UNITED STATES

                       SECURITIES AND EXCHANGE COMMISSION

                             Washington, D.C. 20549

                                  SCHEDULE 13D

                    Under the Securities Exchange Act of 1934

                               (Amendment No. 1)*



                         ACHILLION PHARMACEUTICALS, INC.
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                                (Name of Issuer)

                    Common Stock, par value $0.001 per share
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                         (Title of Class of Securities)

                                   0044Q201
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                                 (CUSIP Number)


                                Mitchell D. Kaye
                         Xmark Opportunity Partners, LLC
                                 90 Grove Street
                                    Suite 201
                              Ridgefield, CT 06877
                                 (203) 244-9503
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                  (Name, Address and Telephone Number of Person
                Authorized to Receive Notices and Communications)

                                  July 16, 2008
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             (Date of Event which Requires Filing of this Statement)



If the filing person has previously  filed a statement on Schedule l3G to report
the  acquisition  that is the subject of this  Schedule  13D, and is filing this
schedule  because of Sections 240.13d-1(e), 240.13d-1(f)  or 240.13d-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 Section 240.13d-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.    0044Q201
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1.  Names of Reporting Persons.  I.R.S. Identification Nos. of above persons
    (entities only):
               Xmark Opportunity Partners, LLC
               20-2052197
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2.  Check the Appropriate Box if a Member of a Group (See Instructions):
         (a)   Not
         (b)   Applicable
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3.  SEC Use Only

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4.  Source of Funds (See Instructions):   AF, WC
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5.  Check if Disclosure  of Legal Proceedings Is Required Pursuant to Items 2(d)
    or 2(e):               Not Applicable
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6.  Citizenship or Place of Organization:    United States

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    Number of                      7. Sole Voting Power:             874,068*
                                     -------------------------------------------
    Shares Beneficially            8. Shared Voting Power:
                                     -------------------------------------------
    Owned by
    Each Reporting                 9. Sole Dispositive Power:        874,068*
                                      ------------------------------------------
    Person With                    10. Shared Dispositive Power:
                                      ------------------------------------------
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11. Aggregate Amount Beneficially Owned by Each Reporting Person:   874,068*

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12. Check if  the Aggregate  Amount in  Row (11) Excludes  Certain  Shares  (See
    Instructions):         Not Applicable
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13. Percent of Class Represented by Amount in Row (11):   5.6%*
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14. Type of Reporting Person (See Instructions):  IA
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* 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   Achillion
Pharmaceuticals,   Inc.,  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 together with  Opportunity Ltd and Opportunity LP,
the  "Funds"),  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 July 16, 2008,  Opportunity LP held 170,954 shares of Common Stock, $0.001
par value per share (the "Common Shares"), of the Company,  Opportunity Ltd held
508,875  Common Shares of the Company and JV Partners held 194,239 Common Shares
of the Company.  Based upon  information  set forth in the Company's most recent
Quarterly  Report  on Form  10-Q,  as filed  with the  Securities  and  Exchange
Commission on May 7, 2008,  there were  15,641,462  Common Shares of the Company
issued and  outstanding  as of May 1, 2008.  As a result of the  foregoing,  for
purposes  of  Reg.  Section  240.13d-3,   Opportunity   Partners  is  deemed  to





beneficially  own 874,068  Common  Shares of the Company,  or 5.6% of the Common
Shares  of the  Company  deemed  issued  and  outstanding  as of July 16,  2008.
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 3.   Source and Amount of Funds or Other Consideration.
          -------------------------------------------------

          Item 3 is hereby amended by adding the following at the end thereof:

          All  funds  used  to  purchase  the  Common  Shares  in  the  purchase
transactions  described in Item 5, as amended,  on behalf of the Funds have come
directly from the assets of the Funds. See Item 5 for further information.


Item 4.   Purpose of Transaction.
          -----------------------

          Item 4 is hereby  amended by deleting  Item 4 in its  entirety  and by
substituting the following in lieu thereof:

          Opportunity   Partners  is  not  clear  on  the  strategic   direction
management  and the Board of Directors  intend to pursue.  Opportunity  Partners
firmly   believes   that  a  sale  of  the   Company,   or  certain   individual
assets/technology, would make more sense than equity dilution.  In that respect,
Opportunity  Partners believes that management and the Board of Directors should
immediately  engage  a  qualified  investment  bank  to  pursue  such  strategic
alternatives.

          On July 11, 2008,  Mitchell D. Kaye, Chief Executive  Officer of Xmark
Capital   Partners,   LLC,  the  Managing   Member  of   Opportunity   Partners,
electronically  transmitted  a letter on  behalf  of the Funds to the  Company's
President/Chief   Executive  Officer  and  two  of  its  other  directors.   The
electronically transmitted letter, in pertinent parts, stated:

          "(1) Xmark has been the  largest  accumulator  of  Achillion
          shares during the past 5 quarters (dating back to 2Q07);

          (2) not a single  insider  --  management  or members of the
          Board -- purchased a single  share of  Achillion  during the
          past 12 months;

          (3)  Achillion  shares  have  declined  by over 50% in value
          during the time that we have been invested;

          (4)  management  is  handsomely  compensated  and  has  been
          awarded  bonuses  during a time  when the  company's  common
          stock price has suffered and cash is scant;

          (5)  we  disagree   with  the  decision  to   in-license  an
          antibiotic  compound  -- an area in which the company has no
          demonstrable track record of development success; and

          (6) the company has missed many  milestones  on which it had
          provided specific prior public guidance."





          Opportunity  Partners  also renewed a prior request that it be allowed
to observe  the next  meeting of the Board of  Directors  of the Company for the
purpose of  "gain[ing] a better  understanding  of (A) the  strategic  vision of
management and the Board,  and (B) the relationship  between  management and the
Board." Although the July 11, 2008 electronically transmitted letter requested a
response by 5:00 p.m.,  July 16,  2008,  the Company  has not  responded  in any
fashion to  Opportunity  Partners' request to  observe  the next  meeting of the
Board of Directors of the Company.  Opportunity Partners is further concerned by
the failure of the Company to respond in any fashion to its request.

          Opportunity  Partners  intends to  continue  to  evaluate  closely the
performance of the Common Shares of the Company, including,  without limitation,
the  continued  analysis  and  assessment  of the  Company's  business,  assets,
operations,  financial condition,  capital structure,  management and prospects.
Opportunity  Partners may, from time to time,  evaluate various options in order
to influence the  performance  of the Company and the activities of its Board of
Directors.  Depending  on various  factors,  Opportunity  Partners may take such
actions  on  behalf  of the Funds as it deems  appropriate,  including,  without
limitation, (i) engaging in further discussions with management and/or the Board
of Directors, (ii) communicating with other shareholders, (iii) making proposals
to the  Company  concerning  the  operations  of the  Company,  (iv)  purchasing
additional  securities of the Company, (v) selling some or all of the securities
of the  Company  held by the Funds,  (v)  seeking to make a  significant  equity
investment in the Company,  all in accordance with applicable  securities  laws,
and/or (vi) formally requesting a seat on the Board of Directors.

          Other than as described  above in this Item 4 as amended,  Opportunity
Partners does not have any present plans or intentions which relate to, or would
result  in,  any of the  transactions  required  to be  described  in  Item 4 of
Schedule 13D.


Item 5.   Interest in Securities of the Issuer.
          ------------------------------------

          Item 5 is hereby  amended by deleting  Item 5 in its  entirety  and by
substituting the following in lieu thereof:

          Based  upon  information  set  forth  in  the  Company's  most  recent
Quarterly  Report  on Form  10-Q,  as filed  with the  Securities  and  Exchange
Commission on May 7, 2008,  there were  15,641,462  Common Shares of the Company
issued and outstanding as of May 1, 2008.

          As of July 16, 2008,  Opportunity LP held 170,954 Common Shares of the
Company,  Opportunity  Ltd held  508,875  Common  Shares of the  Company  and JV
Partners held 194,239 Common Shares of the Company.

          Opportunity  Partners  possesses  sole  power to vote and  direct  the
disposition of all securities of the Company held by the Funds. Messrs. Kaye and
Cavalier  share  voting  and  investment  power with  respect to all  securities
beneficially owned by Opportunity Partners.

          As a result of the foregoing,  for purposes of Reg. Section 240.13d-3,
Opportunity  Partners is deemed to beneficially own 874,068 Common Shares of the
Company,  or  5.6%  of the  Common  Shares  of the  Company  deemed  issued  and
outstanding as of July 16, 2008.

          The following  table details all of the  transactions in Common Shares
of the Company, or securities  convertible into, exercisable for or exchangeable





for Common Shares of the Company,  by the persons  referenced in Item 2 (each of
which were effected by the Funds in ordinary brokerage transactions),  since the
filing of Schedule 13D as of June 26, 2008:


                    Type of        Number of                          Price per
    Date          Transaction       Shares         Security Type      Share ($)
  ---------       -----------      ---------       -------------      ----------

  6/30/2008         Purchase         5,000         Common Shares       $2.2300
  7/08/2008           Sale          50,000         Common Shares       $2.2000
  7/15/2008         Purchase         1,000         Common Shares       $2.1140








                                   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.


July 18, 2008                         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).