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                                                                   EXHIBIT 10.13

[PREDIX PHARMACEUTICALS LOGO]           10 K GILL STREET
                                        Woburn, MA 01801
                                        Telephone: 781-376-0821

September 24, 2004

Stephen R. Donahue, M.D.
[ADDRESS]

Re: Employment Agreement

Dear Steve,

      This letter is to confirm our understanding with respect to your future
employment by Predix Pharmaceutical Holdings Inc. or its parent, subsidiaries or
affiliates (the "Company"). The terms and conditions agreed to in this letter
shall hereinafter be referred to as the "Agreement." In consideration of the
mutual promises and covenants contained in this Agreement, and for other good
and valuable consideration, the receipt and sufficiency of which are hereby
mutually acknowledged, we have agreed as follows:

      1. Employment. Employee will be expected to work Monday through Thursday
at the Company Headquarters (in Woburn or other corporate designated location)
for a period of no more than 8 months from the date of hire. During this period
not to exceed 8 months, employee will be expected to work on Fridays from a
location which he deems appropriate. Thereafter, employee will be expected to
work Monday through Friday at the Company Headquarters (in Woburn or other
corporate designated location)

      2. Commencement Date. This offer for the position of Vice President of
Clinical and Regulatory Affairs reporting to me, Michael Kauffman, CEO and
President, stands open until September 30, 2004. Predix would like you to start
as soon as possible but no later than October 27, 2004. In this position, you
are part of the Executive Team of Predix. You will be expected to attend a
Pre-IND Briefing Meeting with FDA on October 14, 2004, and to be present with
the Predix Team on October 13, 2004, for a rehearsal session.

      3. Employment at Will. Your employment hereunder is terminable by either
party "at will." It is entirely voluntary for both parties and either you or the
Company may terminate the employment relationship at any time for any reason. No
provision of this Agreement shall be deemed to alter the "at will" nature of
this relationship. In the event that employment is terminated without cause by
the Company, employee will be granted six (6) months of severance at the current
salary, including employment benefits during that period. Cause for termination
shall not include economic and restructuring causes.

      4. Compensation. Subject to your acceptance of the position offered to you
upon the terms and conditions herein, the Company will provide you with the
following compensation:

      (a) Salary. You shall be paid from the Company's payroll. For your
services hereunder you shall be paid a base salary of $250,000.00 per year,
which shall be paid in accordance with the Company's payroll practices as in
effect from time to time, less any amounts required to be withheld under
applicable law.

      (b) Moving Allowance: The Employee shall be reimbursed up to $25,000.00
for moving expenses pursuant to the corporate moving policy Section 1
(attached). Please note that all moving reimbursements will be made based on
receipts for moving services rendered pursuant to the Predix Relocation Benefits
Policy.

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      (c) Sign-On Bonus. Signing Bonus - The Employee shall be entitled to a one
time gross signing bonus of twenty five thousand dollars ($25,000.00) due on the
Employment Date.

      (d) Annual Bonus. You will be eligible to participate in the Company's
bonus program, in the amount of up to forty thousand dollars ($40,000.00), which
is based on successful achievement of annual corporate goals and approval by the
Board of Directors.

      Notwithstanding anything contained herein to the contrary, all bonus
payments remain unearned until they become payable in accordance with the
Company's standard policies, practices and procedures. All bonus plans may be
amended, modified or terminated by the Company at any time.

      (e) Stock Options. Subject to the terms and conditions of the Predix
Pharmaceutical Holdings, Inc. Share Option Plan and the execution of an option
agreement executed pursuant thereto with the Employee, the Employee shall be
granted options to purchase 1,090,929 shares of the Company's common stocks, at
an exercise price to be determined within 90 days of the hire date(the
"Options"). The Options granted hereunder shall vest over a period of sixteen
(16) quarters with six and a quarter (6.25) percent of the options become
exercisable every three months beginning three months following the Employment
Date. All stock option grants are contingent upon approval by the Compensation
Committee of the Company's Board of Directors and are priced according to the
grant date, which is your date of hire. The exercise price will be the fair
market value of the stock on the grant date. Notwithstanding the foregoing,
these options shall not be exercisable for any Shares until the date that the
Employee has been employed by the Company for a period of one year. In case the
company merges, acquires and is acquired by another corporation ("M&A") and
there is a change of control in the Company event, the company will accelerate
100% of the vesting in case: (a) The Employee is offered a job with no equal
responsibility and leaves the company within 12 month following the M & A; or
(2) The employee is terminated within 12 month following the M & A; or (3) there
is no rollover of the Company's options into a new option plan following the
M&A.

      Additional option grants to purchase company stock may be considered
annually based upon your satisfaction of performance criteria to be mutually
agreed upon by you and the Company

      5. Employee Benefit Plans. Commencing on your Employment Date, the Company
shall provide the Employee with the option to select hospitalization, medical
and dental insurance pursuant to terms determined by the Company and its
insurer.

      6. Time Off. The Employee shall be entitled to twenty days of paid time
off (WYN days: When You Need It) for reasons determined by the employee during
each calendar year. Five (5) days of unused "as you need it" time may be accrued
and carried over into the next year for an annual total not to exceed 25 days.
The Employee shall also be entitled to paid time off on all holidays recognized
by the Company.

      7. No Conflicting Agreements. You hereby represent and warrant that you
have no commitments or obligations inconsistent with this Agreement and you
hereby agree to indemnify and hold the Company harmless against loss, damage,
liability or expense arising from any claim based upon circumstances alleged to
be inconsistent with such representation and warranty.

      8. Protected Information. As used in this Agreement, the term "Inventions"
means all ideas, discoveries, creations, manuscripts and properties,
innovations, improvements, know-how, inventions, designs, developments,
apparatus, techniques, methods, biological processes, cell lines, laboratory
notebooks and formulae, whether or not patentable or copyrightable, including
all rights to obtain, register, perfect and enforce these proprietary interests.
You shall at all times, both during and after any termination of this Agreement
by either you or the Company, maintain in confidence and shall not, without the
prior written consent of the Company, use, except in the course of performance
of your duties for the Company,

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disclose or give to others any fact or information which was disclosed to or
developed by you during the course of performing services for, or receiving
training from, the Company, and which is not generally available to the public,
including but not limited to information and facts concerning business plans,
customers, future customers, suppliers, licensors, licensees, partners,
investors, affiliates or others, training methods and materials, financial
information, sales prospects, client lists, Inventions, or any other scientific,
technical, trade or business secret or confidential or proprietary information
of the Company or of any third party provided to you during the Term. In the
event you are questioned by anyone not employed by the Company or by an employee
of or a consultant to the Company not authorized to receive such information, in
regard to any such information or any other secret or confidential work of the
Company, or concerning any fact or circumstance relating thereto, you will
promptly notify the president of the Company.

      9. Prohibited Competition.

      (a) Certain Acknowledgments and Agreements.

            (i) You recognize and acknowledge the competitive and proprietary
nature of the Company's business operations. You acknowledge and agree that a
business will be deemed competitive with the Company if it engages in a line of
business in which it performs any of the services, conducts research, or
develops, manufactures or sells any products provided or offered by the Company
or under development by the Company, or any similar products or products
fulfilling the same function, whether or not similar, in the Field of Interest
(such business to be referred to as a "competitive business"). The term "Field
of Interest" currently means the field of diagnosis, prevention or treatment of
diseases or conditions in humans relating to central nervous system and diseases
treated with serotonergic agents. You further understand that the Company may
expand the definition of its Field of Interest at any time by action of its
Board of Directors, which new definition will be binding upon you ten (10) days
after written notice to you of such change.

            (ii) You further acknowledge and agree that during the course of
performing services for the Company, the Company will furnish, disclose or make
available to you confidential and proprietary information related to the
Company's business. You also acknowledge that such confidential information to
be provided by the Company has been developed and will be developed by the
Company through the expenditure by the Company of substantial time, effort and
money and that all such confidential information could be used by you or a
competitive business to compete with the Company.

      (b) Covenants Not to Compete. During the period in which you are employed
by the Company (the "Term") and for a period of one year following the
expiration or termination of the Term, whether such termination is voluntary or
involuntary, you shall not, without the prior written consent of the Company:

            (i) for yourself or on behalf of any other person or entity,
directly or indirectly, either as principal, agent, stockholder, employee,
consultant, representative or in any other capacity, own, manage, operate or
control, or be concerned, connected or employed by, or otherwise associate in
any manner with, engage in or have a financial interest in any competitive
business anywhere in the world (the "Restricted Territory"), except that nothing
contained herein shall preclude you from purchasing or owning securities of any
such business if such securities are publicly traded, and provided that your
holdings do not exceed one (1 %) percent of the issued and outstanding
securities of any class of securities of such business; or

            (ii) either individually or on behalf of or through any third party,
solicit, divert or appropriate or attempt to solicit, divert or appropriate, for
the purpose of competing with the Company or any present or future parent,
subsidiary or other affiliate of the Company which is engaged in a similar
business as the Company, any customers or patrons of the Company, or any
prospective customers or patrons with respect to which the Company has developed
or made a sales presentation (or similar offering

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of services), located within the Restricted Territory; or

            (iii) either individually or on behalf of or through any third
party, directly or indirectly, solicit, entice or persuade or attempt to
solicit, entice or persuade any other employees of or consultants to the Company
or any present or future parent, subsidiary or affiliate of the Company to leave
the services of the Company or any such parent, subsidiary or affiliate for any
reason.

      (c) Reasonableness of Restrictions. You further recognize and acknowledge
that (i) the types of employment which are prohibited by this Section 9 are
reasonable in relation to the skills which represent your principal salable
asset both to the Company and to your other prospective employers, and (ii) the
geographical scope of the provisions of this Section 9 is reasonable, legitimate
and fair to you in light of the Company's need to market its services and sell
its products worldwide in order to have a sufficient customer base to make the
Company's business profitable and in light of the limited restrictions on the
type of employment prohibited herein compared to the types of employment for
which you are qualified to earn your livelihood.

      (d) Survival of Acknowledgments and Agreements. Your acknowledgments and
agreements set forth in this Section 9 shall survive the expiration or
termination of this Agreement and the termination of your employment with the
Company for any reason.

10. Ownership of Ideas, Copyrights and Patents.

      (a) Property of the Company. You agree that all inventions which you may
conceive, reduce to practice or develop during the Term (or, if based on or
related to any confidential or proprietary information of the Company, within
one (1) year after the termination of such employment), alone or in conjunction
with another, or others, whether during or out of regular business hours, and
whether at the request or upon the suggestion of the Company, or otherwise,
shall be the sole and exclusive property of the Company (the "Company
inventions"), that you will promptly disclose all such inventions to the
Company, and that you shall not publish any of the Company inventions without
the prior written consent of the Company. You hereby assign to the Company all
of your right, title and interest in and to all of the foregoing. You further
agree to use your best efforts to ensure that none of the Company Inventions
will violate or infringe upon any right, patent, copyright, trademark or right
of privacy, or constitute libel or slander against or violate any other rights
of any person, firm or corporation.

You have attached hereto as Schedule A (i) a list of all inventions in which you
have any right, title, or interest as of the date of your execution of this
Agreement ("Other inventions"), and (ii) a list of all Other inventions made,
conceived, or developed, in whole or in part, by you prior to the date of your
execution of this Agreement, and you represent that Schedule A is a complete and
accurate list of all Other inventions.

      (b) Cooperation. You agree that you will fully cooperate with the Company,
its attorneys and agents in the preparation and filing of all papers and other
documents as may be required to perfect the Company's rights in and to any of
such Company Inventions, including, but not limited to, performing all acts
deemed necessary or desirable by the Company (both during and after your
employment with the Company) and joining in any proceeding to obtain letters
patent, copyrights, trademarks or other legal rights of the United States and of
any and all other countries on such Company Inventions; provided, that, the
Company will bear the expense of all such proceedings. You hereby agree that any
patent or other legal right covering any Company Invention issued to you
personally, shall be assigned by you to the Company without charge by you.

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      (c) Works Made For Hire. Without limiting the foregoing, you further
acknowledge that all original works of authorship made by you, whether alone or
jointly with others within the scope of your employment with the Company and
which are protectable by copyright are "works made for hire" within the meaning
of the United States Copyright Act, 17 U.S.C. Section 101, as amended, the
copyright of which shall be owned solely, completely and exclusively by the
Company. If any Company invention is considered to be work not included in the
categories of work covered by the United States Copyright Act, 17 U.S.C. Section
101, as amended, such work shall be owned solely by, or hereby assigned or
transferred completely and exclusively to, the Company.

      11. Disclosure to Future Employers. You agree that you will provide, and
that the Company may similarly provide in its discretion, a copy of the
covenants contained in Sections 8, 9 and 10 of this Agreement to any business or
enterprise which you may directly, or indirectly, own, manage, operate, finance,
join, control or in which you participate through the ownership, management,
operation, financing, or control, or with which you may be connected as an
officer, director, employee, partner, principal, agent, representative,
consultant or otherwise.

      12. Representations Regarding Prior Work and Legal Obligations.

      (a) You represent that you have no agreement or other legal obligation
with any prior employer or any other person or entity that restricts your
ability to engage in employment discussions with, employment with, or to perform
any function for the Company.

      (b) You represent that you have been advised by the Company that at no
time should you divulge to or use for the benefit of the Company any trade
secret or confidential or proprietary information of any previous employer. You
acknowledge that you have not divulged or used any such information for the
benefit of the Company.

      (c) You represent that you have not and will not misappropriate any Other
Invention that you played any part in creating while working for any former
employer.

      (d) You acknowledge that the Company is basing important business
decisions on these representations, and affirm that all of the statements
included herein are true.

      13. Records. Upon termination of your relationship with the Company, you
shall deliver to the Company any property of the Company which may be in your
possession including products, materials, memoranda, notes, records, reports,
writings, drawings, diskettes, models and other materials or other documents or
photocopies of the same in any tangible form whatsoever constituting
confidential or proprietary information of the Company, and any of the foregoing
in intangible form.

      14. General.

      (a) Notices. All notices, requests, consents and other communications
hereunder shall be in writing, shall be addressed to the receiving party's
address set forth below or to such other address as a party may designate by
notice hereunder, and shall be either (i) delivered by hand, (ii) made by telex,
telecopy or facsimile transmission, (iii) sent by overnight courier, or (iv)
sent by registered mail, return receipt requested, postage prepaid.

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      If to the Company:

      Predix Pharmaceutical Holdings, Inc.
      10 K Gill Street
      Woburn, MA 01801 Attention: President & CEO Telephone: 781.376.0821
      Facsimile: 781.376.0822

      If to you:

      To the address set forth on the signature page of this Agreement.

      All notices, requests, consents and other communications hereunder shall
be deemed to have been given either (i) if by hand, at the time of the delivery
thereof to the receiving party at the address of such party set forth above,
(ii) if made by telex, te1ecopy or facsimile transmission, at the time that
receipt thereof has been acknowledged by electronic confirmation or otherwise,
(iii) if sent by overnight courier, on the next business day following the day
such notice is delivered to the courier service, or (iv) if sent by registered
mail, on the fifth business day following the day such mailing is made.

      (b) Entire Agreement. This Agreement embodies the entire agreement and
understanding between the parties hereto with respect to the subject matter
hereof and supersedes all prior oral or written agreements and understandings
relating to the subject matter hereof. No statement, representation, warranty,
covenant or agreement of any kind not expressly set forth in this Agreement
shall affect, or be used to interpret, change or restrict, the express terms and
provisions of this Agreement.

      (c) Modifications and Amendments. The terms and provisions of this
Agreement may be modified or amended only by written agreement executed by the
parties hereto.

      (d) Waivers and Consents. The terms and provisions of this Agreement may
be waived, or consent for the departure there for granted, only by written
document executed by the party entitled to the benefits of such terms or
provisions. No such waiver or consent shall be deemed to be or shall constitute
a waiver or consent with respect to any other terms or provisions of this
Agreement, whether or not similar. Each such waiver or consent shall be
effective only in the specific instance and for the purpose for which it was
given, and shall not constitute a continuing waiver or consent.

      (e) Assignment. The Company may assign its rights and obligations
hereunder to any person or entity that succeeds to all or substantially all of
the Company's business or that aspect of the Company's business in which you are
principally involved. Your rights and obligations under this Agreement may not
be assigned by you without the prior written consent of the Company.

      (f) Benefit. All statements, representations, warranties, covenants and
agreements of this Agreement shall be binding on the parties hereto and shall
inure to the benefit of the respective successors and permitted assigns of each
party hereto. Nothing in this Agreement shall be construed to create any rights
or obligations except between the parties hereto, and no person or entity shall
be regarded as a third-party beneficiary of this Agreement, other than any
future parent, subsidiary or affiliate of the Company.

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      (g) Governing Law. This Agreement and the rights and obligations of the
parties hereunder shall be construed in accordance with and governed by the law
of the Commonwealth of Massachusetts, without giving effect to the conflict of
law principles thereof.

      (h) Jurisdiction and Service of Process. Any legal action or proceeding
with respect to this Agreement may be brought solely in the courts of the
Commonwealth of Massachusetts or of the United States of America for the
District of Massachusetts. By execution and delivery of this Agreement, each of
the parties hereto accepts for itself and in respect of its property, generally
and unconditionally, the jurisdiction of the aforesaid courts. Each of the
parties hereto irrevocably consents to the service of process of any of the
aforementioned courts in any such action or proceeding by the mailing of copies
thereof by certified mail, postage prepaid, to the party at its address set
forth in Section 13(a) hereof.

      (i) Severability. The parties intend this Agreement to be enforced as
written. However, (i) if any portion or provision of this Agreement shall to any
extent be declared illegal or unenforceable by a duly authorized court having
jurisdiction, then the remainder of this Agreement, or the application of such
portion or provision in circumstances other than those as to which it is so
declared illegal or unenforceable, shall not be affected thereby, and each
portion and provision of this Agreement shall be valid and enforceable to the
fullest extent permitted by law; and (ii) if any provision, or part thereof, is
held to be unenforceable because of the duration of such provision or the
geographic area covered thereby, the Company and you agree that the court making
such determination shall have the power to reduce the duration and/or geographic
area of such provision, and/or to delete specific words and phrases
("blue-penciling"), and in its reduced or blue-penciled form such provision
shall then be enforceable and shall be enforced.

      (j) Headings and Captions. The headings and captions of the various
subdivisions of this Agreement are for convenience of reference only and shall
in no way modify, or affect the meaning or construction of any of the terms or
provisions hereof.

      (k) Injunctive Relief. You hereby expressly acknowledge that any breach or
threatened breach of any of the terms and/or conditions set forth in this
Agreement will result in substantial, continuing and irreparable injury to the
Company. Therefore, you hereby agree that, in addition to any other remedy that
may be available to the Company, the Company shall be entitled to injunctive or
other equitable relief by a court of appropriate jurisdiction in the event of
any breach or threatened breach of the terms or conditions of this Agreement.
The seeking of such injunction or order shall not affect the Company's right to
seek and obtain damages or other equitable relief on account of any such actual
or threatened breach. Nothing in this paragraph shall be construed to limit the
remedies for any actual or threatened breach that would otherwise be available
to the Company.

      (l) No Waiver of Rights, Powers and Remedies. No failure or delay by a
party hereto in exercising any right, power or remedy under this Agreement, and
no course of dealing between the parties hereto, shall operate as a waiver of
any such right, power or remedy of the party. No single or partial exercise of
any right, power or remedy under this Agreement by a party hereto, nor any
abandonment or discontinuance of steps to enforce any such right, power or
remedy, shall preclude such party from any other or further exercise thereof or
the exercise of any other right, power or remedy hereunder. The election of any
remedy by a party hereto shall not constitute a waiver of the right of such
party to pursue other

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available remedies. No notice to or demand on a party not expressly required
under this Agreement shall entitle the party receiving such notice or demand to
any other or further notice or demand in similar or other circumstances or
constitute a waiver of the rights of the party giving such notice or demand to
any other or further action in any circumstances without such notice or demand.

      (m) Counterparts. This Agreement may be executed in one or more
counterparts, and by different parties hereto on separate counterparts, each of
which shall be deemed an original, but all of which together shall constitute
one and the same instrument.

Yours very truly,

PREDIX PHARMACEUTICAL HOLDINGS, INC.

/s/ MICHAEL G. KAUFFMAN
-----------------------
Michael G. Kauffman, M.D., Ph.D.
President and CEO

September 24, 2004
------------------
Date

ACCEPTED:

/s/ STEPHEN DONAHUE
---------------------
Dr. Stephen R. Donahue

September 27, 2004
------------------
Date

ADDRESS:
[ADDRESS]

Please sign both originals of this Employment Agreement, keep one copy for your
   files and return a copy to Chris Wang at Predix Pharmaceuticals along with
          completed form # F-CHR002 and a signed copy of the Relocation
                               Payback Agreement.

Enclosures:   Predix Relocation Benefits Policy
              Predix Relocation Payback Agreement
              New Employee Form F-CHR002
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                                   SCHEDULE A

                                   INVENTIONS