Document:

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                                                                   Exhibit 10.42

(HYBRIDON LETTERHEAD)

                                 April 18, 2003

PrimeCorp Finance S.A.
17, avenue George V
75008 Paris-France
Attn: Robert Sursock

Pillar Investment Limited
4, rue de Cerisoles
75008 Paris-France
Attn: Youssef El Zein

Gentlemen:

         This letter sets forth the terms and conditions of the engagement of
PrimeCorp Finance S.A. ("PrimeCorp") and Pillar Investment Limited ("Pillar" and
together with PrimeCorp, the "Advisors") as non-exclusive financial advisors to
Hybridon, Inc. (the "Company") in connection with the arrangement and
negotiation of a private placement of the Company's securities outside of the
United States (the "Transaction"). The Advisors, in their capacity as financial
advisors for the Company, will identify potential non-U.S. investors and,
subject to the Company's prior written approval, contact such potential
investors on behalf of the Company and provide such other services in connection
with the Transaction as the Company may from time to time reasonably request.

         Neither of the Advisors shall contact or initiate any discussions with
any party or prospective investor without first identifying such party or
prospective investor to the Company and obtaining the Company's prior written
approval to make such contact or initiate such discussions (such parties and
prospective investors that are approved by the Company, as well as those listed
on Exhibit A hereto which shall be deemed to have been approved by the Company,
are referred to herein as the "Approved Investors"). Neither Advisor shall have
authority under this letter to bind the Company in any way to any party, and
nothing contained in this letter shall require the Company to accept the terms
of any proposal or undertake any other action that would result in the receipt
by the Advisors of a fee hereunder.

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         Each of the Advisors represents, warrants and covenants to the Company
that:

                  (a)      It shall not offer, offer to sell or sell any
         securities of the Company on the basis of any written communications or
         documents relating to the Company or its business other than written
         materials furnished by the Company or previously approved by the
         Company in writing, including without limitation the Company's filings
         under the Securities Exchange Act of 1934, as amended (the "Offering
         Materials"). No communications (whether oral or written) or documents
         relating to the Company or its business made or delivered by such
         Advisor shall be inconsistent with the Offering Materials.

                  (b)      It shall not offer, offer to sell or sell any
         securities of the Company to any investor in the United States or to
         any United States person outside the United States.

                  (c)      It shall not engage in any form of general
         solicitation or general advertising which is prohibited by Regulation D
         ("Regulation D") promulgated under the Securities Act of 1933, as
         amended (the "Securities Act"), in connection with the Transaction or
         any directed selling efforts in the United States (as such term is
         defined in Regulation S ("Regulation S") promulgated under the
         Securities Act). In addition, such Advisor shall not take any action
         that might reasonably be expected to jeopardize the availability for
         the Transaction of the exemption from registration provided by
         Regulation S or the qualification of securities of the Company for
         offer and sale under any applicable foreign securities laws.

                  (d)      It shall make reasonable inquiry to determine that
         each investor is acquiring the securities of the Company for his or its
         own account for investment.

                  (e)      In the performance of its services hereunder, it
         shall comply with the U.S. securities laws and the securities laws in
         effect in any jurisdiction in which securities of the Company are
         offered by it and the rules, regulations and orders of any securities
         administrator existing or adopted thereunder.

                  (f)      It shall not receive, directly or indirectly, any
         remuneration in respect of any issuance and sale by the Company of its
         securities in the United States or to any U.S. person.

         In the event a Transaction with Approved Investors is completed during
the term of this letter, or during the period commencing on the termination or
expiration of this letter and ending on March 31, 2004, the Company will (i) pay
the Advisors, in the aggregate, a fee in an amount equal to 7% of the Aggregate
Value (as defined below) of the Transaction received from Approved Investors and
(ii) issue to the Advisors a warrant or warrants (substantially in the form
issued to Approved Investors in the Transaction, if applicable, and in any event
including antidilution protection for stock splits and other similar events and
other customary provisions as agreed by the Company and the Advisors; provided
that the term of the warrant or warrants shall not exceed five years) to
purchase, in the aggregate, such number of shares of common stock of

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the Company as is equal to 10% of the Issued Shares (as defined below) at an
exercise price per share equal to the greater of (a) the exercise price of the
warrants issued to Approved Investors in the Transaction, if applicable, and (b)
the result obtained by dividing the Aggregate Value by the Issued Shares. The
aggregate fee and warrants to be paid to the Advisors hereunder shall be
allocated between the Advisors, and delivered by the Company, in accordance with
written instructions executed by each of the Advisors and delivered to the
Company not later than three business days prior to the closing of the
Transaction. The Company shall have no liability or obligation whatsoever under
this letter with respect to any investor or other purchaser of the Company's
securities other than an Approved Investor.

         For the purposes of this letter, (i) the term "Aggregate Value" shall
mean the total amount of cash and the fair market value of all other property
paid by Approved Investors to the Company in consideration for the securities of
the Company to be issued in the Transaction (excluding, however, any payment
made in connection with the exercise of any warrants issued to the investors in
the Transaction), and (ii) the term "Issued Shares" shall mean the total number
of shares of common stock of the Company issued to the Approved Investors in the
Transaction (assuming conversion of all securities (excluding warrants and other
similar rights) issued to the Investors that are convertible into common stock).

         In addition to any fees payable to the Advisors under the terms of this
letter, the Company agrees to reimburse the Advisors for their reasonable
out-of-pocket expenses incurred in connection with the Advisors' activities
under this letter, which shall not exceed $20,000, in the aggregate, without the
Company's prior approval.

         The Company hereby acknowledges that, notwithstanding the
representations, warranties and covenants set forth above, PrimeCorp has had
discussions with certain U.S. entities listed on Exhibit B hereto (such U.S.
entities or persons being referred to as the "Permitted U.S. Investors")
regarding a private placement of the Company's securities. PrimeCorp hereby
represents that it has fully complied with, and hereby covenants and agrees that
it will continue to fully comply with, all applicable laws, including U.S.
securities laws, with respect to a proposed investment in the Company's
securities by the Permitted U.S. Investors, including without limitation Rule
15a-6 ("Rule 15a-6") under the Securities Exchange Act of 1934, as amended, to
the extent applicable. PrimeCorp agrees that the Permitted U.S. Investors shall
not be deemed "Approved Investors" hereunder and that the Company shall not have
any liability or obligation to the Advisors in connection with any investment by
the Permitted U.S. Investors in the Company's securities; provided, however,
that notwithstanding the foregoing, non-U.S. affiliates of the Permitted U.S.
Investors shall be deemed Approved Investors hereunder. The Company and
PrimeCorp acknowledge that (i) the Company has engaged SCO Financial Group LLC
("SCO") as a non-exclusive financial advisor and placement agent in connection
with the Transaction, (ii) it is contemplated that PrimeCorp will enter into an
arrangement with SCO with respect to the Permitted U.S. Investors pursuant to
which SCO shall pay to PrimeCorp a portion of any fee SCO receives from the
Company in connection with the sale of securities to the Permitted U.S.
Investors, if any, and (iii) if the sales of securities contemplated by clause
(ii) occur, such sales shall be facilitated by SCO and shall otherwise comply
with all applicable U.S. securities laws, including without limitation Rule
15a-6. If the Company or PrimeCorp terminates it arrangements with SCO, the
Company and/or PrimeCorp shall use reasonable

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efforts to identify a registered broker-dealer in the United States satisfactory
to the Company which would be willing to participate in the Transaction with
PrimeCorp in accordance with Rule 15a-6 and to engage such broker-dealer on
terms comparable to the terms hereof which such broker-dealer would agree to
allow PrimeCorp to participate in the Transaction in accordance with Rule 15a-6.
PrimeCorp hereby agrees that it shall not, directly or indirectly, pay, or
otherwise compensate Pillar or any of its affiliates in respect of any sale by
the Company of its securities to the Permitted U.S. Investors.

         The Company agrees to indemnify the Advisors and each of their
respective affiliates and directors, officers, employees, agents and controlling
persons (each such person being an "Indemnified Party") from and against any and
all losses, claims, damages and liabilities, joint or several, to which such
Indemnified Party may become subject under any applicable federal or state law,
or otherwise, related to or arising out of the engagement of the Advisors
pursuant to, and the performance by the Advisors of the services contemplated
by, this letter and will, subject to the limitation set forth below, reimburse
any Indemnified Party for all expenses (including reasonable counsel fees and
expenses, whether incurred in connection with third party claims or direct
claims against the Company) as they are incurred in connection with the
investigation of, preparation for or defense of any pending or threatened claim
or any action or proceeding arising therefrom, whether or not such Indemnified
Party is a party. The Company will not be liable under the foregoing
indemnification provision to the extent that any loss, claim, damage, liability
or expense is found in a final judgment by a court of competent jurisdiction to
have resulted from an Indemnified Party's breach of this letter, bad faith,
willful misfeasance, gross negligence or reckless disregard of its obligations
or duties. No Indemnified Party shall settle any claim for which indemnification
may be sought by him or it hereunder without the prior written consent of the
Company. The Company's obligations to indemnify pursuant hereto shall be limited
to the Indemnified Party's actual liabilities, losses, damages or expenses
incurred and shall not include any consequential damages or damages for loss of
business or reputation.

         The Company will have the right, at its option, to assume the defense
of any litigation or proceeding in respect of which indemnity may be sought
hereunder, including the employment of counsel reasonably satisfactory to each
of the Advisors (the Advisors hereby agree that Hale and Dorr LLP is
satisfactory to the Advisors) and the payment of the fees and expenses of such
counsel, in which event, except as provided below, the Company shall not be
liable for the fees and expenses of any other counsel retained by any
Indemnified Person in connection with such litigation or proceeding. In any such
litigation or proceeding the defense of which the Company shall have so assumed,
any Indemnified Person shall have the right to participate in such litigation or
proceeding and to retain its own counsel.

         Upon receipt by an Indemnified Person of actual notice of a claim,
action or proceeding against such Indemnified Person in respect of which
indemnity may be sought hereunder, such Indemnified Person shall promptly notify
the Company with respect thereto. In addition, an Indemnified Person shall
promptly notify the Company after any action is commenced (by the way of service
with a summons or other legal process giving information as to the nature and
basis of the claim) against such Indemnified Person in respect of which
indemnity may be sought hereunder. In any event, failure to notify the Company
shall not relieve the Company from any

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liability which the Company may have on account of this indemnity or otherwise,
except to the extent the Company shall have been prejudiced by such failure.

         In the course of their services, each of the Advisors will have access
to Confidential Information (as defined below) concerning the Company. Each
Advisor agrees that all Confidential Information will be treated by the Advisor
as confidential in all respects. The term "Confidential Information" shall mean
all information, whether written or oral, which is disclosed by the Company or
its affiliates, agents or representatives to the Advisor in connection with its
role as financial advisor to the Company which is not in the public domain, but
shall not include: (i) information which, prior to disclosure to the Advisor,
was already in the Advisor's possession and was not otherwise subject to an
obligation of confidentiality; (ii) information which is publicly disclosed
other than by the Advisor in violation of this letter; (iii) information which
is obtained by the Advisor from a third party that (x) the Advisor does not know
to have violated, or to have obtained such information in violation of, any
obligation to the Company or its affiliates with respect to such information,
and (y) does not require the Advisor to refrain from disclosing such
information; and (iv) information which is required to be disclosed by the
Advisor or its outside counsel under compulsion of law (whether by oral
question, interrogatory, subpoena, civil investigative demand or otherwise) or
by order of any court or governmental or regulatory body to whose supervisory
authority the Advisor is subject; provided that, in such circumstance, the
Advisor will give the Company prior written notice of such disclosure and
cooperate with the Company to minimize the scope of any such disclosure. Each
Advisor's obligation under this paragraph shall survive the expiration,
termination or completion of this letter or the Advisor's engagement hereunder.

         The Advisors' engagement hereunder and this letter shall terminate on
the earlier of (i) June 30, 2003 or (ii) written notice of termination by the
Company to the Advisors or by the Advisors to the Company, it being understood
that the provisions relating to the payment of fees and expenses,
confidentiality and indemnification will survive any such termination, provided
that if (i) the Company terminates this letter as a result of a failure by the
Advisor to perform the services reasonably requested by the Company pursuant to
this letter in a professional and businesslike manner, (ii) the Company
terminates this letter as a result of a material breach by an Advisor of its
obligations pursuant to this letter or (iii) the Advisors terminate this letter
other than as a result of a material breach of this letter by the Company, the
provisions relating to the payment of fees after termination of this letter
shall not survive any such termination and the Company shall not have any
further liability or obligation thereunder.

         This letter shall be construed and interpreted in accordance with the
laws of the Commonwealth of Massachusetts. This letter constitutes the entire
agreement of the parties with respect to the subject matter hereof.

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         If the foregoing is in accordance with your understanding, please
confirm acceptance by signing and returning to us the duplicate of this letter
attached herewith.

                                              Sincerely,

                                              HYBRIDON, INC.

                                              By: /s/ Stephen R. Seiler
                                                  ------------------------------
                                                  Name: Stephen R. Seiler
                                                  Title: Chief Executive Officer

AGREED AND ACCEPTED AS OF
THE DATE SET FORTH ABOVE BY:

PRIMECORP FINANCE S.A.

By: /s/ Robert Sursock
    _________________________________

    Title: __________________________

PILLAR INVESTMENT LIMITED

By: /s/ Youssef El-Zein
    _________________________________

    Title:  Director
           __________________________

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                                    EXHIBIT A

Approved Investors

Pillar Biotechnology S.A.

Non-US Affiliates of CIC Group, Inc.

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                                    EXHIBIT B

Permitted Investors

Anthem Venture Partners

CIC Group, Inc.

                                      -8-<PAGE>

                                                                   Exhibit 10.43

                                 HYBRIDON, INC.

                          REGISTRATION RIGHTS AGREEMENT

                           dated as of August 28, 2003
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                                 HYBRIDON, INC.

                          REGISTRATION RIGHTS AGREEMENT

         This Registration Rights Agreement (this "Agreement") is entered into
as of August 28, 2003 by and among Hybridon, Inc., a Delaware corporation (the
"Company"), the persons and entities listed on the Schedule of Purchasers
attached hereto as Exhibit A (the "Purchasers") and the entities listed on the
Schedule of Agents attached hereto as Exhibit B (the "Agents"). The Purchasers
and the Agents shall become parties to this Agreement by the execution and
delivery of counterpart signature pages hereto in a form reasonably satisfactory
to the Company.

         WHEREAS, the Company is conducting an offering of Units (the "Unit
Offering"), with each Unit consisting of shares of the Company's common stock,
$.001 par value per share ("Common Stock"), and warrants to purchase shares of
Common Stock (the "Purchaser Warrants"), as described in the Private Placement
Memorandum dated July 22, 2003;

         WHEREAS, in connection with the Unit Offering, the Company has engaged
the Agents and has agreed to issue to the Agents warrants to purchase Common
Stock (the "Agent Warrants"); and

         WHEREAS, to induce the Purchasers to purchase Units in the Unit
Offering and the Agents to assist the Company in the Unit Offering, the Company
has agreed to provide certain registration rights under the Securities Act (as
defined below) and applicable state securities laws;

         NOW, THEREFORE, in consideration of the promises and the mutual
covenants contained herein and other good and valuable consideration, the
receipt and sufficiency of which are hereby acknowledged, the Company and each
of the Purchasers and the Agents hereby agree as follows:

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

                  (a)      "Business Day" means any day other than Saturday,
Sunday or any other day on which commercial banks in The City of New York are
required by law to remain closed.

                  (b)      "Commission" means the Securities and Exchange
Commission, or any other federal agency at the time administering the Securities
Act.

                  (c)      "Exchange Act" means the Securities Exchange Act of
1934, as amended, or any successor federal statute, and the rules and
regulations of the Commission issued under such Act, as they each may, from time
to time, be in effect.

                  (d)      "Indemnified Party" means a party entitled to
indemnification pursuant to Section 7.

                  (e)      "Indemnifying Party" means a party obligated to
provide indemnification pursuant to Section 7.

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                  (f)      "Person" means an individual, a limited liability
company, a partnership, a joint venture, a corporation, a trust, an
unincorporated organization or association and governmental or any department or
agency thereof.

                  (g)      "Registrable Securities" means (i) the shares of
Common Stock issued as part of the Units issued pursuant to the Unit Offering,
(ii) the shares of Common Stock issued or issuable upon exercise of the
Purchaser Warrants and the Agent Warrants and (iii) any other shares of Common
Stock issued in respect of such shares (as a result of a stock split, stock
dividend, reclassification, recapitalization or other similar transaction
affecting the Common Stock); provided, however, that shares of Common Stock that
are Registrable Securities shall cease to be Registrable Securities upon the
earliest of (A) the date that such shares are eligible to be sold under Rule 144
of the Securities Act without restriction by the volume limitations of Rule
144(e) of the Securities Act, (B) the date that such shares are sold (I)
pursuant to a registration statement, (II) to or through a broker, dealer or
underwriter in a public securities transaction and/or (III) in a transaction
exempt from the registration and prospectus delivery requirements of the
Securities Act such that all transfer restrictions and restrictive legends with
respect thereto, if any, are removed upon the consummation of such sale, or (C)
any sale or transfer to any Person which by virtue of Section 9 of this
Agreement is not entitled to the rights provided by this Agreement. Wherever
reference is made in this Agreement to a request or consent of holders of a
certain percentage of Registrable Securities, the determination of such
percentage shall include shares of Common Stock issuable upon exercise of the
Purchaser Warrants and the Agent Warrants even if such exercise has not been
effected.

                  (h)      "Registration Statement" means a registration
statement of the Company filed under the Securities Act and covering the
Registrable Securities.

                  (i)      "Rightsholders" means the Purchasers, the Agents and
any persons or entities to whom the rights granted under this Agreement are
transferred by any Purchaser, Agent or his or its successors or assigns pursuant
to Section 9 of this Agreement.

                  (j)      "Securities Act" means the Securities Act of 1933, as
amended, or any successor federal statute, and the rules and regulations of the
Commission issued under such Act, as they each may, from time to time, be in
effect.

         2.       Registration

                  (a)      The Company shall use its best efforts to prepare and
file with the Commission a Registration Statement covering the resale of all of
the Registrable Securities and such other shares of Common Stock as the Company
may be required to include pursuant to registration rights agreements with other
Persons within 45 days of the date hereof. The Company shall use its best
efforts to have the Registration Statement declared effective by the Commission
within 90 days after the date the Registration Statement is filed.

                  (b)      The Company shall use its best efforts to cause the
Registration Statement to remain effective until the earlier of (i) the second
anniversary of the date hereof and (ii) the date on which the Rightsholders do
not hold any Registrable Securities.

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                  (c)      The Company agrees that, in the event that the
Company has not filed the Registration Statement with the Commission within 135
days after the date hereof (such event being referred to as a "Registration
Default"), then, within five Business Days after the date 135 days after the
date hereof and after the end of each 30-day period thereafter, to the extent
the Registration Statement has not been filed as of the applicable date, the
Company shall pay to each Purchaser an amount in cash equal to 1% of the
aggregate purchase price paid by the Purchaser for its Units in the Unit
Offering. If the Company breaches Section 2(a) of this Agreement and if the
Company is judged to be liable to a Purchaser for damages as a result of such
breach, the Company shall be entitled to offset the total amount paid to the
Purchaser under this paragraph (c) against any payment of damages it is
obligated to make to such Purchaser.

         3.       Registration Procedures.

                  (a)      In connection with the effectiveness of the
Registration Statement, the Company shall furnish to each Rightsholder such
reasonable numbers of copies of the prospectus and such documents incident
thereto, including any amendment of or supplement to the prospectus, as a
Rightsholder from time to time may reasonably request in order to facilitate the
disposition of such Rightsholder's Registrable Securities under the Registration
Statement in conformity with the requirements of the Securities Act.

                  (b)      The Company shall use its best efforts to register or
qualify the Registrable Securities covered by the Registration Statement under
the securities laws of each state of the United States; provided, however, that
the Company shall not be required in connection with this paragraph (b) to
qualify as a foreign corporation or execute a general consent to service of
process in any jurisdiction.

                  (c)      If the Company has delivered preliminary or final
prospectuses to the Rightsholders and if after having done so the Company
determines that the prospectus and/or the Registration Statement needs to be
amended or supplemented to comply with the requirements of the Securities Act,
the Company shall promptly notify the Rightsholders and, if requested by the
Company, the Rightsholders shall immediately cease making offers or sales of
shares under the Registration Statement and shall return all prospectuses to the
Company. The Company shall as promptly as reasonably practicable prepare and
file with the Commission any required amendment or supplement and following such
filing, and, if applicable, the effectiveness of such filing, shall provide the
Rightsholders with revised or supplemented prospectuses. Following receipt of
the revised or supplemented prospectuses, the Rightsholders shall be free to
resume making offers and sales under the Registration Statement.

                  (d)      The Company shall use its best efforts to cause all
such Registrable Securities registered pursuant to this Agreement to be listed
on each securities exchange on which similar securities issued by the Company
are then listed.

         4.       Limitations on Registration Rights.

                  (a)      The Company may, by written notice to the
Rightsholders, suspend the Registration Statement after effectiveness and
require that the Rightsholders immediately cease sales of shares pursuant to the
Registration Statement, in the event that the Company is engaged

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in any activity or transaction or preparations or negotiations for any activity
or transaction that the Company desires to keep confidential for business
reasons, if the Company determines in good faith that the public disclosure
requirements imposed on the Company under the Securities Act in connection with
the Registration Statement would require disclosure of such activity,
transaction, preparations or negotiations.

                  (b)      If the Company requires the Rightsholders to cease
sales of shares pursuant to paragraph (a) above, the Company shall, as promptly
as practicable following the termination of the circumstance which entitled the
Company to do so, take such actions as may be necessary to reinstate the
effectiveness of the Registration Statement and/or give written notice to all
Rightsholders authorizing them to resume sales pursuant to the Registration
Statement. If as a result thereof the prospectus included in the Registration
Statement has been amended to comply with the requirements of the Securities
Act, the Company shall enclose such revised prospectus with the notice to
Rightsholders given pursuant to this paragraph (b), and the Rightsholders shall
make no offers or sales of shares pursuant to the Registration Statement other
than by means of such revised prospectus.

                  (c)      Notwithstanding the foregoing, the Company may not
suspend the Registration Statement pursuant to paragraph (a) above on more than
two occasions during any 12-month period or for more than 60 days per such
occasion.

         5.       Obligations of the Rightsholders.

                  (a)      The Company shall not be required to include any
Registrable Securities in the Registration Statement unless such Rightsholder
shall have furnished to the Company such information regarding itself, the
Registrable Securities held by it and the intended method of disposition of the
Registrable Securities held by it as shall be reasonably required by the Company
to effect the effectiveness of the Registration Statement and unless such
Rightsholder shall have executed such documents in connection with the
Registration Statement as the Company may reasonably request. Each Rightsholder
shall promptly notify the Company of any material change with respect to such
information previously provided to the Company by such Rightsholder, including
without limitation notice of the sale by the Rightsholder of any Registrable
Securities.

                  (b)      Each Rightsholder agrees to cooperate with the
Company as reasonably requested by the Company in connection with the
preparation and filing of the Registration Statement hereunder.

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         6.       Expenses of Registration. The Company shall pay the expenses
incurred by it in complying with its obligations under this Agreement, including
all registration and filing fees, exchange listing fees, fees and expenses of
counsel for the Company, and fees and expenses of accountants for the Company,
but excluding (i) any brokerage fees, selling commissions or underwriting
discounts incurred by the Rightsholders in connection with sales under the
Registration Statement and (ii) the fees and expenses of any counsel retained by
Rightsholders.

         7.       Indemnification and Contribution.

                  (a)      In the event of any registration of any of the
Registrable Securities under the Securities Act pursuant to this Agreement, the
Company will indemnify and hold harmless each Rightsholder, each of its
officers, directors and partners, and each underwriter of such Registrable
Securities, if any, and each other person, if any, who controls such
Rightsholder or underwriter within the meaning of the Securities Act or the
Exchange Act against any and all losses, claims, damages or liabilities, joint
or several, to which such Rightsholder, underwriter or controlling person may
become subject under the Securities Act, the Exchange Act, state securities or
Blue Sky laws or otherwise, insofar as such losses, claims, damages or
liabilities (or actions in respect thereof) arise out of or are based upon (i)
any untrue statement or alleged untrue statement of any material fact contained
in the Registration Statement, any preliminary prospectus or final prospectus
contained in the Registration Statement, or any amendment or supplement to such
Registration Statement or (ii) the omission or alleged omission to state a
material fact required to be stated therein or necessary to make the statements
therein not misleading; and the Company will reimburse such Rightsholder,
underwriter and each such controlling person for any legal or any other expenses
reasonably incurred by such Rightsholder, underwriter or controlling person in
connection with investigating or defending any such loss, claim, damage,
liability or action; provided, however, that the Company will not be liable in
any such case to the extent that any such loss, claim, damage, liability or
action arises out of or is based upon any untrue statement or omission made in
such Registration Statement, preliminary prospectus or prospectus, or any such
amendment or supplement, in reliance upon and in conformity with information
furnished to the Company by or on behalf of such Rightsholder, underwriter or
controlling person and stated to be specifically for use in connection with the
Registration Statement.

                  (b)      Each Rightsholder, severally and not jointly, will
indemnify and hold harmless the Company, each of its directors and officers and
each underwriter (if any) and each person, if any, who controls the Company or
any such underwriter within the meaning of the Securities Act or the Exchange
Act, against any losses, claims, damages or liabilities, joint or several, to
which the Company, such directors and officers, underwriter or controlling
person may become subject under the Securities Act, Exchange Act, state
securities or Blue Sky laws or otherwise, insofar as such losses, claims,
damages or liabilities (or actions in respect thereof) arise out of or are based
upon (i) any untrue statement or alleged untrue statement of a material fact
contained in the Registration Statement, any preliminary prospectus or final
prospectus contained in the Registration Statement, or any amendment or
supplement to the Registration Statement, or (ii) any omission or alleged
omission to state a material fact required to be stated therein or necessary to
make the statements therein not misleading, if and to the extent (and only to
the extent) that the statement or omission was made in reliance upon and in
conformity with written information relating to such Rightsholder furnished to
the Company by such

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<PAGE>

Rightsholder and stated to be specifically for use in connection with such
Registration Statement, prospectus, amendment or supplement; provided, however,
that the obligations of a Rightsholder hereunder shall be limited to an amount
equal to the net proceeds to such Rightsholder of Registrable Securities sold in
connection with such registration.

                  (c)      Each Indemnified Party shall give notice to the
Indemnifying Party promptly after such Indemnified Party has actual knowledge of
any claim as to which indemnity may be sought, and shall permit the Indemnifying
Party to assume the defense of any such claim or any litigation resulting
therefrom; provided, that counsel for the Indemnifying Party, who shall conduct
the defense of such claim or litigation, shall be approved by the Indemnified
Party (whose approval shall not be unreasonably withheld, conditioned or
delayed); and, provided, further, that the failure of any Indemnified Party to
give notice as provided herein shall not relieve the Indemnifying Party of its
obligations under this Section 7 except to the extent that the Indemnifying
Party is adversely affected by such failure. The Indemnified Party may
participate in such defense at such party's expense; provided, however, that the
Indemnifying Party shall pay such expense if the Indemnified Party reasonably
concludes based upon written advice of its counsel that representation of such
Indemnified Party by the counsel retained by the Indemnifying Party would be
inappropriate due to actual or potential differing interests between the
Indemnified Party and any other party represented by such counsel in such
proceeding; provided further that in no event shall the Indemnifying Party be
required to pay the expenses of more than one law firm per jurisdiction as
counsel for the Indemnified Party. The Indemnifying Party also shall be
responsible for the expenses of such defense if the Indemnifying Party does not
elect to assume such defense. No Indemnifying Party, in the defense of any such
claim or litigation shall, except with the consent of each Indemnified Party,
consent to entry of any judgment or enter into any settlement which does not
include as an unconditional term thereof the giving by the claimant or plaintiff
to such Indemnified Party of a release from all liability in respect of such
claim or litigation, and no Indemnified Party shall consent to entry of any
judgment or settle such claim or litigation without the prior written consent of
the Indemnifying Party, which consent shall not be unreasonably withheld,
conditioned or delayed.

                  (d)      In order to provide for just and equitable
contribution in circumstances in which the indemnification provided for in this
Section 7 is due in accordance with its terms but for any reason is held to be
unavailable to an Indemnified Party in respect to any losses, claims, damages
and liabilities referred to herein, then the Indemnifying Party shall, in lieu
of indemnifying such Indemnified Party, contribute to the amount paid or payable
by such Indemnified Party as a result of such losses, claims, damages or
liabilities to which such party may be subject in such proportion as is
appropriate to reflect the relative fault of the Company on the one hand and the
Rightsholders on the other in connection with the statements or omissions which
resulted in such losses, claims, damages or liabilities, as well as any other
relevant equitable considerations. The relative fault of the Company and the
Rightsholders shall be determined by reference to, among other things, whether
the untrue or alleged untrue statement of material fact related to information
supplied by the Company or the Rightsholders and the parties' relative intent,
knowledge, access to information and opportunity to correct or prevent such
statement or omission. The Company and the Rightsholders agree that it would not
be just and equitable if contribution pursuant to this Section 7(d) were
determined by pro rata allocation or by any other method of allocation which
does not take account of the equitable considerations referred to above.
Notwithstanding the provisions of this Section 7(d), (i) in no case shall any

                                       6
<PAGE>

one Rightsholder be liable or responsible for any amount in excess of the gross
proceeds received by such Rightsholder from the offering of Registrable
Securities and (ii) the Company shall be liable and responsible for any amount
in excess of such proceeds; provided, however, that no person guilty of
fraudulent misrepresentation (within the meaning of Section 11(f) of the
Securities Act) shall be entitled to contribution from any person who was not
guilty of such fraudulent misrepresentation. Any party entitled to contribution
will, promptly after receipt of notice of commencement of any action, suit or
proceeding against such party in respect of which a claim for contribution may
be made against another party or parties under this Section 7(d), notify such
party or parties from whom contribution may be sought, but the omission so to
notify such party or parties from whom contribution may be sought shall not
relieve such party from any other obligation it or they may have thereunder or
otherwise under this Section 7(d) except to the extent that the party or parties
from whom contribution may be sought are adversely affected. No party shall be
liable for contribution with respect to any action, suit, proceeding or claim
settled without its prior written consent, which consent shall not be
unreasonably withheld, conditioned or delayed.

         8.       Reporting. With a view to making available to the
Rightsholders the benefits of Rule 144 promulgated under the Securities Act or
any other similar rule or regulation of the Commission that may at any time
permit the Rightsholders to sell securities of the Company to the public without
registration ("Rule 144"), for so long as Rightsholders continue to own
Registrable Securities, the Company shall use its reasonable efforts to:

                  (a)      make and keep public information available, as those
terms are understood and defined in Rule 144 and file with the Commission in a
timely manner all reports and other documents required of the Company under the
Securities Act and the Exchange Act; and

                  (b)      furnish to each Rightsholder, for so long as such
Rightsholder owns Registrable Securities, promptly upon request, (i) a written
statement by the Company, if true, that it has complied with the applicable
reporting requirements of Rule 144, the Securities Act and the Exchange Act,
(ii) a copy of the most recent annual or quarterly report of the Company and
(iii) such other information as may be reasonably requested to permit the
Rightsholders to sell such securities pursuant to Rule 144 without registration.

         9.       Assignment of Registration Rights. The rights under this
Agreement shall not be assigned by any Rightsholder except in connection with
the transfer of Registrable Securities by such Rightsholder to an affiliate of
such Rightsholder, provided that (i) the Rightsholder agrees in writing with the
transferee or assignee to assign such rights, and a copy of such agreement is
furnished to the Company; (ii) the Company is furnished with written notice of
(a) the name and address of such transferee or assignee, and (b) the securities
with respect to which such rights are being transferred or assigned; (iii) at or
before the time the Company receives the written notice contemplated by clause
(ii) of this sentence, the transferee or assignee agrees in writing with the
Company to be bound by all of the obligations of an Rightsholder under this
Agreement; and (iv) such transfer shall have been conducted in accordance with
all applicable federal and state securities laws.

         10.      Amendment of Registration Rights.

                                       7
<PAGE>

                  (a)      Any provision of this Agreement may be amended and
the observance of any provision of this Agreement may be waived (either
generally or in a particular instance and either retroactively or
prospectively), only with the written consent of the Company and Rightsholders
who then hold at least a majority of the Registrable Securities. Any amendment
or waiver effected in accordance with this Section 10 shall be binding upon each
Rightsholder and the Company. No such amendment shall be effective to the extent
that it applies to less than all of the holders of the Registrable Securities.
No consideration shall be offered or paid to any Person to amend or consent to a
waiver or modification of any provision of any of this Agreement unless the same
consideration also is offered to all of the parties to this Agreement.

                  (b)      In the event that the Company issues and sells Units
as part of the Unit Offering after the date hereof, the Company shall have the
right to amend this Agreement without the consent of the Rightsholders to
include the purchasers of such Units in this Agreement as Purchasers and
Rightsholders and any placement agent or selected dealer that receives warrants
in connection with the sale of such Units as an Agent and Rightsholder and in
connection therewith to modify the Schedule of Purchasers to include such
Purchaser and the Schedule of Agents to include such Agent.

         11.      Miscellaneous.

                  (a)      A Person is deemed to be a holder of Registrable
Securities whenever such Person owns or is deemed to own of record such
Registrable Securities. If the Company receives conflicting instructions,
notices or elections from two or more Persons with respect to the same
Registrable Securities, the Company shall act upon the basis of instructions,
notice or election received from the record owner of such Registrable
Securities.

                  (b)      Any notices, consents, waivers or other
communications required or permitted to be given under the terms of this
Agreement must be in writing and will be deemed to have been delivered: (i) upon
receipt, when delivered personally; (ii) upon receipt, when sent by facsimile;
or (iii) two (2) Business Days after deposit with a reputable overnight delivery
service, in each case properly addressed to the party to receive the same. The
addresses and facsimile numbers for such communications shall be:

         If to the Company:

                                    Hybridon, Inc.
                                    345 Vassar Street
                                    Cambridge, MA 02139-4818
                                    Telephone:   617-679-5500
                                    Facsimile:   617-679-5592
                                    Attention:   Chief Executive Officer

         with a copy to:

                                    Hale and Dorr LLP
                                    60 State Street

                                       8
<PAGE>

                                    Boston, MA 02109
                                    Telephone: 617-526-6000
                                    Facsimile: 617-526-5000
                                    Attention: David E. Redlick, Esq.

         If to a Rightsholder, to its address and facsimile number set forth on
         the Schedule of Purchasers or on the Schedule of Agents, as the case
         may be, or to such other address and/or facsimile number and/or to the
         attention of such other Person as the recipient party has specified by
         written notice given to each other party five (5) days prior to the
         effectiveness of such change.

         Written confirmation of receipt (A) given by the recipient of such
notice, consent, waiver or other communication, (B) mechanically or
electronically generated by the sender's facsimile machine containing the time,
date, recipient facsimile number and an image of the first page of such
transmission, or (C) provided by a courier or overnight courier service shall be
rebuttable evidence of personal service, receipt by facsimile or receipt from a
reputable overnight delivery service in accordance with clause (i), (ii) or
(iii) above, respectively.

                  (c)      Failure of any party to exercise any right or remedy
under this Agreement or otherwise, or delay by a party in exercising such right
or remedy, shall not operate as a waiver thereof.

                  (d)      All questions concerning the construction, validity,
enforcement and interpretation of this Agreement shall be governed by the
internal laws of the State of Delaware, without giving effect to any choice of
law or conflict of law provision or rule (whether of the State of Delaware or
any other jurisdictions) that would cause the application of the laws of any
jurisdictions other than the State of Delaware.

                  (e)      This Agreement and the documents referenced herein
constitute the entire agreement among the parties hereto with respect to the
subject matter hereof and thereof. There are no restrictions, promises,
warranties or undertakings, other than those set forth or referred to herein and
therein. This Agreement and the documents referenced herein supersede all prior
agreements and understandings among the parties hereto with respect to the
subject matter hereof.

                  (f)      Subject to the requirements of Section 9 of this
Agreement, this Agreement shall inure to the benefit of and be binding upon the
permitted successors and assigns of each of the parties hereto.

                  (g)      The headings in this Agreement are for convenience of
reference only and shall not limit or otherwise affect the meaning hereof.

                  (h)      This Agreement may be executed in identical
counterparts, each of which shall be deemed an original but all of which shall
constitute one and the same agreement. This Agreement, once executed by a party,
may be delivered to the other parties hereto by facsimile

                                       9
<PAGE>

transmission of a copy of this Agreement bearing the signature of the party so
delivering this Agreement.

                  (i)      Each party shall do and perform, or cause to be done
and performed, all such further acts and things, and shall execute and deliver
all such other agreements, certificates, instruments and documents, as the other
party may reasonably request in order to carry out the intent and accomplish the
purposes of this Agreement and the consummation of the transactions contemplated
hereby.

                  (j)      All consents and other determinations required to be
made by the Rightsholders pursuant to this Agreement shall be made, unless
otherwise specified in this Agreement, by Rightsholders holding at least a
majority of the Registrable Securities.

                  (k)      The language used in this Agreement will be deemed to
be the language chosen by the parties to express their mutual intent and no
rules of strict construction will be applied against any party.

                  (l)      This Agreement is intended for the benefit of the
parties hereto and their respective permitted successors and assigns, and is not
for the benefit of, nor may any provision hereof be enforced by, any other
Person.

         IN WITNESS WHEREOF, the parties have caused this Registration Rights
Agreement to be duly executed as of the day and year first above written.

                                           COMPANY:

                                           HYBRIDON, INC.

                                           By: /s/ Stephen R. Seiler
                                               -------------------------------
                                               Name:  Stephen R. Seiler
                                               Title: Chief Executive Officer

                                           PURCHASERS:

                                           Counterpart signature pages attached.

                                           AGENTS:

                                           Counterpart signature pages attached.

                                       10
<PAGE>

                                    Exhibit A

                             Schedule of Purchasers

<TABLE>
<CAPTION>
Name and Address                                       Registrable Securities
----------------                                       ----------------------
<S>                                                    <C>
</TABLE>

<PAGE>

                                    Exhibit B

                               Schedule of Agents

<TABLE>
<CAPTION>
Name and Address                                       Registrable Securities
----------------                                       ----------------------
<S>                                                    <C>
</TABLE>

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