UNITED STATES

                       SECURITIES AND EXCHANGE COMMISSION

                             Washington, D.C. 20549

                                  SCHEDULE 13D

                   Under the Securities Exchange Act of 1934




                         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                             with a copy to:
    Xmark Opportunity Partners, LLC              Peter D. Greene, Esq.
    90 Grove Street                              Lowenstein Sandler PC
    Suite 201                                    1251 Avenue of the Americas
    Ridgefield, CT  06877                        New York, NY  10020
    (203) 244-9503                               (646) 414-6908

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                  (Name, Address and Telephone Number of Person
                Authorized to Receive Notices and Communications)

                                  June 26, 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.  [X]

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:             923,068*
                                     -------------------------------------------
    Shares Beneficially            8. Shared Voting Power:
                                     -------------------------------------------
    Owned by
    Each Reporting                 9. Sole Dispositive Power:        923,068*
                                      ------------------------------------------
    Person With                    10. Shared Dispositive Power:
                                      ------------------------------------------
--------------------------------------------------------------------------------
11. Aggregate Amount Beneficially Owned by Each Reporting Person:   923,068*

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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):   5.9%*
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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 June 26, 2008,  Opportunity LP held 180,427 shares of Common Stock, $0.001
par value per share (the "Common Shares"), of the Company,  Opportunity Ltd held
537,278  Common Shares of the Company and JV Partners held 205,363 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 923,068  Common  Shares of the Company,  or 5.9% of the Common
Shares  of the  Company  deemed  issued  and  outstanding  as of June 26,  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 1.   Security and Issuer.
          -------------------

          The class of equity  securities  to which this Schedule 13D relates is
the common stock, par value $0.001 per share (the "Common Shares"), of Achillion
Pharmaceuticals,  Inc., a Delaware  corporation (the  "Company").  The principal
executive  offices of the Company are located at 300 George  Street,  New Haven,
Connecticut 06511.


Item 2.   Identity and Background.
          -----------------------

          The person filing this statement is Xmark Opportunity Partners, LLC, a
Delaware limited liability  company  ("Opportunity  Partners"),  whose principal
address  is 90 Grove  Street,  Suite  201,  Ridgefield,  CT  06877.  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 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  LP  and
Opportunity  Ltd, the "Funds"),  and, as such,  possesses sole power to vote and
direct the disposition of all securities of the Company held by JV Partners. The
Funds are engaged in the investment in property of all kinds, including, without
limitation,  capital stock,  depository receipts,  investment companies,  mutual
funds,  subscriptions,  warrants,  bonds, notes,  debentures,  options and other
securities of whatever kind and nature.

          Mitchell D. Kaye and David C. Cavalier,  the Chief  Executive  Officer
and Chief Operating 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.  None of
Opportunity  Partners,  the  Funds,  Mr. Kaye  or  Mr. Cavalier  ever  has  been
convicted in any criminal  proceeding  (excluding  traffic violations or similar
misdemeanors),  and none ever has been a party to any civil proceeding commenced
before a judicial or administrative  body of competent  jurisdiction as a result
of which  it/he was or is now  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.  Messrs. Kaye and Cavalier are citizens of the United States.


Item 3.   Source and Amount of Funds or Other Consideration.
          -------------------------------------------------

          All funds used to  purchase  the Common  Shares 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.
          -----------------------

          The Funds  acquired the Common Shares of the Company  reported on this
Schedule 13D because  Opportunity  Partners  believes that the Common Shares are
undervalued and represent an attractive investment opportunity.

          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.





          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  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,  and/or (v)  seeking  to make a  significant  equity
investment in the Company, all in accordance with applicable securities laws.

          Other than as  described  above in this Item 4,  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.
          ------------------------------------

          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 June 26, 2008,  Opportunity LP held 180,427 Common Shares of the
Company,  Opportunity  Ltd held  537,278  Common  Shares of the  Company  and JV
Partners held 205,363 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 923,068 Common Shares of the
Company,  or  5.9%  of the  Common  Shares  of the  Company  deemed  issued  and
outstanding as of June 26, 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 purchases effected by the Funds in ordinary brokerage  transactions),
during the sixty (60) day period occurring on or prior to June 26, 2008.





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

       4/29/2008          11,500         Common Shares           $3.9900
       4/30/2008          20,000         Common Shares           $4.4500
       4/30/2008           1,000         Common Shares           $3.9650
       5/05/2008           3,000         Common Shares           $3.9310
       5/22/2008           8,000         Common Shares           $3.4928
       5/23/2008           5,000         Common Shares           $3.1882
       5/23/2008           4,400         Common Shares           $2.9856
       5/27/2008             400         Common Shares           $3.7250
       5/29/2008          10,000         Common Shares           $2.9356
       5/30/2008          11,600         Common Shares           $3.1452
       6/03/2008           2,000         Common Shares           $2.9245
       6/04/2008           2,000         Common Shares           $2.8247
       6/10/2008          18,000         Common Shares           $2.5275
       6/10/2008           5,000         Common Shares           $2.3600
       6/11/2008           1,200         Common Shares           $2.8042
       6/13/2008           6,100         Common Shares           $3.0000
       6/16/2008          22,700         Common Shares           $2.9473
       6/17/2008           2,300         Common Shares           $2.9500
       6/19/2008           4,109         Common Shares           $2.9500
       6/20/2008         295,805         Common Shares           $2.0788
       6/24/2008          22,403         Common Shares           $2.1512
       6/24/2008          10,000         Common Shares           $2.1015



Item 6.   Contracts, Arrangements, Understandings or Relationships with  Respect
          to Securities of the Issuer.
          ----------------------------------------------------------------------

          Except as described herein, no contracts, arrangements, understandings
or similar  relationships  exist with respect to the  securities  of the Company
between the persons  referenced in Item 2 of this Schedule 13D and any person or
entity.



Item 7.   Material to be Filed as Exhibits.
          --------------------------------
          Not applicable.








                                   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.


June 30, 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).