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


                                  SCHEDULE 13G


                    Under the Securities Exchange Act of 1934
                               (Amendment No. 2)*



                              REPLIGEN CORPORATION
--------------------------------------------------------------------------------
                                (Name of Issuer)

                    COMMON STOCK, $0.01 PAR VALUE PER SHARE
--------------------------------------------------------------------------------
                         (Title of Class of Securities)

                                    759916109
--------------------------------------------------------------------------------
                                 (CUSIP Number)

                                December 31, 2006
--------------------------------------------------------------------------------
             (Date of Event Which Requires Filing of this Statement)


Check the appropriate box to designate the rule pursuant to  which this Schedule
13G is filed:

         [ ] Rule 13d-1(b)
         [X] Rule 13d-1(c)
         [ ] Rule 13d-1(d)





*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 the
disclosures provided in a prior cover page.

The information required in 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.   759916109
--------------------------------------------------------------------------------

(1)  Names of Reporting Persons.   I.R.S.  Identification  Nos. of Above Persons
     (entities only):

              Xmark Opportunity Partners, LLC
              20-2052197
--------------------------------------------------------------------------------

(2)  Check the Appropriate Box if a Member of a Group (See Instructions)
        (a)
        (b)
--------------------------------------------------------------------------------

(3)     SEC Use Only
--------------------------------------------------------------------------------

(4)     Citizenship or Place of Organization:  Delaware, United States
--------------------------------------------------------------------------------
Number of Shares Beneficially Owned
   by Each Reporting Person With       (5) Sole Voting Power:        1,675,645**
                                           -------------------------------------
                                       (6) Shared Voting Power:               **
                                           -------------------------------------
                                       (7) Sole Dispositive Power:   1,675,645**
                                           -------------------------------------
                                       (8) Shared Dispositive Power:          **
                                           -------------------------------------
--------------------------------------------------------------------------------
 (9) Aggregate Amount Beneficially Owned by Each Reporting Person:

           1,675,645**
--------------------------------------------------------------------------------

(10) Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See
     Instructions):   **
--------------------------------------------------------------------------------

(11) Percent of Class Represented by Amount in Row (9):  5.5%**
--------------------------------------------------------------------------------

(12) Type of Reporting Person (See Instructions): IA
--------------------------------------------------------------------------------
** Xmark Opportunity Partners,  LLC ("Opportunity  Partners") is the sole member
of the investment  manager of Xmark  Opportunity  Fund, L.P., a Delaware limited
partnership  ("Opportunity  LP"),  and Xmark  Opportunity  Fund,  Ltd., a Cayman
Islands exempted company ("Opportunity Ltd"), and, as such, possesses sole power
to vote and direct the disposition of all securities of Repligen Corporation,  a
Delaware  corporation  (the  "Company"),  held by Opportunity LP and Opportunity
Ltd.  Opportunity  Partners  is the  investment  manager of Xmark JV  Investment
Partners,  LLC, a Delaware limited  liability  company ("JV Partners"),  and, as
such,  possesses sole power to vote and direct the disposition of all securities
of the Company held by JV Partners.  David C. Cavalier and Mitchell D. Kaye, the
Chief Operating  Officer and Chief  Executive  Officer,  respectively,  of Xmark
Capital Partners, LLC, the Managing Member of Opportunity Partners, share voting
and  investment  power with  respect  to all  securities  beneficially  owned by
Opportunity Partners.

As of December 31, 2006,  Opportunity LP held 357,590  common shares,  $0.01 par
value per share (the  "Common  Shares"),  of the Company,  Opportunity  Ltd held
633,918  Common Shares of the Company and JV Partners held 684,137 Common Shares
of the Company.





Based on  information  from the Company,  as of December  31,  2006,  there were
30,395,635  Common Shares of the Company issued and outstanding.  As a result of
the foregoing,  for purposes of Reg. Section 240.13d-3,  Opportunity Partners is
deemed to beneficially  own 1,675,645  Common Shares of the Company,  or 5.5% of
the Common Shares of the Company  deemed issued and  outstanding  as of December
31, 2006.  Opportunity  Partners' interest in the securities  reported herein is
limited to the extent of its pecuniary  interest in Opportunity LP,  Opportunity
Ltd and JV Partners, if any.











Item 1(a).  Name Of Issuer:  Repligen Corporation

Item 1(b).  Address of  Issuer's  Principal Executive Offices:

            41 Seyon Street, Building #1, Suite 100
            Waltham, MA 02453


Item 2(a).  Name of Person Filing:

            Xmark Opportunity Partners, LLC

Item 2(b).  Address of Principal Business Office or, if None, Residence:

            301 Tresser Boulevard, Suite 1320
            Stamford, CT 06901

Item 2(c).  Citizenship:

            Xmark Opportunity Partners, LLC is a Delaware limited liability
            company.

Item 2(d).  Title of Class of Securities:  Common Stock, $0.01 par value
            per share

Item 2(e).  CUSIP No.:  759916109


Item 3.     If This Statement Is Filed Pursuant to Sections 240.13d-1(b) or
            240.13d-2(b) or (c), check whether the Person Filing is a:

            Not Applicable.


Item 4.  Ownership

         (a) Amount Beneficially Owned:

             1,675,645**


         (b) Percent of Class:

             5.5%**


         (c) Number of Shares as to which the person has:

             (i)  sole power to vote or to direct the vote           1,675,645**

             (ii) shared power to vote or to direct the vote                  **

            (iii) sole power to dispose or to direct the
                  disposition of                                     1,675,645**





             (iv) shared power to dispose or to direct the
                  disposition of                                              **


Item 5.  Ownership of Five Percent or Less of a Class

          If this  statement  is being  filed to report  the fact that as of the
date  hereof the  reporting  person has ceased to be the owner of more than five
percent of the class of securities, check the following: [ ]


Item 6.  Ownership of More Than Five Percent on Behalf of Another Person

         Not Applicable.


Item 7.  Identification  and  Classification  of the  Subsidiary Which  Acquired
         the Security Being Reported on by the Parent Holding Company or Control
         Person

         Not Applicable.


Item 8.  Identification and Classification of Members of the Group

         Not Applicable.


Item 9.  Notice of Dissolution of Group

         Not Applicable.


-------------------
** Xmark Opportunity Partners,  LLC ("Opportunity  Partners") is the sole member
of the investment  manager of Xmark  Opportunity  Fund, L.P., a Delaware limited
partnership  ("Opportunity  LP"),  and Xmark  Opportunity  Fund,  Ltd., a Cayman
Islands exempted company ("Opportunity Ltd"), and, as such, possesses sole power
to vote and direct the disposition of all securities of Repligen Corporation,  a
Delaware  corporation  (the  "Company"),  held by Opportunity LP and Opportunity
Ltd.  Opportunity  Partners  is the  investment  manager of Xmark JV  Investment
Partners,  LLC, a Delaware limited  liability  company ("JV Partners"),  and, as
such,  possesses sole power to vote and direct the disposition of all securities
of the Company held by JV Partners.  David C. Cavalier and Mitchell D. Kaye, the
Chief Operating  Officer and Chief  Executive  Officer,  respectively,  of Xmark
Capital Partners, LLC, the Managing Member of Opportunity Partners, share voting
and  investment  power with  respect  to all  securities  beneficially  owned by
Opportunity Partners.

As of December 31, 2006,  Opportunity LP held 357,590  common shares,  $0.01 par
value per share (the  "Common  Shares"),  of the Company,  Opportunity  Ltd held
633,918  Common Shares of the Company and JV Partners held 684,137 Common Shares
of the Company.

Based on  information  from the Company,  as of December  31,  2006,  there were
30,395,635  Common Shares of the Company issued and outstanding.  As a result of
the foregoing,  for purposes of Reg. Section 240.13d-3,  Opportunity Partners is
deemed to beneficially  own 1,675,645  Common Shares of the Company,  or 5.5% of
the Common Shares of the Company  deemed issued and  outstanding  as of December
31, 2006.  Opportunity  Partners' interest in the securities  reported herein is
limited to the extent of its pecuniary  interest in Opportunity LP,  Opportunity
Ltd and JV Partners, if any.





Item 10.  Certification

          By  signing  below I certify  that,  to the best of my  knowledge  and
belief, the securities  referred to above were not acquired and are not held for
the purpose of or with the effect of changing or influencing  the control of the
issuer of the  securities  and were not acquired and are not held in  connection
with or as a participant in any transaction having that purpose or effect.







                                    SIGNATURE
                                    ---------

          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.


February 2, 2007
                                              XMARK OPPORTUNITY PARTNERS, LLC
                                              By:   XMARK CAPITAL PARTNERS, LLC,
                                                    its Managing Member


                                              By:   /s/ Mitchell D. Kaye
                                                 -------------------------------
                                              Name:  Mitchell D. Kaye
                                              Title: Chief Executive Officer



      Attention:  Intentional misstatements or omissions of fact constitute
                Federal criminal violations (See 18 U.S.C. 1001)