UNITED STATES
                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549

                                  SCHEDULE 13D

                    Under the Securities Exchange Act of 1934
                                (Amendment No. 1)

                                  REPLIGEN CORP
                      ------------------------------------
                                (Name of Issuer)

                          Common Stock, $.01 Par Value
                    -----------------------------------------
                         (Title of Class of Securities)

                                   759916 10 9
                                  ------------
                                 (CUSIP Number)

                              Howard Friedman, Esq.
                   6745 N. Kilpatrick, Lincolnwood, IL 60712
                                  847-983-4950
                            -------------------------
                  (Name, Address and Telephone Number of Person
                Authorized to Receive Notices and Communications)

                                July 13, 2007
                                -----------------
             (Date of Event which Requires Filing of this Statement)

   If the filing person has previously filed a statement on Schedule 13G to
   report the acquisition that is the subject of this Schedule 13D, and is
   filing this schedule because of Rule 13d-1(e), 13d-1(f) or 13d-1(g), check
   the following box. [ ]

   Note: Schedules filed in paper format shall include a signed original and
   five copies of the schedule, including all exhibits. See Rule 13d-7 for other
   parties to whom copies are to be sent.

   * The remainder of this cover page shall be filled out for a reporting
   person's initial filing on this form with respect to the subject class of
   securities, and for any subsequent amendment containing information which
   would alter disclosures provided in a prior cover page.

   The information required on the remainder of this cover page shall not be
   deemed to be "filed" for the purpose of Section 18 of the Securities Exchange
   Act of 1934 ("Act") or otherwise subject to the liabilities of that section
   of the Act but shall be subject to all other provisions of the Act (however,
   see the Notes).




CUSIP No.         759916 10 9
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                1. Names of Reporting Persons.
                   I.R.S. Identification Nos. of above persons (entities only).

                   Individual Retirement Accounts for the benefit of
                   Ronald L. Chez and Ronald L. Chez Individually

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                2. Check the Appropriate Box if a Member of a Group (See
                   Instructions)
                   (a)     [ ]
                   (b)     [ ]

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                3. SEC Use Only

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                4. Source of Funds (See Instructions)                        PF

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                5. Check if Disclosure of Legal Proceedings Is Required Pursuant
                   to Items 2(d) or 2(e)    N/A

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                6. Citizenship or Place of Organization
                                                        United States of America

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Number of
Shares
Beneficially
Owned by
Each
Reporting
Person With

                        7. Sole Voting Power                           1,926,906

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                        8. Shared Voting Power                               -0-

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                        9. Sole Dispositive Power                      1,926,906

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                       10. Shared Dispositive Power                          -0-

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               11. Aggregate Amount Beneficially Owned by
                   Each Reporting Person                               1,926,906

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               12. Check if the Aggregate Amount in Row (11) Excludes Certain
                   Shares (See Instructions)  N/A

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               13. Percent of Class Represented by Amount in Row (11)       6.3%

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               14. Type of Reporting Person (See Instructions)
                                                                              IN

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CUSIP No. 759916109                                               Page 3 of 6

     Pursuant to Rule 13d-1 of Regulation 13D-G of the General Rules and
Regulations under the Securities Exchange Act of 1934, as amended (the "Act"),
the undersigned Ronald L. Chez (the "Reporting Person") hereby files this
Amendment No.1 to Schedule 13D.

     Unless otherwise indicated herein all capitalized terms used herein shall
have the same meanings ascribed to them in the original Schedule 13D filed by
this Reporting Person with report to this Issuer. Also unless otherwise
indicated herein, there are no material changes in the information set forth in
the original Schedule 13D filed by this Reporting Person except for the number
of shares now owned and the effect thereof on percentages and similar matters.

Item 1.     Security and Issuer.

     This statement relates to the Common Stock, $.01 Par Value (the Stock") of
Repligen Corporation. a Delaware corporation (the "Issuer"), 41 Seyon Street,
Building #1, Suite 100, Waltham, Massachusetts 02453.

Item 2      Identity and Background.

     (a), (b), (c)

     The Reporting Person is an individual whose principal occupation is an
investor and his business address is 1524 N. Astor St. Chicago, IL 60610.

     (d) The Reporting Person has not, during the last five years, been
convicted in a criminal proceeding (excluding traffic violations or similar
misdemeanors).

     (e) The Reporting Person has not, during the last five years, been a party
to a civil proceeding of a judicial or administrative body of competent
jurisdiction and as a result of such proceeding was or is subject to a judgment,
decree or final order enjoining future violations of, or prohibiting or
mandating activities subject to, federal or state securities laws or finding any
violation with respect to such laws.

     (f) The Reporting Person is a citizen of the United States of America.

Item 3.     Source and Amount of Funds or other Consideration.

     The source and amount of funds used by the Reporting Person to purchase
shares of the Stock that require the filing of this Schedule 13D are personal
funds including amounts held by individual retirement accounts for the benefit
of the Reporting Person.

Item 4.     Purposes of Transaction.

     The shares of the Stock of the Issuer to which this statement relates have
been acquired by the Reporting Person for investment purposes. The Reporting
Person has been pleased with the progress of the Issuer. It is possible that at
a future date the Reporting Person might decide to seek a more active role in
management and/or to sell shares of the Stock or to acquire



CUSIP No. 759916109                                               Page 4 of 6


additional shares of the Stock through open market or privately negotiated
transactions. Any such future decisions will be made by the Reporting Person in
light of the then current financial conditions and prospects of the Issuer, the
market value of the Stock, the financial condition of the Reporting Person and
other relevant factors. Except as noted above, the Reporting Person has not
formulated any plans or proposals which relate to or would result in:

     (a) The acquisition by any person of additional securities of the Issuer,
or the disposition of securities of the Issuer;

     (b) An extraordinary corporate transaction, such as a merger,
reorganization or liquidation, involving the Issuer or any of its subsidiaries;

     (c) A sale or transfer of a material amount of assets of the Issuer or any
of its subsidiaries;

     (d) Any change in the present board of directors or to fill any existing
vacancies on the board;

     (e) Any material change in the present capitalization or dividend policy of
the Issuer;

     (f) Any other material change in the Issuer's business or corporate
structure including but not limited to, if the Issuer is a registered closed-end
investment company, any plans or proposals to make any changes in its investment
policy for which a vote is required by Section 13 of the Investment Company Act
of 1940;

     (g) Changes in the Issuer's charter, bylaws or instruments corresponding
thereto or other actions which may impede the acquisition of control of the
Issuer by any person;

     (h) Causing a class of securities of the Issuer to be delisted from a
national securities exchange or to cease to be authorized to be quoted in an
inter-dealer quotation system of a registered national securities association;

     (i) A class of equity securities of the Issuer becoming eligible for
termination of registration pursuant to Section 12(g)(4) of the Act; or

     (j) Any action similar to any of those enumerated above.

            However, the Reporting Person may at some future date propose any of
the foregoing changes or actions which he hereafter considers desirable in light
of his examination of the Issuer and its assets and operations, future prospects
and of the circumstances prevailing at the time.




CUSIP No. 759916109                                               Page 5 of 6


Item 5.     Interest in the Securities of the Issuer.

     (a) The aggregate number of shares of the Stock owned beneficially by the
Reporting Person is 1,926,906 (the "Shares") constituting approximately 6.3% of
the outstanding shares of the Stock. The percentages in this Item 5(a) are based
on 30,477,635 shares of the Stock outstanding, as reported in the Issuer's Form
10K filed on June 7, 2007.

     (b) The Reporting Person has the sole power (and no shared power) to vote
or dispose of or direct the disposition of Shares owned by such Reporting
Person.

     (c) Except as described immediately below (and as set forth in Exhibit A
hereto), the Reporting Person has not effected any transaction in shares of the
Stock during the 61 days prior to and including July 13, 2007. The Reporting
Person purchased Shares of Stock as follows:

     Please see Exhibit A hereto entitled: Exhibit A to Amendment No.1 to
Schedule 13D Issuer Repligen Corporation, Reporting Person Ronald L. Chez.

     All purchases were made in the open market. The Reporting Person had
previously owned 1,833,508 shares of Stock, all acquired in the open market at
various prices more than 61 days prior to and including July 13, 2007.

     (d) No person other than the Reporting Person has the right to receive or
the power to direct the receipt of dividends from, or the proceeds from the sale
of, the Shares owned by such Reporting Person.

     (e) It is inapplicable, for purposes of this statement, to state the date
on which the Reporting Person ceased to be the owner of more than 5% of the
Shares of the Stock of the Issuer.

Item 6.     Contracts, Arrangements, Understanding or Relationships with
            Respect to Securities of the issuer.

            None




CUSIP No. 759916109                                               Page 6 of 6


Item 7.     Material to be Filed as Exhibits.

            Exhibit A is schedule of Shares of Stock purchased by the Reporting
Person within the 61 days prior to and including July 13, 2007.

            After reasonable inquiry and to the best of my knowledge and belief,
I certify that the information set forth in this statement is true, complete and
correct.

            Dated:    July 16, 2007

                                             /s/ Ronald L. Chez
                                             ------------------------------
                                             Ronald L. Chez



   EXHIBIT A TO SCHEDULE 13D ISSUER REPLIGEN CORPORATION
   REPORTING PERSON RONALD L. CHEZ (SS# ###-##-####)

                              Approximate
Purchase           No of         Cost              Total
   Date           Shares       Per Share           Cost



05/15/2007         2,000        $   3.51       $   7,023.80
05/15/2007         1,000        $   3.51       $   3,509.90
05/15/2007         1,000        $   3.52       $   3,517.00
05/31/2007         1,500        $   3.91       $   5,863.95
05/31/2007         1,500        $   3.91       $   5,865.00
05/31/2007           878        $   3.90       $   3,424.20
05/31/2007         1,600        $   3.88       $   6,212.00
06/01/2007         1,370        $   3.70       $   5,073.00
06/01/2007        10,500        $   3.86       $  40,535.25
06/04/2007         2,000        $   3.70       $   7,399.80
06/04/2007         1,800        $   3.70       $   6,660.70
06/04/2007           200        $   3.71       $     742.00
06/07/2007           200        $   3.53       $     705.50
06/15/2007         2,500        $   3.96       $   9,899.00
07/11/2007        10,000        $   3.99       $  39,876.50
07/13/2007        25,000        $   4.01       $ 100,250.00
07/16/2007        25,350        $   4.07       $ 103,174.50
07/16/2007         5,000        $   4.12       $  20,600.00