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                                                                   EXHIBIT 10.20

                       RESTRICTED STOCK PURCHASE AGREEMENT
                               (Ram Sasisekharan)

         This Restricted Stock Purchase Agreement dated as of June 13, 2001
(this "Agreement") is made by and between Mimeon, Inc., a Delaware corporation
(the "Company"), and Ram Sasisekharan (the "Purchaser").

         1.       DEFINITIONS.  As used in this Agreement, the following terms
shall have the following meanings:

                  QUALIFIED OFFERING: A firm commitment underwritten public
offering of the Company's Common Stock pursuant to an effective registration
statement under the Securities Act of 1933, as amended (the "Act"), in which the
price per share is at least $5.00 (subject to equitable adjustment in the event
of stock splits and the like) and the aggregate gross proceeds to the Company
from such offering are not less than $10,000,000.

                  QUALIFIED SALE: The sale of all or substantially all of assets
or issued and outstanding capital stock of the Company, or merger or
consolidation involving the Company in which stockholders of the Company
immediately before such merger or consolidation do not own immediately after
such merger or consolidation capital stock or other equity interests of
surviving corporation or entity representing more than fifty percent in voting
power of capital stock or other equity interests of such surviving corporation
or entity outstanding immediately after such merger or consolidation.

                  SERVICE:  Service as an employee, officer or director of, or a
consultant or advisor the Company or its successors.

                  SHARES: The shares of Common Stock issued to Purchaser
hereunder and any other securities of the Company which may be issued in
exchange for or in respect of such shares of Common Stock, whether by way of
stock split, stock dividend, combination of shares, reclassification,
recapitalization, reorganization or any other means.

                  UNVESTED SHARES: Any Shares that are not Vested Shares.

                  VESTED: Released from the Company's Repurchase Option (as
defined in Section 5(a)).

                  VESTED SHARES: Any Shares that have vested in accordance with
Section 5(b).

         2.       PURCHASE AND SALE OF SHARES. The Company hereby sells to
Purchaser, and Purchaser hereby purchases from the Company, 809,800 shares of
the Company's common stock, $0.0001 par value per, share ("Common Stock"), for a
purchase price per share of $0.0001, and an aggregate purchase price of $80.98.
The Company acknowledges receipt from Purchaser of $80.98 in cash, in full
payment of such purchase price. Purchaser and the Company hereby agree that the
fair market value of the Shares on the date hereof is $0.0001 per share.

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         3.       REPRESENTATIONS OF PURCHASER.  Purchaser understands that the
Shares are not registered under the Act, and represents to the Company, and
agrees that the Company is entitled to rely on such representations, as follows:

                  (a)      Purchaser understands that the Shares have not been
registered under the Act, or registered or qualified under the securities or
"Blue Sky" laws of any jurisdiction, and are being sold pursuant to exemptions
contained in the Act and exemptions contained in other applicable securities or
"Blue Sky" laws. Purchaser understands further that the Company's reliance on
these exemptions is based in part on the representations made by Purchaser in
the Agreement. In this connection, Purchaser represents and warrants that the
offer and sale of the Shares were made solely in Massachusetts.

                  (b)      Purchaser understands the term "accredited investor"
as used in Regulation D promulgated under the Act and represents and warrants to
the Company that he is an "accredited investor" for purposes of acquiring the
Shares. The nature and amount of Purchaser's investment in the Shares is
consistent with Purchaser's investment objectives, abilities, and resources.
Purchaser understands that the Shares are an illiquid investment, which will not
become freely transferable by reason of any "change of circumstances" whatever.
Purchaser has adequate means of providing for Purchaser's current needs and
possible contingencies and has no need for liquidity in Purchaser's investment.

                  (c)      Purchaser is acquiring the Shares for Purchaser's own
account for investment, and not for, with a view to, or in connection with the
resale or distribution thereof. Purchaser has no present intention to sell,
hypothecate, distribute or otherwise transfer the Shares or any portion thereof
or any interest therein.

                  (d)      Purchaser understands that the Shares will constitute
"restricted securities" thin-the meaning of Rule 144 promulgated under the Act
and that, as such, the Shares must be held indefinitely unless they are
subsequently registered under the Act or unless an exemption from the
registration requirements thereof is available. Purchaser has been advised that
Rule 144, which permits the resale, subject to various terms and conditions, of
small amounts of such "restricted securities" after they have been held for one
year, does not now apply to the Company, because the Company is not now required
to file, and does not file, current reports under the Securities Exchange Act of
1934, and because information concerning the Company substantially equivalent to
that which would be available if the Company were required to file such reports
is not now publicly available. The Company may become a reporting entity at some
future date, but no assurance can be given that it will do so.

                  (e)      In connection with Purchaser's acquisition of the
Shares, Purchaser accepts the condition that the Company may maintain "stop
transfer" orders with respect to the Shares and that each certificate or other
document evidencing the Shares will bear conspicuous legends in substantially
the form set forth in Section 7 of this Agreement.

                  (f)      Purchaser has consulted Purchaser's attorney or
accountant with respect to Purchaser's purchase of the Shares. Purchaser has
fully investigated the Company and its business and financial condition and has
knowledge of the Company's current activities. Purchaser acknowledges that the
Company has granted Purchaser and Purchaser's attorney or

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accountant access to all information about the Company which they have requested
and has offered each of them access to all further information which they deemed
relevant to an investment decision with respect to the Shares. Purchaser and
Purchaser's attorney or accountant have had the opportunity to ask questions of,
and receive answers from, representatives of the Company concerning such
information and the Company's financial condition and prospects.

         4.       RESTRICTIONS ON TRANSFER.  The following restrictions on
transfer of the Shares shall apply:

                  (a)      SECURITIES LAWS. Except for purchases of Unvested
Shares by the Company as contemplated by Section 5, no Shares, nor any interest
therein, may be sold, assigned, pledged or otherwise transferred at any time or
under any circumstances unless (i) the Shares proposed to be transferred have
been registered under the Act and qualified under applicable state securities
laws, or (ii) the Company has received, or agreed to waive, an opinion of
counsel acceptable to the Company to the effect that such transfer may be
effected without registration under the Act or qualification under the
securities laws of relevant states and the proposed transferee has made such
representations and agreements as the Company shall require to assure compliance
with the Act and such laws.

                  (b)      TERMINATION OF REPURCHASE OPTION. Except for
purchases of Unvested Shares by the Company as contemplated by Section 5, no
Shares, nor any interest therein, may be sold, assigned, pledged or otherwise
transferred until the Repurchase Option shall have terminated with respect to
such Shares.

                  (c)      RIGHT OF FIRST OFFER. In the event that at any time
Purchaser desires to sell, assign or otherwise transfer any of the Vested Shares
then held by Purchaser, he shall first offer the Vested Shares desired to be
transferred to the Company by giving written notice of the proposed transfer.
The notice shall state the number of Vested Shares proposed to be transferred,
the name of the person or persons to whom it is proposed to transfer the Vested
Shares, the price at which the Vested Shares are intended to be transferred and
all other terms of the transaction, which must be bona fide. Such notice shall
constitute an offer by Purchaser to the Company for the Company to purchase such
Vested Shares on such terms and at a price per Vested Share equal to the price
stated in the notice. The Company may accept the offer as to all, but not less
than all, of the Vested Shares offered by notifying Purchaser in writing within
30 days after receipt of such notice of its acceptance of the offer. If the
offer is accepted, Purchaser shall sell the offered Vested Shares to the Company
on the terms and at the price per Vested Share as aforesaid, free of all
encumbrances, and shall deliver the certificates representing such Vested
Shares, duly endorsed in blank by Purchaser or with duly executed stock powers
attached thereto, all in form suitable for the transfer of such Vested Shares to
the Company, within 30 days of the date of acceptance of the offer to sell,
against payment therefor at the same price per Vested Share and according to the
same terms as were offered by the proposed transferee. If within the applicable
time period Purchaser does not receive notice of the Company's intention to
purchase the offered Vested Shares, the offer shall be deemed to have been
rejected. In such event, Purchaser may transfer title to the offered Vested
Shares within 90 days from the date of his written notice to the Company of his
intention to sell, but such transfer shall be made only to the proposed
transferee or transferees and at the proposed price and on such other terms as
stated

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in such notice. Vested Shares that are so transferred to such transferee or
transferees shall remain subject to this Section 3 and as a condition to any
transfer Purchaser shall obtain a written agreement from the transferee by which
the transferee agrees to be bound by this Section 3.

                  (d)      PERMITTED TRANSFERS. Any portion or all of the Vested
Shares may, without compliance with the provisions of Section 4(c), be
transferred by Purchaser to a member of his immediate family or to a family
partnership or family trust, or on Purchaser's death may be transferred to
Purchaser's estate or to those entitled to a distribution of the Vested Shares
under the laws of descent and distribution, provided that Shares that are so
transferred shall remain subject to this Section 4 and as a condition to any
transfer Purchaser shall obtain a written agreement from the transferee by which
the transferee agrees to be bound by this Section 4.

                  (e)      REMEDIES. No sale, assignment, pledge or other
transfer of Shares shall be effective or given effect on the books of the
Company unless all of the applicable provisions of this Section 4 have been duly
complied with. If any transfer of Shares is made or attempted in violation of
the foregoing restrictions, or if Shares are not offered to the Company as
required hereby, the Company shall have the right to purchase such Shares from
the purported owner thereof or his transferee at any time before or after the
transfer, as herein provided. In addition to any other legal or equitable
remedies which it may have, the Company may enforce its rights by actions for
specific performance (to the extent permitted by law) and may refuse to
recognize any transferee as one of its stockholders for any purpose, including,
without limitation, for purposes of dividend and voting rights, until all
applicable provisions hereof have been complied with.

                  (f)      LOCK-UP. Purchaser agrees that for a period of up to
180 days from the effective date of any registration of securities of the
Company (upon request of the, Company or the underwriters managing any
underwritten offering of the Company's securities), he will not sell make any
short sale or loan of, grant any option for the purchase of, or otherwise
dispose of any Shares held by him without the prior written consent of the
Company or such underwriters, as the case may be.

                  (g)      TERMINATION OF RESTRICTIONS.  Sections 4(c) and 4(d)
shall terminate upon the earlier to occur of: (i) immediately prior to the
consummation of a Qualified Sale; or (ii) the closing of a Qualified Offering.

         5.       REPURCHASE OF UNVESTED SHARES.

                  (a)      REPURCHASE OPTION.

                           (i)      In the event of the termination of
Purchaser's Service by Purchaser or the Company for any reason, with or without
cause, the Company shall upon the date of such termination (the "Termination
Date") have an irrevocable, exclusive option (the "Repurchase Option") for a
period of 90 days from such date to repurchase all or any portion of the
Unvested Shares at the per share repurchase price of $0.0001 per share,
appropriately adjusted in the event of a stock dividend, stock split,
recapitalization, combination of shares or similar event occurring subsequent to
the date of this Agreement.

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                           (ii)     Unless the Company notifies Purchaser within
90 days from the date of termination of Purchaser's Service that it does not
intend to exercise its Repurchase Option with respect to some or all of the
Unvested Shares, the Repurchase Option shall be deemed automatically exercised
by the Company as of the 90th day following such termination, provided that the
Company may notify Purchaser that it is exercising its Repurchase Option as of a
date prior to such 90th day. Unless Purchaser is otherwise notified by the
Company pursuant to the preceding sentence that the Company does not intend to
exercise its Repurchase Option as to some or all of the Unvested Shares to which
it applies at the time of termination, execution of this Agreement by Purchaser
constitutes written notice to Purchaser of the Company's intention to exercise
its Repurchase Option with respect to all Unvested Shares to which such
Repurchase Option applies. The Company, at its choice, may satisfy its payment
obligation to Purchaser with respect to exercise of the Repurchase Option by
either (A) delivering a check to Purchaser in the amount of the purchase price
for the Unvested Shares being repurchased, or (B) in the event Purchaser is
indebted to the Company, canceling an amount of such indebtedness equal to the
purchase price for the Unvested Shares being repurchased, or (C) by a
combination of (A) and (B) so that the combined payment and cancellation of
indebtedness equals such purchase price, provided that the Company shall use
good faith efforts to satisfy its payment obligation to Purchaser within 15 days
after Company's notice of exercise of the Repurchase Option (or deemed
exercise), and that if such check is not delivered or such cancellation is not
effective within such 15 days from such date, the amount of the Company's
unsatisfied payment obligation shall bear interest at a rate of nine percent
(9%) per annum until the Company has satisfied its payment obligation under this
paragraph (ii). In the event of any deemed automatic exercise of the Repurchase
Option pursuant to this Section 5(a)(ii) and Purchaser is then indebted to the
Company, the amount of such indebtedness equal to the purchase price of the
Unvested Shares being repurchased shall be deemed automatically canceled as of
the date of Company's notice of exercise of the Repurchase Option (or deemed
exercise). As a result of any repurchase of Unvested Shares pursuant to this
Section 5(a), the Company shall become the legal and beneficial owner of the
Unvested Shares being repurchased and shall have all rights and interest therein
or related thereto, and the Company shall have the right to transfer to its own
name the number of Unvested Shares being repurchased by the Company, without
further action by Purchaser.

                  (b)      VESTING.

                           (i)      The Shares will become vested as follows:

                                    (A)      one sixteenth (1/16) of the Shares
shall vest on September 13, 2001; and

                                    (B)      an additional one sixteenth (1/16)
of the Shares shall vest on the last day of each consecutive three-month period
thereafter until 100% of the Shares have become vested; PROVIDED, however, that
the vesting of Shares on any such vesting date shall be conditioned upon
Purchaser's continuing Service with the Company from the date hereof through
such vesting date. Fractional shares shall be rounded down to the nearest whole
share.

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                           (ii)     Notwithstanding Section 4(b)(i), all Shares
shall be deemed to have vested immediately prior to the consummation of a
Qualified Sale.

         6.       CUSTODY OF CERTIFICATES. In order to facilitate the exercise
of the Repurchase Option, the Company or its counsel shall hold all certificates
representing Unvested Shares, together with an adequate number of undated and
otherwise blank stock powers executed by Purchaser. The Company shall have the
right to cause transfers of Unvested Shares to be effected pursuant to Section
5. After any Shares become Vested Shares, the Company shall, on request of
Purchaser, deliver to Purchaser a certificate or certificates representing such
vested Shares. After the Company sends Purchaser a notice that it does not
intend to exercise its Repurchase Option as to certain Unvested Shares, the
Company shall, upon request of Purchaser, deliver to Purchaser a certificate or
certificates representing such Unvested Shares.

         7.       LEGENDS.  Each certificate representing Shares shall
prominently bear legends in substantially the following forms:

         These securities have not been registered under the Securities Act of
         1933. They may not be sold, offered for sale, pledged or hypothecated
         in the absence of a registration statement in effect with respect to
         the securities under such Act or an opinion of counsel satisfactory to
         the Corporation that such registration is not required.

         The securities represented by this certificate have been acquired for
         investment and have not been registered or qualified under the
         securities or "Blue Sky" laws of any jurisdiction. They may not be
         offered or sold without an opinion of counsel to the Corporation to the
         effect that the proposed transaction will be exempt from registration,
         qualification, and filings in all applicable jurisdictions.

         The Corporation is authorized to issue more than one class or series of
         stock. The powers, designations, preferences and relative
         participating, optional or other special rights, and the
         qualifications, limitations or restrictions of such preferences and/or
         rights of each class of stock or series of any class set forth in the
         Certificate of Incorporation of the Corporation. The Corporation will
         furnish a copy of the Certificate of Incorporation of the Corporation
         to the holder of this certificate without charge upon request.

         The securities represented by this certificate are subject to
         restrictions on transfer and repurchase rights pursuant to the terms of
         a Restricted Stock Purchase Agreement, as amended from time to time,
         between the owner of this certificate and the Corporation. The
         Corporation will finish a copy of this agreement to the holder hereof
         without charge upon written request.

         8.       MISCELLANEOUS.

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                  (a)      ENTIRE AGREEMENT. This Agreement constitutes the
entire agreement between the parties with respect to the subject matter hereof
and supersedes all prior agreements, negotiations, representations and
proposals, written or oral, relating to such subject matter.

                  (b)      AMENDMENTS.  Neither this Agreement nor any provision
hereof may be changed or modified except by an agreement in writing executed by
Purchaser and on behalf of the Company.

                  (c)      BINDING EFFECT OF THE AGREEMENT. This Agreement shall
inure to the benefit of, and be binding upon, the Company, Purchaser and their
respective estates, heirs, executors, transferees, successors, assigns and legal
representatives.

                  (d)      PROVISIONS SEVERABLE. In the event that any provision
of this Agreement shall be determined to be invalid, illegal or otherwise
unenforceable by any court of competent jurisdiction, the validity, legality and
enforceability of the other provisions of this Agreement shall not be affected
thereby. Any invalid, illegal or unenforceable provision of this Agreement shall
be severed, and after any such severance, all other provisions hereof shall
remain in full force and effect.

                  (e)      NOTICES. All notices under this Agreement shall be
effective (i) upon personal or facsimile delivery, (ii) two business days after
deposit in the United States mail as registered or certified mail postage fully
prepaid, or (iii) one business day after pickup by any overnight commercial
courier service, in each case sent or addressed to the Company at its principal
office and to Purchaser at his record address as carried in the stock records of
the Company or at such other address as he may from time to time designate in
writing to the Company.

                  (f)      CONSTRUCTION. A reference to a Section shall mean a
Section of this Agreement unless otherwise expressly stated. The titles and
headings herein are for reference purposes only and shall not in any manner
limit the construction of this Agreement which shall be considered as a whole.
The words "include," "includes" and "including" when used herein shall be deemed
in each case to be followed by the words "without limitation." Whenever the
context may require, any pronouns used herein shall include the corresponding
masculine, feminine or neuter forms, and the singular form of names and pronouns
shall include the plural and vice-versa.

                  (g)      NO EMPLOYMENT AGREEMENT.  This Agreement shall not be
construed as an agreement by the Company to employ Purchaser, nor is the Company
obligated to employ Purchaser by reason of this Agreement or the issuance of the
Shares to Purchaser.

                  (h)      SECTION 83(b) ELECTION. Purchaser will furnish to the
Company a copy of any election made by Purchaser under Section 83(b) of the
Internal Revenue Code of 1986, as amended, with respect to his acquisition of
the Shares.

                  (i)      APPLICABLE LAW. This Agreement shall be construed and
enforced in accordance with the laws of The Commonwealth of Massachusetts,
without regard to its Principles of conflicts of laws. Purchaser consents to
jurisdiction and venue in any state or

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federal court in The Commonwealth of Massachusetts for the purposes of any
action relating to or arising out of this Agreement or any breach or alleged
breach hereof, and to service of process in any such action by certified or
registered mail, return receipt requested.

                  (j)      DISPOSITION OF SHARES; PURCHASE BY NOMINEE OR
DESIGNEE. Any Shares that the Company elects to purchase hereunder may be
disposed of by it in such manner as it deems appropriate with or without
restrictions on the transfer thereof, and the Company may require their transfer
to a nominee or designee as part of any purchase of the Shares from Purchaser.

                  (k)      WITHHOLDING TAXES. Purchaser acknowledges and agrees
that the Company has the right to deduct from payments of any kind otherwise due
to Purchaser any federal, state or local taxes of any kind required by law to be
withheld with respect to the purchase of the Shares by Purchaser.

         IN WITNESS WHEREOF, the parties hereto have executed this Restricted
Stock Purchase Agreement as of the date first above written.

                                        MIMEON, INC.


                                        By:   /s/ Ganesh Venkataraman
                                           -------------------------------------
                                        Name:
                                        Title:


                                              /s/ Ram Sasiekharan
                                        ----------------------------------------
                                        Ram Sasisekharan

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