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                                                                   Exhibit 10.12

                                   SYNTApharma

                                                           April 8, 2004

Dr. Mitsunori Ono
[ADDRESS]

Dear Dr. Ono:

     The purpose of this letter agreement is to set forth our mutual
understanding and agreement with respect to your resignation from employment
with Synta Pharmaceuticals Corp. (the "Company"). In consideration of the mutual
covenants set forth herein, the receipt and sufficiency of which you
acknowledge, we have agreed as follows:

1.   RESIGNATION FROM EMPLOYMENT. Your resignation from employment shall be
     effective as of the close of business, Thursday, January 1, 2004 (your
     "resignation date"), and you shall have relinquished as of that date any
     and all positions that you have held with the Company, including but not
     limited to President and Chief Operating Officer of the Company. You shall
     not be considered an employee of the Company for any purpose after that
     date and shall have no authority to act on behalf of the Company.

2.   TERMINAL PAY. You agree that you have received all compensation to which
     you are entitled in connection with your employment through your
     resignation date. You agree to make no claims for further compensation from
     the Company of any type, including bonus payments, commission payments, and
     vacation pay. You acknowledge that, except to the extent provided herein,
     the Company is under no obligation to provide you with the consideration
     described below, including, but not limited to, the severance payment.

3.   SEVERANCE PAYMENT. The Company shall pay you a lump sum amount of
     $200,000.00 (less all applicable federal, state or local tax withholding,
     F.I.C.A., and any other applicable payroll deductions), subject to the
     Company's receipt of this letter agreement and General Release signed by
     you and the expiration of the seven (7) day revocation period contained in
     paragraph 16 hereof. The Company shall also pay you monthly installments
     totaling $250,000.00 commencing in March 2004 for eighteen (18) months
     consisting of seventeen (17) payments in the amount of $13,888.89 and the
     final payment in the amount of $13,888.87. In the event that your death
     shall precede the Company's full payment of any of the amounts set forth in
     this Section 3. then any unpaid amounts under this section shall continue
     to be paid upon the schedule set forth above to your surviving spouse or,
     if there is no surviving spouse, to your estate, unless the Company
     receives a written instrument signed by you designating a beneficiary and
     providing the beneficiary's social security number (if any) and home
     address.

4.   STOCK OPTIONS. You acknowledge that, pursuant to Incentive Stock Option
     Agreement No. 004 dated December 13, 2002 ("ISOA No. 004") and Incentive
     Stock Option

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     Agreement No. 099 dated June 17, 2003 ("ISOA No. 099"), your right to
     exercise ISOA No. 004 for an aggregate of 812,500 shares (the "Original
     Vested Shares") of the Company's common stock, $.0001 par value per share
     (the "Common Stock"), vested as of your resignation date. You agree that as
     of your resignation date and subject to the approval of the Compensation
     Committee of the Company's Board of Directors:

     (i)      ISOA No. 099 shall terminate; and

     (ii)     the terms of ISOA No. 004 shall be amended as follows:

              a.   All references to "1,500,000 shares" on the face of ISOA No.
                   004 will be deleted and replaced with "1,000,000 shares";

              b.   Section 3 of ISOA No. 004 will be deleted in its entirety
                   and replaced with the following:

                        "3. VESTING SCHEDULE. The Option shall be exercisable
                   with respect to 1,000,000 of the Stock Rights Shares
                   immediately."

              c.   Section 4 of the ISOA No. 004 will be deleted in its
                   entirety and replaced with the following:

                        "4. TERM OF OPTION. Subject to earlier termination as
                   provided in this Agreement or the Plan, the Option shall
                   expire on the tenth anniversary of the Grant Date."

     (iii)    upon the effective date of the amendments set forth in paragraph
              4(ii) above, ISOA No. 004 shall cease to be treated as an
              incentive stock option for purposes of the Company's 2001 Stock
              Option Plan and for tax purposes pursuant to Section 422 of the
              Internal Revenue Code of 1986, as amended.

     (iv)     the first aggregate of 187,500 shares you exercise in connection
              with ISOA No. 004, as amended, shall not consist of any Original
              Vested Shares.

              A copy of the First Amendment To Incentive Stock Option Agreement
              No. 004 is attached hereto as Exhibit A.

5.   MEDICAL INSURANCE CONTINUATION. At your option, you may continue to be
     covered under the Company's group medical insurance plan for up to eighteen
     (18) months after your resignation date, subject to the terms and
     conditions provided for in the Consolidated Omnibus Budget Reconciliation
     Act of 1985, as amended ("COBRA"). The Company shall, at your option, pay
     the entire cost of your group medical insurance premiums during the entire
     eighteen (18) months of your COBRA continuation period or until such
     earlier time as you become eligible for alternate medical insurance
     coverage from a new employer provided you have timely and properly elected
     COBRA coverage in accordance with the Company's COBRA election procedures,
     notice of which shall be

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     sent to you under separate cover. You agree promptly to notify the Company
     in writing if and when you become eligible for alternate medical coverage
     during this eighteen (18) month period.

6.   TRANSFER OF RESPONSIBILITIES. You shall cooperate fully with the Company
     and its personnel to provide an orderly transfer of your duties and
     responsibilities. This cooperation includes but is not limited to timely
     compliance with all reasonable requests for information.

7.   CONFIDENTIALITY. You agree, to the extent permitted by law, to keep
     confidential and not to disclose the existence or terms of this letter
     agreement or sums paid under this letter agreement to anyone or to any
     organization, except you may disclose such information to your spouse,
     attorney, and financial advisor, provided you have received in advance
     their promises to maintain this information in strict confidence; provided,
     however, that nothing in this Agreement will prevent you from cooperating
     with or participating in any proceeding before the EEOC, the MCAD or any
     other federal, state or local agency or entity. You also agree that you
     will not, without the Company's prior written consent, reveal or disclose
     to any person or entity outside of the Company or use for your own benefit
     or for the benefit of any other person or entity, any fact or information
     which was disclosed to or developed by you during the course of your
     employment with the Company, and is not generally available to the public,
     including but not limited to information and facts concerning the business
     plans, customers, future customers, suppliers, licensors, licensees,
     partners, investors, affiliates, training methods and materials, financial
     information, sales prospects, client lists, Inventions (as defined in
     paragraph 11), or any other scientific, technical, trade or business secret
     or confidential or proprietary information of the Company ("Confidential
     Information").

8.   RETURN OF PROPERTY. You acknowledge that you have returned to the Company
     all property of the Company that is in your possession or under your
     control, including, without limitation, the laptop computer, printer, other
     computer accessories, pager, corporate credit card, telephone card, Company
     keys, and any and all written or digital-based files, documents,
     communications with the Company's directors, officers, employees and
     consultants, and other information with respect to the Company's
     management, business operations or customers, including all files,
     documents, or other information containing Confidential Information. You
     further acknowledge and agree that payment of your final severance payment
     at the end of the eighteen (18) month severance period will be conditioned
     upon your return of the company-issued automobile to the Company, in good
     condition, normal wear and tear excluded, by July 1, 2005. It is understood
     that you may continue to use the company-issued automobile for the duration
     of the eighteen (18) month lease period.

9.   COOPERATION IN LITIGATION. At the Company's request, you agree to assist,
     consult with, and cooperate with the Company in any litigation or
     administrative proceeding or inquiry that involves the Company, subject to
     reimbursement for your reasonable out of pocket expenses, such as travel,
     meals, or lodging.

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10.  NON-DISPARAGEMENT. You further agree, to the extent permitted by law, that
     you will not, at any time after the date hereof, make any remarks or
     comments, orally or in writing, to the Company's shareholders, officers,
     directors, employees, agents, or others, which or who have, or could
     reasonably be anticipated to have, business dealings with the Company,
     which remarks or comments reasonably could be construed to be derogatory or
     disparaging to the Company or any of its shareholders, officers, directors,
     employees, attorneys or agents, or which reasonably could be anticipated to
     be damaging or injurious to the Company's reputation or good will or to the
     reputation or good will of any person associated with the Company. The
     Company agrees, to the extent permitted by law, that neither the Chief
     Executive Officer or Scientific Founder will, at any time after the date
     hereof, make any remarks or comments, orally or in writing, to third
     parties which remarks or comments reasonably could be construed to be
     derogatory or disparaging to you which reasonably could be anticipated to
     be damaging or injurious to your reputation or good will.

11.  OWNERSHIP OF INVENTIONS AND DEVELOPMENTS. You agree that all ideas,
     discoveries, creations, manuscripts and properties, innovations,
     improvements, know-how, inventions, designs, developments, apparatus,
     techniques, algorithms, software, mask works, methods, and formulae which
     you worked upon, conceived, made, developed or improved, during your
     employment by the Company whether or not reduced to practice and whether or
     not patentable, copyrightable, or otherwise protectable, alone or in
     conjunction with any other party, and whether or not at the request or upon
     the suggestion of the Company (all of the foregoing being hereinafter
     referred to as the "Inventions"), are the sole and exclusive property of
     the Company. The Company is and will be the sole owner of all patents,
     copyrights and other proprietary rights in and with respect to such
     Inventions. To the fullest extent permitted by law, such Inventions will be
     deemed works made for hire. You hereby transfer and assign to the Company
     any proprietary rights which you may have or acquire in any such Inventions
     and you waive any moral rights or other special rights which you may have
     or accrue therein. You agree to execute any documents and take any actions
     that may be required to effect and confirm such transfer and assignment and
     waiver. In the event that the Company is unable for any reason to secure
     your signature to any lawful and necessary document required to perfect its
     rights in and to any Inventions as set forth in this paragraph 12, you
     hereby designate the Company as your agent for, and grant to the Company a
     power of attorney with full power of substitution, which power of attorney
     shall be deemed coupled with an interest, for the purpose of effecting the
     foregoing assignments from you to the Company.

12.  NON-SOLICITATION. You agree that you will not, while receiving severance
     payments from the Company, directly or indirectly, whether as owner,
     partner, shareholder, consultant, agent, employee, co-venturer or
     otherwise, engage or participate in the hiring of any Company employees or
     consultants or engage or participate in the solicitation of or attempt to
     solicit any Company employees or consultants resulting in the termination
     of their employment or relationship with the Company or resulting in their
     working for any other business, person or company. For purposes of this
     Paragraph 12, employees and consultants shall include those who were
     employed or had a relationship with the Company during the twelve month
     period prior to the resignation date.

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13.  BREACH OF AGREEMENT. You understand and agree that any material breach of
     your obligations under this letter agreement will immediately render the
     Company's obligations and agreements hereunder null and void, all payments
     pursuant to paragraphs 3 and 5 shall immediately cease, and, to the extent
     permitted by law, you shall repay to the Company all sums you have been
     paid or sums paid on your behalf pursuant to paragraphs 3 and 5.

14.  MUTUAL GENERAL RELEASE. You, for yourself and your heirs, legal
     representatives, beneficiaries, assigns and successors in interest, hereby
     knowingly and voluntarily release, remise and forever discharge the Company
     and its successors, assigns, former or current shareholders, officers,
     directors, employees, agents, attorneys and representatives ("Company
     Released Parties") whether in their individual or official capacities, from
     any and all actions or causes of action, suits, debts, claims, complaints,
     contracts, controversies, agreements, promises, damages, claims for
     attorneys' fees, costs, interest, punitive damages or reinstatement,
     judgments and demands whatsoever, in law or equity, you now have, may have
     or ever had, known or unknown from the beginning of the world to this date,
     including, without limitation, any claims under the Age Discrimination in
     Employment Act, 29 U.S.C. Section 621 ET SEQ.; Title VII of the Civil
     Rights Act of 1964, 42 U.S.C. Section 2000e ET SEQ.; the Employee
     Retirement Income Security Act of 1974, as amended, 29 U.S.C. Section 1000
     ET SEQ.; Massachusetts General Laws, Chapters 149, 151B, 214; the
     Massachusetts Civil Rights Act; the Massachusetts Equal Rights Act; the
     Americans with Disabilities Act, 42 U.S.C. Section 12101 et seq.; claims
     for breach of contract or based on tort; and any other statutory,
     regulatory or common law causes of action ("the Released Claims"). The
     Company hereby knowingly and voluntarily releases, remises and forever
     discharges you from any and all actions or causes of action, suits, debts,
     claims, complaints, contracts, controversies, agreements, promises,
     damages, claims for attorneys' fees, costs, interest or punitive damages,
     judgments and demands whatsoever, in law or equity, the Company now has,
     may have or ever had, known or unknown from the beginning of the world to
     this date, including, without limitation, claims for breach of contract or
     based on tort, and any other statutory, regulatory or common law causes of
     action. THE PARTIES HEREBY ACKNOWLEDGE AND UNDERSTAND THAT THIS IS A
     GENERAL RELEASE.

15.  MUTUAL COVENANT NOT TO SUE. To the extent permitted by law, you
     specifically agree not to commence any legal action against any of the
     Company Released Parties arising out of or in connection with the Released
     Claims. To the extent permitted by law, you expressly agree that if you
     commence such an action in violation of this Agreement, you shall indemnify
     the Company Released Parties for the full and complete costs of defending
     such an action and enforcing this Agreement, including reasonable
     attorneys' fees (whether incurred in a third party action or in an action
     to enforce this Agreement), court costs, and other related expenses. You
     further agree that, to the extent permitted by law, if you commence such an
     action despite the provisions of this Agreement, you shall be obligated to
     return to the Company the full amount of all sums paid to you, or on your
     behalf, pursuant to paragraphs 3 and 5. To the extent permitted by law, the
     Company specifically agrees not to commence any legal action against you
     arising out of or in connection with the Released Claims. To the extent
     permitted by law, the Company

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     expressly agrees that if the Company commences such an action in violation
     of this Agreement, the Company shall indemnify you for the full and
     complete costs of defending such an action and enforcing this Agreement,
     including reasonable attorneys' fees (whether incurred in a third party
     action or in an action to enforce this Agreement), court costs, and other
     related expenses.

16.  ACKNOWLEDGMENT. You acknowledge and agree that you understand the meaning
     of this letter agreement and that you freely and voluntarily enter into it
     and the General Release contained herein. You agree that no fact, evidence,
     event, or transaction, whether known or unknown, shall affect in any manner
     the final and unconditional nature of the agreements and releases set forth
     herein. You acknowledge that you have been advised that you have twenty-one
     (21) days to consider this General Release and to consult with an attorney
     prior to executing it. For a period of seven (7) days after executing this
     General Release, you may revoke this General Release by providing written
     notice of said revocation to Dr. Safi R. Bahcall at the address of the
     Company set forth above and this General Release shall not become effective
     or enforceable until said seven-day period has expired.

17.  MISCELLANEOUS. This letter agreement shall be construed in accordance with
     the laws of the Commonwealth of Massachusetts without regard to choice or
     conflict of law principles. A waiver of any breach of or failure to comply
     fully with any provision of this letter agreement by either party shall not
     operate or be construed as a waiver of any subsequent breach thereof or
     failure so to comply. If any portion or provision of this letter agreement
     shall to any extent be deemed invalid or unenforceable, the remainder of
     this letter agreement shall not be affected thereby and each portion and
     provision of this letter agreement shall be valid and enforceable to the
     fullest extent permitted. To avoid any possible misunderstanding, the
     Company intends this letter agreement to be a comprehensive statement of
     the terms of your resignation. This letter agreement supersedes any prior
     understanding or statement made to you by the Company regarding your
     positions with the Company or your arrangements with the Company for the
     period after your resignation. For the same reason, any modifications of
     the terms set forth in this letter agreement must be in writing and signed
     by you and by me on behalf of the Company.

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     Please indicate your agreement to the terms of this letter agreement by
signing and dating the last page of the enclosed copy of this letter agreement,
and return it to me not later than the close of business on May 12, 2004, which
you acknowledge to be twenty-one (21) days from the date of your receipt of this
letter agreement.

                                          Sincerely,


                                          /s/ DR. SAFI R. BAHCALL
                                          ---------------------------
                                          Dr. Safi R. Bahcall
                                          Chief Executive Officer


AGREED TO AND EXECUTED UNDER SEAL THIS 21 day of April, 2004.


                                          /s/ DR. MITSUNORI ONO
                                          ---------------------------
                                          Dr. Mitsunori Ono

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                                    EXHIBIT A

           FIRST AMENDMENT TO INCENTIVE STOCK OPTION AGREEMENT NO. 004

This First Amendment to Incentive Stock Option Agreement No. 004 (the
"Amendment") is dated as of April 8. 2004 and is entered into by and between
SYNTA PHARMACEUTICALS CORP., a Delaware corporation (the "Company"), and
MITSUNORI ONO, an individual (the "Grantee").

Reference is made to that certain Incentive Stock Option Agreement No. 004 dated
December 13, 2002, by and between the Company and the Grantee (the "Option") as
issued pursuant to the 2001 Stock Plan adopted by the Company (the "Stock
Plan"). Reference also is made to that certain Separation Agreement dated as of
April 8, 2004 and by and between the Company and the Grantee (collectively, the
"Separation Agreement"). Capitalized terms used, but not defined, herein shall
have the meanings as set forth in the Option.

WHEREAS, Grantee's employment with the Company terminated upon January 1, 2004
(the "Resignation Date");

WHEREAS, pursuant to the terms of the Option, Grantee's right to exercise the
Option for an aggregate of 812,500 shares (the "Original Vested Shares") vested
upon the Resignation Date; and

WHEREAS, the Company and the Grantee wish to amend the Option pursuant to the
terms of the Separation Agreement.

NOW, THEREFORE, for good and valuable consideration, the receipt and sufficiency
of which is hereby acknowledged by the parties, the parties hereby agree as
follows:

1.   The Option is hereby amended as follows:

     a.   All references to "1,500,000 shares" on the face of the Option are
hereby deleted and replaced with "1,000,000 shares".

     b.   Section 3 of the Option is deleted in its entirety and hereby replaced
with the following:

          "3. VESTING SCHEDULE. The Option shall be exercisable with respect to
     1,000,000 of the Stock Rights Shares immediately, provided that the first
     aggregate of 187,500 Stock Rights Shares that the Participant exercises
     shall not be Original Vested Shares."

     c.   Section 4 of the Option is deleted in its entirety and hereby replaced
with the following:

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          "4. TERM OF OPTION. Subject to earlier termination as provided in this
     Agreement or the Plan, the Option shall expire on the tenth anniversary of
     the Grant Date."

2.   All other terms and conditions of the Option remain in full force and
effect, and are not modified in any way except as expressly set forth herein.
However, in the event of a conflict between the terms and conditions of the
Option and this Amendment, the terms of this Amendment shall govern.

3.   The Grantee hereby acknowledges that the Option as amended by this
Amendment will not be an incentive stock option that complies with Section 422
of the Internal Revenue Code of 1986, as amended.

4.   The Grantee further acknowledges that the exercise of the amended Option by
the Grantee could have negative tax consequences for the Grantee, and that the
Grantee has had sufficient opportunity to discuss the tax consequences of the
foregoing amendments to the Option with his tax advisor(s) prior to entering
into this Amendment.

5.   This Amendment shall be governed by and construed in accordance with the
laws of the State of Delaware, without regard to conflict of laws principles.


IN WITNESS WHEREOF, the parties hereto have executed this Amendment as of the
date first set forth above.


GRANTEE                                   COMPANY

                                          SYNTA PHARMACEUTICALS CORP.


 /s/ MITSUNORI ONO                        /s/ SAFI R. BAHCALL
 ---------------------                    ------------------------
Mitsunori Ono                             By:  Safi R. Bahcall
                                          Its: Chief Executive Officer