EXHIBIT 4.2.1 AMENDED AND RESTATED INVESTOR RIGHTS AGREEMENT

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                                                                   Exhibit 4.2.1

                           SYNTA PHARMACEUTICALS CORP.

                 AMENDED AND RESTATED INVESTOR RIGHTS AGREEMENT

     THIS AMENDED AND RESTATED INVESTOR RIGHTS AGREEMENT (this "Agreement")
dated as of December 13, 2002, is by and among Synta Pharmaceuticals Corp., a
Delaware corporation (the "Company"), Robert A. Day and Mountain Trail
Investments, LLC, (singly, a "DAY INVESTOR", and collectively, the "DAY
INVESTOR"), Keith R. Gollust, Gollust Trust II, and Wyandanch Partners, L.P.
(singly, a "GOLLUST INVESTOR", and collectively, the "GOLLUST INVESTOR") and
Cxsynta LLC, an affiliate of Caxton Corporation (the "Caxton Investor") (each an
"INVESTOR" and, collectively, the "INVESTORS").

     Reference is hereby made to that certain Investor Rights Agreement dated
May 16, 2002 by and between the Company and Cxsynta LLC, which agreement is
hereby amended and restated in its entirety and shall be without further force
or effect as of the date hereof

                                    RECITALS

     WHEREAS, the Investors have purchased the number of shares (the "SHARES")
of the Common Stock, $.0001 par value, of the Company (the "COMMON STOCK") set
forth opposite their respective names on SCHEDULE A hereto;

     WHEREAS, concurrently with the execution of this Agreement, the Investors
have executed the Amended and Restated Stockholders' Agreement of even date
herewith (the "STOCKHOLDERS' AGREEMENT") with the Company pursuant to which all
parties have agreed to provide for certain board of director representation and
meeting requirements, co-sale rights and rights of first refusal with respect to
the Common Stock; and

     WHEREAS, the Company wishes to provide the Investors with certain
information rights, registration rights and rights of first refusal in
conjunction with the purchase of their respective Shares;

     NOW, THEREFORE, in consideration of the mutual covenants and agreements
contained herein, and for other good and valuable consideration, the receipt and
sufficiency of which are hereby acknowledged, the parties hereto agree as
follows:

1.   DEFINITIONS. For purposes of this Agreement:

          (a)  The term "AFFILIATE" means any general or limited partner of any
     person that is a partnership, any member or manager of any person that is a
     limited liability company or any person or entity that, directly or
     indirectly, through one or more intermediaries, controls or is controlled
     by, or is under common control with, such person.

          (b)  The term "ACT" means the Securities Act of 1933, as amended, or
     any similar federal statute, and the rules and regulations of the SEC, all
     as shall be in effect at the time.

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          (c)  The term "BUDGET" has the meaning set forth in SECTION 3.3
     hereof.

          (d)  The term "COMPANY INDEMNIFIED PARTIES" has the meaning set forth
     in SECTION 2.6(a) hereof.

          (e)  The term "DEMAND PERIOD" has the meaning set forth in SECTION
     2.2(a) hereof.

          (f)  The term "FORM S-3" means such form under the Act as in effect on
     the date hereof or any registration form under the act subsequently adopted
     by the SEC that permits inclusion or incorporation of substantial
     information by reference to other documents filed by the Company with the
     SEC.

          (g)  The term "GAAP" means generally accepted accounting principles as
     consistently applied by the Company.

          (h)  The term "HOLDER" means any person owning or having the right to
     acquire Registrable Securities, or any assignee thereof, in accordance with
     SECTION 2.8 hereof.

          (i)  The term "INITIAL OFFERING" means the Company's first firm
     commitment underwritten public offering of its Common Stock under the Act.

          (j)  The term "1934 ACT" means the Securities Exchange Act of 1934, as
     amended, or any similar federal statute, and the rules and regulations of
     the SEC, all as shall be in effect at the time.

          (k)  The term "NEW SECURITIES" shall mean any equity securities of the
     Company, whether now authorized or not, and rights, options, or warrants to
     purchase said equity securities, and securities of any type whatsoever that
     are, or may become, convertible into said equity securities; PROVIDED,
     HOWEVER that "New Securities" does not include: (i) securities offered to
     the public pursuant to an Initial Offering; (ii) securities issued pursuant
     to the acquisition of another corporation or entity by the Company by
     merger, purchase of substantially all of the assets, or other
     reorganization whereby the Company acquires a majority of the voting power
     of such corporation or entity; (iii) up to ten million (10,000,000) shares
     of Common Stock issued or issuable to employees, consultants or directors
     of the Company pursuant to the Company's 2001 Stock Plan (including shares
     issued or issuable upon exercise of options previously granted), which
     number of shares may be adjusted upward by the affirmative vote of the
     Company's Board of Directors; (iv) up to three hundred sixty eight thousand
     eight hundred ninety four (368,894) shares of Common Stock issuable to
     employees of the Company pursuant to the Company's 2002 Employee Stock
     Purchase Plan; (v) securities issued to strategic partners of the Company,
     such as biotechnology, pharmaceutical, drug manufacturing or clinical
     research companies; (vi) securities issued to licensors of technology to
     the Company; or (vii) securities issued in connection with any bank lines
     of credit, equipment lease transactions, or real estate transactions; in
     each case as approved by the Company's Board of Directors.

          (1)  The term "NOTICE OF ACCEPTANCE" has the meaning set forth in
     SECTION 4.3

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     hereof.

          (m)  The term "OFFER" has the meaning set forth in SECTION 4.2 hereof.

          (n)  The term "PRO RATA AMOUNT" has the meaning set forth in SECTION
     4.1 hereof.

          (o)  The term "REFUSED SECURITIES" has the meaning set forth in
     SECTION 4.4 hereof.

          (p)  The term "REGISTER," "REGISTERED," and "REGISTRATION" refer to a
     registration effected by preparing and filing a registration statement or
     similar document in compliance with the Act, and the declaration or
     ordering of effectiveness of such registration statement or document.

          (q)  The term "REGISTRABLE SECURITIES" means (i) any shares of Common
     Stock held by an Investor (ii) any shares of capital stock of the Company
     acquired by an Investor (or any transferee of an Investor) after the date
     hereof pursuant to the Stockholders' Agreement and (iii) any Common Stock
     issued as (or issuable upon the conversion or exercise of any warrant,
     right or other security) a dividend or other distribution with respect to
     or because of stock splits, stock dividends, reclassifications,
     recapitalizations, or similar events, or in exchange for, or in replacement
     of the shares referenced in (i) and (ii) above, excluding in all cases,
     however, any Registrable Securities sold by a person in a transaction in
     which his, her or its rights under SECTION 2 hereof are not assigned.
     Registrable Securities shall exclude any shares which (A) have been
     registered under the Securities Act pursuant to an effective registration
     statement filed thereunder and disposed of in accordance with the
     registration statement covering them, or (B) may be publicly sold pursuant
     to and in compliance with SEC Rule 144 in any ninety (90) day period,
     provided that such shares shall not be excluded if (x) the number of shares
     proposed to be sold by such Investor is larger than the number of shares
     that may be sold in any single 90-day period pursuant to Rule 144 or (y)
     such Investor believes in good faith that a sale pursuant to Rule 144 will
     be less advantageous to it than a sale pursuant to Section 2.1 or 2.2.

          (r)  The term "REQUESTING HOLDERS" has the meaning set forth in
     SECTION 2.2(a) hereof.

          (s)  The term "SEC" means the Securities and Exchange Commission.

          (t)  The term "SELLING HOLDER" has the meaning set forth in SECTION
     2.1(a) hereof.

          (u)  The term "SELLING HOLDER INDEMNIFIED PARTIES" has the meaning set
     forth in SECTION 2.6(b) hereof.

          (v)  The term "VIOLATION" has the meaning set forth in SECTION 2.6(a)
     hereof.

2.   Registration Rights. The Company covenants and agrees as follows:

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     2.1  PIGGYBACK REGISTRATION.

           (a)  If (but without any obligation to do so) the Company proposes to
register (including for this purpose a registration effected by the Company for
stockholders other than the Holders) any of its stock or other securities under
the Act in connection with the public offering of such securities (other than a
registration statement on Form S-8 or Form S-4, or their successors, or any
registration statement covering only securities proposed to be issued in
exchange for securities or assets of another corporation), the Company shall, at
such time, promptly give each Holder written notice of such registration. Upon
the written request of each Holder (a "SELLING HOLDER") given within thirty (30)
days after mailing of such notice by the Company in accordance with the
provisions hereof, the Company shall, subject to the provisions of SECTION
2.1(c), use all reasonable efforts to cause to be registered under the Act all
of the Registrable Securities that each such Holder has requested to be
registered.

           (b)  The Company shall have the right to terminate or withdraw any
registration initiated by it under this SECTION 2.1 prior to the effectiveness
of such registration whether or not any Holder has elected to include securities
in such registration. The expenses of such withdrawn registration shall be borne
by the Company in accordance with SECTION 2.5 hereof.

           (c)  In connection with any offering involving an underwriting of
shares of the Company's capital stock, the Company shall not be required under
this SECTION 2.1 to include any of the Holders' securities in such underwriting
unless they accept the terms of the underwriting as agreed upon between the
Company and the underwriters selected by it, which terms shall not contravene
any of the terms hereof without the consent of the Selling Holders holding fifty
percent (50%) of the Registrable Securities requested to be included in such
registration statement, and enter into such an underwriting agreement in
customary form with an underwriter or underwriters selected by the Company. In
connection with any such underwriting agreement, no Selling Holder shall be
required to make representations and warranties other than representations and
warranties regarding such Selling Holder's ownership and title to the
Registrable Securities being sold by it and its plan of distribution with
respect to its Registrable Securities. The number of securities which shall be
included in such registration shall be in such quantity as the managing
underwriter determines in its sole discretion will not materially and adversely
affect the offering by the Company. If the total number of securities, including
Registrable Securities, requested by stockholders to be included in such
offering exceeds the number of securities that the managing underwriter
determines in its sole discretion will not materially and adversely affect the
offering, then the Company shall be required to include in the offering only
that number of such securities, including Registrable Securities, that the
managing underwriter determines in writing in its sole discretion will not
materially and adversely affect the offering (the securities so included to be
apportioned pro rata among the Selling Holders according to the total amount of
securities entitled to be included therein owned by each Selling Holder or in
such other proportions as shall mutually be agreed to by such Selling Holders).
Notwithstanding anything to the contrary contained in this Agreement, to the
extent the stockholders of the Company that have requested to have securities
included in such registration include stockholders other than Holders exercising
contractual demand registration rights, then the Company will include in such
registration, to the extent of the number and type which the Company is so
advised can be sold in such offering, (i) FIRST all Registrable Securities
requested for inclusion held by the Holders exercising contractual demand

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registration rights, and

(ii) SECOND such securities requested to be included in such registration
statement by all other stockholders. For purposes of the foregoing parenthetical
concerning apportionment, for any selling stockholder that is a Holder of
Registrable Securities and that is a partnership, limited liability company or
corporation, the partners, members, retired partners, retired members and
stockholders of such Holder, or the estates and family members of any such
partners, members, retired partners and retired members and any trusts for the
benefit of any of the foregoing persons shall be deemed to be a single Selling
Holder, and any pro rata reduction with respect to such Selling Holder shall be
based upon the aggregate amount of Registrable Securities owned by all such
related entities and individuals.

           (d)  In the event a Selling Holder (i) is unable to include in a
registration by the Company under this SECTION 2.1 all of the Registrable
Securities that such Holder has requested, or (ii) disapproves the terms of the
underwriting as agreed upon between the Company and the underwriters selected by
it, the Selling Holder will have the demand registration rights set forth in
SECTION 2.2.

     2.2  DEMAND REGISTRATION.

           (a)  Commencing at least one hundred eighty (180) days from the
effective date of a registration statement that could have included Registrable
Securities under SECTION 2.1 and expiring two (2) years from such effective date
or until such time as the registration statement delayed pursuant to Section
2.2.(b) has been granted effectiveness (the "DEMAND PERIOD"), any Holders
entitled to demand registration rights under Section 2.1(d) (the "REQUESTING
HOLDERS") shall be entitled to request in writing during the Demand Period that
the Company effect the registration, qualification or compliance of the
Registrable Securities owned by such Requesting Holders; PROVIDED, HOWEVER, that
the expected aggregate price to the public of the Registrable Securities will
equal or exceed five million dollars ($5,000,000). If the Requesting Holders
intend to distribute the Registrable Securities by means of an underwriting,
they shall so advise the Company in their request. The underwriter shall be
reasonably acceptable to the Company.

           (b)  The Company shall file a registration statement covering the
Registrable Securities so requested to be registered as soon as practicable
after receipt of the request or requests of the Requesting Holders, and shall
use its commercially reasonable efforts to effect such registration,
qualification or compliance (including, without limitation, the execution of an
undertaking to file post-effective amendments, appropriate qualification under
applicable blue sky or other state securities laws and appropriate compliance
with applicable regulations issued under the Securities Act and any other
governmental requirements or regulations) as may be so requested and as would
permit or facilitate the sale and distribution of all or such portion of such
Registrable Securities as are specified in such request; PROVIDED, HOWEVER, that
if: (i) in the good faith judgment of the Board of Directors of the Company,
such registration would be seriously detrimental to the Company and the Board of
Directors of the Company concludes, as a result, that it is essential to defer
the filing of such registration statement at such time, and (ii) the Company
shall furnish to such Holders a certificate signed by the Chief Executive
Officer of the Company stating that in the good faith judgment of the Board of
Directors of the Company, it would be seriously detrimental to the Company for
such registration statement to be filed in the near future

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and that it is, therefore, essential to defer the filing for a period of not
more than one hundred twenty (120) days after receipt of the request of the
Requesting Holders; and FURTHER PROVIDED, that the Company shall not defer its
obligation in this manner more than once in any twelve-month period.

           (c) The Company shall not be required to effect more than one (1)
registration pursuant to this SECTION 2.2 during the Demand Period.

     2.3  OBLIGATIONS OF THE COMPANY. Whenever required under this SECTION 2 to
effect the registration of any Registrable Securities, the Company shall, at the
earliest possible date:

           (a)  prepare and file with the SEC a registration statement with
respect to such Registrable Securities and use all reasonable efforts to cause
such registration statement to become effective, and, upon the request of the
Holders of a majority of the Registrable Securities registered thereunder, keep
such registration statement effective for one hundred twenty (120) days from the
effective date or, if earlier, until the distribution contemplated in the
Registration Statement has been completed;

           (b)  as promptly as possible prepare and file with the SEC such
amendments and supplements to such registration statement and the prospectus
used in connection with such registration statement as may be necessary to
comply with the provisions of the Act with respect to the disposition of all
securities covered by such registration statement;

           (c)  as promptly as possible furnish to the Holders such numbers of
copies of the registration statement and amendments thereto, a prospectus,
including a preliminary prospectus, in conformity with the requirements of the
Act, and such other documents as they may reasonably request in order to
facilitate the disposition of Registrable Securities owned by them;

           (d)  as promptly as possible use all reasonable efforts to register
and qualify the securities covered by such registration statement under such
other securities or blue sky or other state securities laws of such
jurisdictions as shall be reasonably requested by the Holders, provided that the
Company shall not be required in connection therewith or as a condition thereto
to qualify to do business or to file a general consent to service of process in
any such states or jurisdictions;

           (e)  as promptly as possible in