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                                                                    Exhibit 10.4

                                 ARCHEMIX CORP.
                               1 Hampshire Street
                               Cambridge, MA 02139

November 14, 2003

Mr. Gregg Beloff
91 Middle Road
Southborough, MA 01772

Dear Gregg:

     I am very pleased to provide you with a summary of the terms and conditions
of your anticipated employment by Archemix Corp. ("Archemix"). The following
sets forth the proposed terms and conditions of your offer of employment. We
hope that you choose to join the Archemix team and look forward to a mutually
beneficial relationship.

1. POSITION Your initial position will be Chief Financial Officer, reporting to
Dr. Errol De Souza, President and Chief Executive Officer. As you progress with
Archemix, your position and assignments are, of course, subject to change. As an
Archemix employee, we expect that you will perform any and all duties and
responsibilities normally associated with your position in a satisfactory manner
and to the best of your abilities at all times. Your performance will be
reviewed formally sometime around December, 2004, and on a periodic basis
thereafter as long as you remain employed by Archemix.

2. JOB DESCRIPTION As Chief Financial Officer, you will have the customary
responsibilities associated with this role which include the development of a
financial strategy, fund raising responsibilities, establishing and maintaining
internal financial controls, establishing and maintaining the corporate budget,
as well as Sarbanes-Oxley Act of 2002 compliance, regulatory compliance, and SEC
filings.

2. STARTING DATE/NATURE OF RELATIONSHIP If you accept this offer, your
employment with Archemix will begin on or before DECEMBER 15, 2004. You should
report to Archemix on that date, and you will be expected to devote all of your
working time to the performance of your duties at Archemix throughout your
employment with Archemix. No provision of this letter shall be construed to
create an express or implied employment contract, or a promise of employment for
any specific period of time. Your employment with Archemix is at-will
employment, which may be terminated by you or Archemix at any time for any
reason.

3. COMPENSATION AND BENEFITS Your initial base pay shall be at a rate of
$15,833.33 per month, minus customary deductions for federal and state taxes and
the like. You

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will be eligible for an annual bonus target equal to 25% of your base salary,
based on overall company and individual performance. Potential bonus amounts may
be compensated in the form of equity or cash or a combination of each and are
subject to approval by the Board of Directors (or an appropriate Committee
appointed by the Board of Directors). Archemix will grant you 350,000 incentive
stock options in Archemix, pursuant to the terms of a formal stock agreement and
option plan. It is anticipated that the Board of Directors will grant such
options within sixty (60) days following your start date with Archemix.

     In addition to your compensation, you may take advantage of various
benefits offered by Archemix, including group medical plans and paid vacation
time. Currently, Archemix provides fifteen vacation days per year, accruable at
the rate of 1 1/4 days per month. In addition Archemix recognizes 8 scheduled
and 3 floating holidays per calendar year. These benefits, of course, may be
modified or changed from time to time at the sole discretion of Archemix, and
the provision of such benefits to you in no way changes or impacts your status
as an at-will employee. Archemix's present benefit structure and other important
information about the benefits for which you may be eligible will be described
to you upon the commencement of your employment. Where a particular benefit is
subject to a formal plan (for example, medical insurance or life insurance),
eligibility to participate in and receive any particular benefit is governed
solely by the applicable plan document. Should you ever have any questions about
Archemix benefits, you should ask the Human Resources Director for a copy of the
applicable plan document.

4. BEFORE YOU START Your employment with Archemix is conditioned on your
eligibility to work in the United States. On your first day, you must complete
an I-9 Form and provide Archemix with any of the accepted forms of
identification specified on the I-9 Form.

5. CONFIDENTIALITY Archemix considers the protection of its confidential
information, proprietary materials and goodwill to be extremely important. Given
the confidential nature of various aspects of our business, you may not discuss
the fact or terms of this offer or any employment discussions with anyone other
than a member of the Archemix management team and members of your immediate
family (and, if relevant, your recruiting firm, financial advisor or lawyer). In
addition, as is true of all Archemix employees, you will be required to sign an
agreement relating to confidentiality, non-competition, non-solicitation and
work product on your first day of work as a condition of this offer of
employment.

6. MISCELLANEOUS This letter constitutes our entire offer regarding the terms
and conditions of your prospective employment with Archemix. It supersedes any
prior agreements, or other promises or statements (whether oral or written)
regarding the offered terms of employment. The terms of your employment shall be
governed by the law of the Commonwealth of Massachusetts. By accepting this
offer of employment, you agree that any action, demand, claim or counterclaim in
connection with any aspect of your employment with Archemix, or any separation
of employment (whether voluntary or involuntary) from Archemix, shall be
resolved by a judge alone, and you waive and forever renounce your right to a
trial before a civil jury.

     You may accept this offer of employment and the terms and conditions hereof
by signing the enclosed additional copy of this letter. Your signature on the
copy of this letter and your


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submission of the signed copy to me will evidence your agreement with the terms
and conditions set forth herein. This offer will expire on NOVEMBER 21, 2003,
unless accepted by you prior to such date.

     We are excited to offer you the opportunity to join Archemix, and we look
forward to having you aboard. We are pleased that you have chosen to join the
Archemix team, and confident that you will make an important contribution to our
unique and exciting enterprise.

                                        Sincerely,


                                        Errol De Souza
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                                        President and Chief Executive Officer

Acknowledgment:

     I ____________________________ have read, understand, and accept employment
on the terms and conditions outlined in this letter. I am not relying on any
representations made to me by anyone other than as set forth above.


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Signature

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Date


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