----------------------------------------------------------------------------
Time Period                      Price
-----------------------------------------------------------------------------
Lease Year 2:                    $13,430,000 plus unamortized amounts of the
June 1, 2003 - May 31, 2004      Tenant Improvement Allowance and the
                                 Additional T.I. Allowance actually received
                                 by Tenant
-----------------------------------------------------------------------------
Lease Year 3:                    $13,750,000 plus unamortized amounts of the
June 1, 2004 - May 31, 2005      Tenant Improvement Allowance and the
                                 Additional T.I. Allowance actually received
                                 by Tenant
-----------------------------------------------------------------------------

     Should you have any questions please do not hesitate to call me or our
asset manager, Dianna Wilhelm. All capitalized terms used but not defined in
this letter shall have the meaning given them in the Lease.

                                           Very truly yours,

                                           /s/ Maria Sequeira Rubi

                                           Maria Sequeira Rubi

MSR/lms

CC:  David Townsend
     Dianna Wilhelm
     Steven M. Cohen, Esquire
            Morgan, Lewis & Bockius, LLP
            1701 Market Street
            Philadelphia, PA 19103
     General Counsel
            Lexicon Genetics Incorporated
            8800 Technology Forest Place
            The Woodlands, TX 77393-2167



