Document:

EX-10.15

 Exhibit 10.15 

SYNDAX PHARMACEUTICALS, INC. 

2015 EMPLOYEE STOCK PURCHASE PLAN 

The Board of Directors of the Company has adopted this 2015 Employee Stock Purchase Plan to enable eligible employees of the Company and its
Participating Affiliates, through payroll deductions or other cash contributions, to purchase shares of Common Stock. The Plan is for the benefit of the employees of the Company and any Participating Affiliates. The Plan is intended to benefit the
Company by increasing the employees’ interest in the Company’s growth and success and encouraging employees to remain in the employ of the Company or its Participating Affiliates. 

It is the intention of the Company to have this Plan satisfy the requirements for “employee stock purchase plans” that are set forth
under Section 423 of the Code, although the Company makes no undertaking to, nor represents that it will, maintain the qualified status of this Plan. 

The provisions of the Plan are set forth below: 
  

	 	1.	DEFINITIONS 

 (a) “Account” means a bookkeeping account maintained on
behalf of a Participant by the Custodian for the purpose of investing in Common Stock and engaging in other transactions permitted under the Plan. 

(b) “Administrator” means the person or persons designated to administer the Plan under Section 3(a). 

(c) “Board” means the Board of Directors of the Company. 

(d) “Code” means the Internal Revenue Code of 1986, as amended. References in the Plan to any Code Section shall be deemed to
include, as applicable, regulations and guidance promulgated under such Code Section. 
 (e) “Committee” means a committee
of, and designated from time to time by resolution of, the Board. 
 (f) “Common Stock” means the Company’s common
stock, par value $0.0001 per share. 
 (g) “Company” means Syndax Pharmaceuticals, Inc., a Delaware corporation, or its
successors. 
 (h) “Custodian” means
[                    ] or a successor thereto, or such other person as may be designated from time to time by the Board. 

(i) “Effective Date” means the date of the closing of the Company’s initial public offering. 

(j) “Enrollment Date” means the first day of each Offering Period. 

(k) “Enrollment Form” means the agreement(s) between the Company and a Participant, in such written, electronic, or other
format and/or pursuant to such written, electronic, or other 

 
process as may be established by the Administrator from time to time, pursuant to which an employee who satisfies the eligibility criteria set forth in Section 4 elects to participate in the
Plan or elects to make changes with respect to such participation as permitted by the Plan. 
 (l) “Fair Market Value”
means the value of each share of Common Stock subject to the Plan on a given date determined as follows: if on such date the shares of Common Stock are listed on an established national or regional stock exchange or are publicly traded on an
established securities market, the Fair Market Value of the shares of Common Stock shall be the closing price of the shares of Common Stock on such exchange or in such market (the exchange or market selected by the Board if there is more than one
such exchange or market) on such date or, if such date is not a trading day, on the trading day immediately preceding such date, or, if no sale of the shares of Common Stock is reported for such trading day, on the next preceding day on which any
sale shall have been reported. If the shares of Common Stock are not listed on such an exchange or traded on such a market, the Fair Market Value shall be determined by the Board in good faith. 

(m) “Offering Period” means the period determined by the Administrator pursuant to Section 7, which period shall not
exceed twenty-seven (27) months, during which payroll deductions or other cash payments are accumulated for the purpose of purchasing Common Stock under the Plan. 

(n) “Participating Affiliate” means any company or other trade or business that is a subsidiary of the Company (determined in
accordance with the principles of Section 424(f) of the Code and the regulations thereunder). 
 (o) “Participant”
means an Employee of the Company or a Participating Affiliate who satisfies the eligibility criteria set forth in Section 4 and who has properly elected to participate in the Plan pursuant to Section 5. 

(p) “Plan” means this Syndax Pharmaceuticals, Inc. 2015 Employee Stock Purchase Plan, as it may be amended from time to time.

 (q) “Purchase Period” means the period designated by the Administrator on the last trading day of which purchases of
Common Stock are made under the Plan. 
 (r) “Purchase Price” means the purchase price of each share of Common Stock
purchased under the Plan. 
  

	 	2.	SHARES SUBJECT TO THE PLAN 

 (a) Subject to adjustment as provided in Section 25,
the aggregate number of shares of Common Stock that may be made available for purchase by Participants under the Plan is [                ] shares. In addition, the
number of shares of Common Stock available for purchase by Participants under the Plan shall automatically increase on January 1st of each year, commencing January 1, 2017 and continuing until the expiration of the Plan, in an amount equal
to the lesser of (a) one percent (1%) of the total number of shares of Common Stock outstanding on December 31st of the preceding calendar year, or (b) 250,000 shares of Common Stock. Notwithstanding the foregoing, the Board may act
prior to the first day of any calendar year to provide that there shall be no increase in the share reserve for such calendar year or that the increase in the share reserve for such calendar year shall be a lesser number of shares of Common Stock
than would otherwise occur pursuant to the preceding sentence. 
 (b) The shares of Common Stock issuable under the Plan may, in the
discretion of the Board, be authorized but unissued shares, treasury shares or shares purchased on the open market. 

  
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	 	3.	ADMINISTRATION 

 (a) Administrator. The Plan shall be administered by an
Administrator, which shall be the Board or a Committee. Subject to the express provisions of the Plan, the Administrator will have full authority (i) to adopt, amend, suspend, waive and rescind such rules and regulations and appoint
such agents as it may deem necessary or advisable to administer the Plan, (ii) to correct any defect or supply any omission or reconcile any inconsistency in the Plan, (iii) to construe and interpret the Plan and rules and regulations
thereunder, (iv) to furnish to the Custodian such information as the Custodian may require, and (v) to make all other decisions and determinations necessary or advisable in administering the Plan (including, without limitation,
determinations relating to eligibility, minimum and maximum contribution rates, limits on the number of shares of Common Stock a Participant may elect to purchase with respect to any Offering Period, the Purchase Price, the timing and length of
Offering Periods and Purchase Periods). No person acting in connection with the administration of the Plan will, in that capacity, participate in deciding any matter relating to his or her participation in the Plan. The Administrator’s
determinations under the Plan shall be final, conclusive, and binding on all persons. 
 (b) Custodian. The Custodian shall act as
custodian under the Plan and shall perform such duties as are set forth in the Plan and in any agreement between the Company and the Custodian. The Custodian will establish and maintain, as agent for each Participant, an Account and any
subaccounts as may be necessary or desirable for the administration of the Plan. 
 (c) Other Administrative Provisions. The Company
will furnish information to the Custodian from its records as directed by the Administrator, and such records will be conclusive on all persons unless determined by the Administrator to be incorrect. Each Participant and other person claiming
benefits under the Plan must furnish to the Company in writing an up-to-date mailing address and any other information as the Administrator or Custodian may reasonably request. Any communication, statement or notice mailed with postage prepaid
to any such Participant or other person at the last mailing address filed with the Company will be deemed sufficiently given when mailed and will be binding upon the named recipient. The Plan will be administered on a reasonable and
nondiscriminatory basis, and Plan provisions and rules thereunder will apply in a uniform manner to all persons similarly situated. 

(d) No Liability. No member of the Board or the Committee, nor any of their agents or designees, shall be liable to any person
(i) for any act, failure to act, or determination made in good faith with respect to the Plan or (ii) for any tax (including any interest and penalties) by reason of the failure of the Plan to satisfy the requirements of Section 423
of the Code, the failure of the Participant to satisfy the requirements of Section 423 of the Code, or otherwise asserted with respect to the Plan or shares of Common Stock purchased or deemed purchased under the Plan.

 

	 	4.	ELIGIBLE EMPLOYEES 

 Any employee of the Company or any of its Participating Affiliates
may participate in the Plan, except the following, who are ineligible to participate: (a) an employee whose customary employment is less than twenty (20) hours per week; and (b) an employee who, after exercising his or her rights to
purchase shares of Common Stock under the Plan, would own (directly or by attribution pursuant to Section 424(d) of the Code) shares of Common Stock (including shares that may be acquired under any outstanding options) representing five percent
(5%) or more of the total combined voting power of all classes of stock of the Company. For purposes of the Plan, the employment relationship will be treated as continuing while the individual is on sick leave or other leave of absence that the
Company or any of its Participating Affiliates approves or which meets the requirements of Treasury Regulations Section 1.421-1(h)(2). Where the period of leave exceeds three (3) months and the individual’s right to reemployment is

  
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not guaranteed either by statute or by contract, the employment relationship will be deemed to have terminated three (3) months and one (1) day following the commencement of such leave.
The Administrator may, at any time in its sole discretion, if it deems it advisable to do so, exclude the participation of the employees of a particular Participating Affiliate from eligibility to participate in a future Offering Period. 

 

	 	5.	ENROLLMENT IN THE PLAN 

 (a) Initial Enrollment. An employee who is or who will
become eligible under Section 4 on or before a given Enrollment Date may, after receiving current information about the Plan, enroll in the Plan by executing a properly completed Enrollment Form, including thereon the employee’s election
as to the rate of payroll deductions or, if authorized by the Administrator, payment of the Purchase Price by means of periodic cash payments, for the Offering Period. For an employee’s enrollment to be effective for any Offering Period, such
Enrollment Form must be submitted as directed by the Administrator at least [             (    )] days before the Enrollment Date for the Offering Period. 

(b) Automatic Reenrollment for Subsequent Offering Periods. Following the end of each Offering Period, each then-current
Participant shall be automatically reenrolled in the next Offering Period unless (i) the Participant terminates participation in the Plan before the Enrollment Date for the next Offering Period in accordance with Section 19 or
(ii) on such Enrollment Date he or she is ineligible to participate in the Plan under Section 4. The rate of payroll contributions or periodic cash payments for a Participant who is automatically reenrolled for an Offering Period will
be the same as the rate of payroll contributions or periodic cash payments in effect at the end of the preceding Offering Period, unless the Participant submits, as directed by the Administrator, a new Enrollment Form at least
[             (    )] days before the Enrollment Date for the Offering Period and designates a different rate of payroll contributions or, if authorized by the
Administrator, periodic cash payments. 
  

	 	6.	CONTRIBUTIONS 

 (a) Payroll Deductions. Subject to Section 6(b), a
Participant’s contributions under the Plan shall be by means of after-tax payroll deductions from each payroll period which ends during the Offering Period, at the rate elected by the Participant in his or her Enrollment Form. Notwithstanding
the foregoing and any election of a Participant, a Participant’s rate of payroll deductions will be adjusted downward by the Company at any time or from time to time as necessary to ensure that the limit on the amount of Common Stock purchased
with respect to an Offering Period set forth in Section 10 is not exceeded. Unless otherwise permitted by the Administrator, a Participant may not during any Offering Period change his or her percentage of payroll deductions for that Offering
Period, nor may a Participant withdraw any contributed funds, other than in accordance with Sections 15 through 19. 
 (b) Periodic Cash
Payments. Pursuant to Section 5, if authorized by the Administrator, a Participant may elect on the Enrollment Form to make contributions under the Plan by means of periodic cash payments to the Plan during an Offering Period. Unless
otherwise permitted by the Administrator, a Participant may not during any Offering Period change his or her amount of periodic cash payments for that Offering Period, nor may a Participant withdraw any contributed funds, other than in accordance
with Sections 15 through 19. Notwithstanding the foregoing, under any of the circumstances contemplated by the Plan, where the purchase of shares of Common Stock will be made through periodic cash payments in lieu of payroll deductions, the failure
to make any such payments shall reduce, to the extent of the deficiency in such payments, the number of shares purchasable under this Plan by the Participant. 

  
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	 	7.	OFFERING PERIODS AND PURCHASE PERIODS 

 The Administrator shall determine the Offering
Periods and Purchase Periods. The first Offering Period under the Plan shall commence on the date determined by the Administrator. Each Offering Period shall consist of one or more Purchase Periods, as determined by the Administrator. 

 

	 	8.	RIGHTS TO PURCHASE COMMON STOCK; PURCHASE PRICE 

 Rights to purchase shares of Common
Stock will be deemed granted to Participants as of the first trading day of each Offering Period. The Purchase Price of each share of Common Stock shall be determined by the Administrator; provided, however, that the Purchase Price shall not
be less than the lesser of eighty-five percent (85%) of the Fair Market Value of the Common Stock (i) on the first trading day of the Offering Period or (ii) on the last trading day of the Purchase Period; provided further,
that in no event shall the Purchase Price be less than the par value of the Common Stock. 
  

	 	9.	TIMING OF PURCHASE 

 Unless a Participant has given prior written notice terminating such
Participant’s participation in the Plan pursuant to Section 15, or the Participant’s participation in the Plan has otherwise been terminated pursuant to Sections 16 through 19, such Participant will be deemed to have
automatically exercised his or her right to purchase shares of Common Stock on the last trading day of the Purchase Period (except as provided in Section 15) for the number of shares of Common Stock (including fractional shares) that the
accumulated funds in the Participant’s Account at that time will purchase at the Purchase Price, subject to the participation adjustment provided for in Section 14 and subject to adjustment under Section 25. 

 

	 	10.	PURCHASE LIMITATIONS 

 Notwithstanding any other provision of the Plan, no Participant
may purchase in any one calendar year under the Plan and all other “employee stock purchase plans” of the Company and its Participating Affiliates shares of Common Stock having an aggregate Fair Market Value in excess of $25,000,
determined as of the first trading date of the Offering Period as to shares purchased during such period. In addition, in no event may a Participant purchase more than
[                ] shares of Common Stock in any one Offering Period; provided, however, that the Administrator may in its discretion, prior to the start of an
Offering Period, set a different limit on the number or value of shares of Common Stock a Participant may purchase during the Offering Period. Effective upon the last trading day of the Purchase Period, a Participant will become a stockholder with
respect to the shares of Common Stock purchased during such period and will thereupon have all dividend, voting and other ownership rights incident thereto except as otherwise provided in Section 11. Notwithstanding the foregoing, no shares of
Common Stock shall be sold pursuant to the Plan unless the Plan is approved by the Company’s stockholders in accordance with Section 24. 
  

	 	11.	STOCK ISSUANCE AND SALE OF PLAN SHARES 

 On the last trading day of the Purchase Period,
a Participant will be credited with the number of shares of Common Stock purchased for his or her Account under the Plan during such Purchase Period. Shares of Common Stock purchased under the Plan will be held by the Custodian. The Custodian may
hold the shares of Common Stock purchased under the Plan by book entry or in the form of stock certificates in nominee names and may commingle shares held in its custody in a single account without identification as to individual Participants. 

  
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 The Administrator shall have the right to require any or all of the following with respect to
shares of Common Stock purchased under the Plan: 
 (a) that a Participant may not request that all or part of the shares of Common Stock be
reissued in the Participant’s own name and shares be delivered to the Participant until two (2) years (or such shorter period of time as the Administrator may designate) have elapsed since the first day of the Offering Period in which the
shares were purchased and one (1) year has elapsed since the day the shares were purchased (the “Holding Period”); 

(b) that all sales of shares of Common Stock during the Holding Period applicable to such purchased shares be performed through a licensed
broker acceptable to the Company; and 
 (c) that Participants abstain from selling or otherwise transferring shares of Common Stock
purchased pursuant to the Plan for a period lasting up to two (2) years from the date the shares of Common Stock were purchased pursuant to the Plan. 
  

	 	12.	WITHHOLDING OF TAXES 

 To the extent that a Participant recognizes ordinary income in
connection with a sale or other transfer of any shares of Common Stock purchased under the Plan, the Company may withhold amounts needed to cover such taxes from any payments otherwise due and owing to the Participant or from shares that would
otherwise be issued to the Participant under the Plan. Any Participant who sells or otherwise transfers shares of Common Stock purchased under the Plan within two (2) years after the beginning of the Offering Period in which the shares were
purchased must within thirty (30) days of such transfer notify the Company’s Payroll Department in writing of such transfer. 
  

	 	13.	ACCOUNT STATEMENTS 

 The Custodian will reflect contributions, purchases, dividends and
distributions and reinvestment thereof, withdrawals and transfers of shares of Common Stock and other Plan transactions by appropriate adjustments to the Participant’s Account. The Custodian will, not less frequently than semi-annually, provide
or cause to be provided a written statement to the Participant showing the transactions in his or her Account and the date thereof, the number of shares of Common Stock purchased, the aggregate Purchase Price paid, the Purchase Price per share, the
brokerage fees and commissions paid (if any), the total shares of Common Stock held for the Participant’s Account (computed to at least three decimal places) and other information. 

 

	 	14.	PARTICIPATION ADJUSTMENT 

 If in any Purchase Period the number of unsold shares that may
be made available for purchase under the Plan pursuant to Section 2 is insufficient to permit exercise of all rights deemed exercised by all Participants pursuant to Section 9, a participation adjustment will be made, and the number of
shares of Common Stock purchasable by all Participants will be reduced proportionately. Any funds then remaining in a Participant’s Account after such exercise will be refunded to the Participant. 

 

	 	15.	CHANGES IN ELECTIONS TO PURCHASE 

 (a) Ceasing Payroll Deductions or Periodic
Payments. A Participant may, at any time prior to the last trading day of the Purchase Period, by written notice to the Administrator, direct the Administrator to cease payroll deductions (or, if the payment for shares is being made through
periodic 

  
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cash payments, notify the Administrator that such payments will be terminated), in accordance with the following alternatives: 

(i) the Participant’s option to purchase shall be reduced to the number of shares that may be purchased, as of the last day of the
Purchase Period, with the amount then credited to the Participant’s Account; or 
 (ii) withdraw the amount in such Participant’s
Account and terminate such Participant’s option to purchase. 
 (b) Decreasing or Increasing Payroll Deductions. A Participant
may decrease his or her rate of payroll deduction once during a Purchase Period (but not below ten dollars ($10.00) per pay period) by delivering to the Company a new Enrollment Form. If and only if expressly permitted by the Administrator, as
determined in its sole discretion, for an Offering Period, a Participant may increase the rate of his or her payroll deduction once during the Offering Period. 

(c) Modifying Payroll Deductions or Periodic Payments at the Start of an Offering Period. Any Participant may increase or decrease his
or her payroll deductions or periodic cash payments, to take effect on the first day of the next Offering Period, by delivering to the Company a new Enrollment Form. 
  

	 	16.	TERMINATION OF EMPLOYMENT 

 In the event a Participant’s employment with the Company
and all Participating Affiliates terminates, or is deemed terminated, for any reason other than death prior to the last day of the Purchase Period, the amount in the Participant’s Account will be distributed, and the Participant’s option
to purchase will terminate. 
  

	 	17.	AUTHORIZED LEAVE OF ABSENCE OR DISABILITY 

 Payroll deductions for shares for which a
Participant has an option to purchase may be suspended during any period of absence of the Participant from work due to an authorized leave of absence or disability or, if the Participant so elects, periodic payments for such shares may continue to
be made in cash. 
 If such Participant returns to active service prior to the last day of the Purchase Period, the Participant’s
payroll deductions will be resumed, and if such Participant did not make periodic cash payments during the Participant’s period of absence, the Participant shall, by written notice to the Administrator within ten (10) days after the
Participant’s return to active service, but not later than the last day of the Purchase Period, elect: 
 (a) to make up any deficiency
in the Participant’s Account resulting from a suspension of payroll deductions by an immediate cash payment; 
 (b) not to make up such
deficiency, in which event the number of shares to be purchased by the Participant shall be reduced to the number of shares which may be purchased with the amount, if any, then credited to the Participant’s Account plus the aggregate amount, if
any, of all payroll deductions to be made thereafter; or 
 (c) to withdraw the total amount in the Participant’s Account and terminate
the Participant’s option to purchase. 
 A Participant on authorized leave of absence or disability on the last day of the Purchase
Period who is still an eligible employee under Section 4 shall deliver written notice to the Administrator on or before the last day of the Purchase Period, electing one of the alternatives provided in the foregoing

  
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Sections 17(a), 17(b) and 17(c). If any Participant fails to deliver such written notice within ten (10) days after the Participant’s return to active service or by the last day of the
Purchase Period, whichever is earlier, the Participant shall be deemed to have elected Section 17(c). 
  

	 	18.	DEATH 

 In the event of the death of a Participant while the Participant’s option to
purchase shares under the Plan is in effect, the legal representatives of such Participant may, within three (3) months after the Participant’s death (but no later than the last day of the Purchase Period) by written notice to the
Administrator, elect one of the following alternatives: 
 (a) the Participant’s option to purchase shall be reduced to the number of
shares that may be purchased, as of the last day of the Purchase Period, with the amount then credited to the Participant’s Account; or 

(b) withdraw the amount in such Participant’s Account and terminate such Participant’s option to purchase. 

In the event the legal representatives of such Participant fail to deliver such written notice to the Administrator within the prescribed
period, the election to purchase shares shall terminate, and the amount then credited to the Participant’s Account shall be paid to such legal representatives. 
  

	 	19.	TERMINATION OF PARTICIPATION 

 A Participant will be refunded all moneys in his or her
Account, and his or her participation in the Plan will be terminated if (a) the Board elects to terminate the Plan as provided in Section 24, (b) the Participant ceases to be eligible to participate in the Plan under Section 4,
or (c) in accordance with Sections 16 and 17. As soon as practicable following termination of a Participant’s participation in the Plan, the Administrator will deliver to the Participant a check representing the amount in the
Participant’s Account and a book entry statement representing the number of shares of Common Stock held in the Participant’s Account. Once terminated, participation may not be reinstated for the then-current Offering Period, but, if
otherwise eligible, the employee may elect to participate in any subsequent Offering Period. 
  

	 	20.	TRANSFER; ASSIGNMENT 

 No Participant may transfer or assign his or her rights to
purchase shares of Common Stock under the Plan, whether voluntarily, by operation of law or otherwise. Any payment of cash or issuance of shares of Common Stock under the Plan may be made only to the Participant (or, in the event of the
Participant’s death, to the Participant’s estate). During a Participant’s lifetime, only such Participant may exercise his or her rights to purchase shares of Common Stock under the Plan. Once a book entry or stock certificate has
been issued to the Participant or the Participant’s estate for his or her Account, such book entry or stock certificate may be assigned the same as any other book entry or stock certificate. 

 

	 	21.	APPLICATION OF FUNDS 

 All funds received or held by the Company under the Plan may be
used for any corporate purpose until applied to the purchase of shares of Common Stock or refunded to the Participant. Participants’ Accounts need not be segregated. 

  
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	 	22.	NO RIGHT TO CONTINUED EMPLOYMENT 

 Neither the Plan nor any right to purchase Common
Stock under the Plan confers upon any employee or Participant any right to continued employment with the Company or any of its Participating Affiliates, nor will a Participant’s participation in the Plan restrict or interfere in any way with
the right of the Company or any of its Participating Affiliates to terminate the Participant’s employment at any time. 
  

	 	23.	AMENDMENT OF THE PLAN 

 The Board may, at any time, amend the Plan in any respect
(including an increase in the percentage specified in Section 8 used in calculating the Purchase Price); provided, however, that without approval of the stockholders of the Company, no amendment shall be made (a) increasing the
number of shares specified in Section 2 that may be made available for purchase under the Plan (except as provided in Section 25) or (b) changing the eligibility requirements for participating in the Plan. No amendment may be made
that impairs the rights of Participants that have vested at the time of amendment. 
  

	 	24.	TERM; TERMINATION OR SUSPENSION OF THE PLAN 

 The Plan shall be effective as of the
Effective Date. If stockholder approval of the Plan is not obtained within twelve (12) months of the Effective Date, any shares of Common Stock purchased under the Plan following the Effective Date shall automatically be deemed forfeited and
cancelled. The Board may suspend or terminate the Plan at any time and for any reason or for no reason, provided that such suspension or termination shall not impair any rights of Participants that have vested at the time of suspension or
termination. In any event, the Plan shall, without further action of the Board, terminate on the day before the tenth (10th) anniversary of the date of adoption of the Plan by the Board or, if earlier, at such time as all shares of Common Stock
that may be made available for purchase under the Plan pursuant to Section 2 have been issued. 
  

	 	25.	CHANGES IN CAPITALIZATION 

 (a) Changes in Common Stock. If the number of
outstanding shares of Common Stock is increased or decreased or the shares of Common Stock are changed into or exchanged for a different number or kind of shares or other securities of the Company by reason of any recapitalization, reclassification,
stock split, reverse stock split, spin-off, combination of shares, exchange of shares, stock dividend, or other distribution payable in capital stock, or other increase or decrease in such shares effected without receipt of consideration by the
Company occurring after the Effective Date, the number and kinds of shares that may be purchased under the Plan shall be adjusted proportionately and accordingly by the Administrator. In addition, the number and kind of shares for which rights are
outstanding shall be similarly adjusted so that the proportionate interest of a Participant immediately following such event shall, to the extent practicable, be the same as immediately prior to such event. Any such adjustment in outstanding rights
shall not change the aggregate Purchase Price payable by a Participant with respect to shares subject to such rights but shall include a corresponding proportionate adjustment in the Purchase Price per share. Notwithstanding the foregoing, in the
event of a spin-off that results in no change in the number of outstanding shares of Common Stock, the Company may, in such manner as the Company deems appropriate, adjust (i) the number and kind of shares for which rights are outstanding under
the Plan and (ii) the Purchase Price per share. 
 (b) Reorganization in Which the Company is
the Surviving Corporation. Subject to Section 25(c), if the Company shall be the surviving corporation in any reorganization, merger or consolidation of the Company with one or more other corporations, all outstanding rights under the
Plan shall pertain to and apply to the securities to which a holder of the number of shares of Common Stock subject to such rights would have been entitled immediately following such reorganization, merger or

  
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consolidation, with a corresponding proportionate adjustment of the Purchase Price per share so that the aggregate Purchase Price thereafter shall be the same as the aggregate Purchase Price of
the shares subject to such rights immediately prior to such reorganization, merger or consolidation. 
 (c)
Reorganization in Which the Company is Not the Surviving Corporation, Sale of Assets or Stock, and Other Corporate Transactions. Upon any dissolution or liquidation of the Company, or upon a merger,
consolidation or reorganization of the Company with one or more other corporations in which the Company is not the surviving corporation, or upon a sale of all or substantially all of the assets of the Company to another corporation, or upon any
transaction (including, without limitation, a merger or reorganization in which the Company is the surviving corporation) approved by the Board that results in any person or entity owning more than fifty percent (50%) of the combined voting
power of all classes of stock of the Company, the Plan and all rights outstanding hereunder shall terminate, except to the extent provision is made in writing in connection with such transaction for the continuation of the Plan and/or the assumption
of the rights theretofore granted, or for the substitution for such rights of new rights covering the stock of a successor corporation, or a parent or subsidiary thereof, with appropriate adjustments as to the number and kinds of shares and purchase
prices, in which event the Plan and rights theretofore granted shall continue in the manner and under the terms so provided. In the event of any such termination of the Plan, the Offering Period and the Purchase Period shall be deemed to have ended
on the last trading day prior to such termination, and in accordance with Section 11 the rights of each Participant then outstanding shall be deemed to be automatically exercised on such last trading day. The Administrator shall send written
notice of an event that will result in such a termination to all Participants at least ten (10) days prior to the date upon which the Plan will be terminated. 

(d) Adjustments. Adjustments under this Section 25 related to stock or securities of the Company shall be made by the
Administrator, whose determination in that respect shall be final, binding, and conclusive. 
 (e)
No Limitations on Company. The grant of a right pursuant to the Plan shall not affect or limit in any way the right or power of the Company to make adjustments, reclassifications, reorganizations or changes of its capital or
business structure or to merge, consolidate, dissolve or liquidate, or to sell or transfer all or any part of its business or assets. 
  

	 	26.	GOVERNMENTAL REGULATION 

 The Company’s obligation to issue, sell and deliver shares
of Common Stock pursuant to the Plan is subject to such approval of any governmental authority and any national securities exchange or other market quotation system as may be required in connection with the authorization, issuance or sale of such
shares. 
  

	 	27.	STOCKHOLDER INFORMATION RIGHTS 

 The Company will deliver to each Participant who
purchases shares of Common Stock under the Plan, as promptly as practicable by mail or otherwise, all notices of meetings, proxy statements, proxies and other materials distributed by the Company to its stockholders. There will be no charge to
Participants in connection with such notices, proxies and other materials. Any shares of Common Stock held by the Custodian for a Participant’s Account will be voted in accordance with the Participant’s duly delivered and signed proxy
instructions. 
  

	 	28.	VOTING RIGHTS 

 Each Participant will be entitled to vote the number of shares of Common
Stock credited to his or her Account (including any fractional shares credited to such Account) on any matter as to which the 

  
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approval of the Company’s stockholders is sought. If a Participant does not vote or grant a valid proxy with respect to shares credited to his or her Account, such shares will be voted by
the Custodian in accordance with any stock exchange or other rules governing the Custodian in the voting of shares held for customer accounts. Similar procedures will apply in the case of any consent solicitation of the Company’s stockholders.

  

	 	29.	DIVIDEND REINVESTMENT 

 Cash dividends on any Common Stock credited to a
Participant’s Account will be automatically reinvested in additional shares of Common Stock; such amounts will not be available in the form of cash to Participants. All cash dividends paid on Common Stock credited to Participants’
Accounts will be paid over by the Company to the Custodian at the dividend payment date. The Custodian will aggregate all purchases of Common Stock in connection with the Plan for a given dividend payment date. Purchases of Common Stock
for purposes of dividend reinvestment will be made as promptly as practicable (but not more than thirty (30) days) after a dividend payment date. The Custodian will make such purchases, as directed by the Administrator, either (i) in
transactions on any securities exchange upon which Common Stock is traded, otherwise in the over-the-counter market or in negotiated transactions, or (ii) directly from the Company at 100% of the Fair Market Value of a share of Common Stock on
the dividend payment date. Any shares of Common Stock distributed as a dividend or distribution in respect of shares of Common Stock or in connection with a split of the Common Stock credited to a Participant’s Account will be credited to
such Account. In the event of any other non-cash dividend or distribution in respect of Common Stock credited to a Participant’s Account, the Custodian will, if reasonably practicable and at the direction of the Administrator, sell any
property received in such dividend or distribution as promptly as practicable and use the proceeds to purchase additional shares of Common Stock in the same manner as cash paid over to the Custodian for purposes of dividend reinvestment.

 

	 	30.	FRACTIONAL SHARES 

 Unless otherwise determined by the Administrator, purchases of Common
Stock under the Plan executed by the Custodian may result in the crediting of fractional shares of Common Stock to a Participant’s Account. Such fractional shares will be computed to at least three decimal places. Fractional shares
will not, however, be issued by the Company, and certificates representing fractional shares will not be delivered to Participants under any circumstances. If at any time fractional shares will not be credited to Participants’ Accounts,
the Administrator shall determine whether a Participant’s payroll deductions remaining after the purchase of the greatest possible number of whole shares on a given purchase date will be refunded or will be retained and applied to purchases in
the next Offering Period. 
  

	 	31.	RULE 16B-3 

 Transactions under this Plan are intended to comply with all applicable
conditions of Rule 16b-3 or any successor provision under the Securities Exchange Act of 1934, as amended. If any provision of the Plan or action by the Board or the Committee fails to so comply, it shall be deemed null and void to the extent
permitted by applicable law and deemed advisable by the Board. Moreover, in the event the Plan does not include a provision required by Rule 16b-3 to be stated in this Plan, such provision (other than one relating to eligibility requirements or the
price and amount of awards) shall be deemed automatically to be incorporated by reference into the Plan. 
  

	 	32.	PAYMENT OF PLAN EXPENSES 

 The Company will bear all costs of administering and carrying
out the Plan. 

  
 11 

	 	33.	GOVERNING LAW.

 The Plan shall be governed by, and construed in accordance with, the laws
of the State of Delaware (except its choice-of-law provisions) and applicable U.S. federal laws. 

  
 12EX-10.16

 Exhibit 10.16 

EXECUTIVE EMPLOYMENT AGREEMENT 

This EXECUTIVE EMPLOYMENT AGREEMENT (this “Agreement”) is entered into as of the 30th day of September, 2015 (the “Execution Date”), between Briggs W. Morrison, M.D. (“Executive”) and SYNDAX PHARMACEUTICALS, INC. (the
“Company”). Certain capitalized terms used in this Agreement are defined in Article 7. 
 RECITALS 

A. The Company is a biopharmaceutical company. 

B. The Company desires to employ Executive, or to continue Executive’s employment, in the position set forth below, and Executive
wishes to be employed, or continue to be employed, by the Company in such position, upon the terms and conditions set forth in this Agreement. 

AGREEMENT 
 NOW,
THEREFORE, in consideration of the mutual promises contained herein, the Company and Executive agree as follows: 
 ARTICLE 1 

PRELIMINARY MATTERS 

1.1. Prior Agreement. This Agreement, on its Effective Date, amends, restates and supersedes the Prior Employment Agreement. 

1.2. Effectiveness of Agreement. This Agreement shall be effective and shall supersede the Prior Employment Agreement concurrently with
the Effective Date. Notwithstanding the foregoing, this Agreement shall not become effective, shall be deemed null and void and shall not supersede the Prior Employment Agreement if (i) the Effective Date does not occur prior to
December 31, 2016 or (ii) Executive’s employment with the Company is terminated by the Company or by Executive for any reason (including death or disability) prior to the Effective Date. If this Agreement does not become effective,
the Prior Employment Agreement shall remain in full force and effect in accordance with its terms. 
 ARTICLE 2 

TERMS OF EMPLOYMENT 

2.1. Appointment. Executive shall serve as the Chief Executive Officer, reporting to the Board. As Chief Executive Officer, Executive
will be the most senior officer of the Company and have such duties and responsibilities typically associated with such senior officer. During Executive’s employment with the Company, Executive shall (i) devote substantially all of
Executive’s business efforts to the Company, provided, however, that Executive may continue to serve as a managing director of MPM Asset Management, LLC and as a member of corporate boards of directors, so long as such activities do not
materially interfere with the discharge of Executive’s duties as Chief Executive Officer, and (ii) faithfully and to the best of Executive’s abilities and experience, and in accordance with the standards and ethics of the business in
which the Company is engaged, perform all duties that may be required of Executive by this Agreement, the Company’s policies and procedures, and such other duties and responsibilities as may be assigned to Executive from time to time, as well
as the directives of the Board. During Executive’s employment with the Company, Executive shall not engage in any activity that conflicts with or is detrimental to the Company’s best interests, as determined by the Board. 

 2.2. Employment Term. Executive will be employed by the Company on an “at-will”
basis. This means that either the Company or Executive may terminate Executive’s employment at any time, for any reason, with or without Cause, and with or without advance notice (provided that Resignation for Good Reason (as defined below)
requires certain advanced notice by Executive of Executive’s termination of employment). It also means that Executive’s job title, duties, responsibilities, reporting level, compensation and benefits, as well as the Company’s
personnel policies and procedures, may be changed with or without notice at any time in the Company’s sole discretion. This at-will employment relationship shall not be modified by any conflicting actions or representations of any Company
employee or other party before or during the term of Executive’s employment. 
 2.3. Compensation. 

a) Annual Base Salary. Executive’s annual base salary shall be $501,000 per year (“Annual Base Salary”),
payable in equal installments, less applicable deductions and withholdings, in accordance with the Company’s standard payroll practices. Upon the IPO, the Annual Base Salary shall be increased to $531,000. Executive’s Annual Base Salary
shall be subject to review by the Company’s compensation committee and may be increased, from time to time. 
 b) Benefits.
Executive will be entitled to participate in all of the employee benefits and benefit plans that the Company generally makes available to its full-time employees and executives and for which Executive is eligible in accordance with the
Company’s policies as in effect from time to time. These benefits are subject to the terms, conditions, and eligibility requirements that govern or apply to them. Notwithstanding the foregoing, if applicable, the Company shall make a group
health plan available to Executive, which provides applicable coverage at both Executive’s permanent residence and Executive’s principal place of employment. From time to time and as the Board deems appropriate, Executive may be eligible
to receive options to purchase the Company’s common stock. 
 c) Bonus. In addition to Annual Base Salary, Executive shall be
eligible to earn an annual performance bonus of up to forty percent (40%) of Executive’s Annual Base Salary, which bonus shall be earned upon Executive’s attainment of objectives to be determined by the Board (or the compensation
committee thereof, as such determination may be delegated by the Board to the compensation committee) and continued employment with the Company as described below (the “Target Performance Bonus”). For clarity, it is expected
that Executive will propose corporate objectives for each calendar year that will be Executive’s individual performance milestones. The Board will work with Executive to revise and refine these annual corporate objectives until a mutually
acceptable set of corporate objectives is approved by the Board. The Board may award Executive a Target Performance Bonus for partial achievement of objectives and may grant Executive a higher bonus for exceptional performance. At Executive’s
request, these annual corporate objectives may be revised in the discretion of the Board during the course of the year depending on changed circumstances. The amount of and Executive’s eligibility for the Target Performance Bonus shall be
determined in the sole discretion of the Board (or the compensation committee thereof, as such determination may be delegated by the Board to the compensation committee). If earned, any Target Performance Bonus shall be paid to Executive, less
authorized deductions and applicable withholdings, on or before the February 15th following the calendar year during which such bonus was earned. For the 2015 calendar year, if applicable,
the Target Performance Bonus will be pro-rated based on Executive’s June 22, 2015 start date with the Company. Except as provided in Sections 3.2 and 4.2, Executive shall be eligible to earn the Target Performance Bonus only if Executive
is actively employed and in good standing with the Company on both the determination and payment dates for the Target Performance Bonus. 

d) Other Compensation. Upon the IPO, Executive will be eligible to receive a one-time bonus equal to $100,000. Additionally, upon a
successful transaction that leads to a Change in Control with an aggregate purchase price of $640 million, Executive will be eligible to receive an additional one-time bonus equal to the Annual Base Salary, payable within thirty (30) days
following the closing date of such Change in Control. 

  
 2 

 2.4. Reimbursement of Expenses. Subject to Section 5.10(c), the Company shall
reimburse Executive for Executive’s necessary and reasonable business expenses incurred in connection with Executive’s duties in accordance with the Company’s generally applicable policies. Executive and the Company acknowledge that
Executive will be required to spend a certain amount of time each month at the Company’s Waltham headquarters. Accordingly, the Company will reimburse, or pay for, all reasonable expenses incurred by Executive in connection with commuting
between the Company’s Waltham office and Executive’s current principal residence in New Jersey, including Executive’s actual and reasonable living expenses incurred in the Waltham area and Executive’s actual and reasonable
commuting expenses incurred between Waltham and Executive’s current principal residence in New Jersey. Executive will not be expected to relocate his residence to Waltham, but should Executive choose to relocate his residence to Waltham, the
Company will pay for Executive’s relocation. The foregoing provisions of this Section 2.4 are subject to Section 5.10(c). 

ARTICLE 3 
 CHANGE IN
CONTROL SEVERANCE BENEFITS 
 3.1. Severance Benefits. Upon a Change in Control Termination, and subject to the limitations and
conditions set forth in this Agreement, Executive shall be eligible to receive the benefits set forth in this Article 3. The receipt of any severance payments or benefits pursuant to this Agreement is subject to Executive signing and not revoking a
separation agreement and general release of claims (the “Release”), in substantially the form attached hereto and incorporated herein as Exhibit A, Exhibit B or Exhibit C, as appropriate, which Release
must become effective and irrevocable no later than the sixtieth (60th) day following Executive’s termination of employment (the “Release Deadline Date”). If the
Release does not become effective and irrevocable by the Release Deadline Date, Executive will forfeit any right to any severance payments or benefits under this Agreement. In no event will severance payments or benefits be paid or provided until
the Release actually becomes effective and irrevocable. 
 3.2. Salary and Pro-Rata Bonus Payment. In consideration of
Executive’s execution and non-revocation of the Release by the Release Deadline Date, in a form provided by the Company and in accordance with Article 5, the Company shall pay Executive a severance payment equal to (i) the sum of
Executive’s Monthly Base Salary and Pro-Rata Bonus multiplied by (ii) the number of months in the Change in Control Severance Period, less applicable withholdings. The severance payment shall be payable (except as set forth in Article 5)
in a lump sum on the first regularly-scheduled payroll date occurring on or after the Release Deadline Date. 
 3.3. Health Continuation
Coverage. 
 a) Provided that Executive is eligible and has made the necessary elections for continuation coverage pursuant to
COBRA under a health, dental or vision plan sponsored by the Company, the Company shall pay the applicable premiums (inclusive of premiums for Executive’s dependents for such health, dental or vision plan coverage as in effect immediately prior
to the date of the Change in Control Termination) for such continued health, dental or vision plan coverage following the date of the Change in Control Termination for up to the number of months equal to the Change in Control Benefits Period (but in
no event after such time as Executive is eligible for coverage under a health, dental or vision insurance plan of a subsequent employer or as Executive and Executive’s dependents are no longer eligible for COBRA coverage); provided that if
continued payment by the Company of the applicable premiums would result in a violation of the nondiscrimination rules of Section 105(h)(2) of the Internal Revenue Code of 1986, as amended, or any statute or regulation of similar effect
(including, without limitation, the 2010 Patient Protection and Affordable Care Act, as amended by the 2010 Health 

  
 3 

 
Care and Education Reconciliation Act), then in lieu of providing such continued payment, the Company will instead pay Executive on the first day of each month a fully taxable cash payment equal
to the applicable premiums for that month, subject to applicable tax withholdings, for the remainder of the Change in Control Benefits Period. Such coverage shall be counted as coverage pursuant to COBRA. The Company shall have no obligation in
respect of any premium payments (or any other payments in respect of health, dental or vision coverage from the Company) following the effective date of Executive’s coverage by a health, dental or vision insurance plan of a subsequent employer.
Executive shall be required to notify the Company immediately if Executive becomes covered by a health, dental or vision insurance plan of a subsequent employer. If Executive and Executive’s dependents continue coverage pursuant to COBRA
following the conclusion of the Change in Control Benefits Period, Executive will be responsible for the entire payment of such premiums required under COBRA for the duration of the COBRA period. 

b) For purposes of this Section 3.3, (i) references to COBRA shall be deemed to refer also to analogous provisions of state law, and
(ii) any applicable insurance premiums that are paid by the Company shall not include any amounts payable by Executive under a Code Section 125 health care reimbursement plan, which amounts, if any, are the sole responsibility of
Executive. 
 3.4. Stock Awards. Upon a Change in Control Termination, (i) the vesting and exercisability of all outstanding
options to purchase the Company’s common stock (or stock appreciation rights or other rights with respect to the stock of the Company issued pursuant to any equity incentive plan of the Company) that are held by Executive on the Termination
Date shall be accelerated in full, (ii) any reacquisition or repurchase rights held by the Company with respect to common stock issued or issuable (or with respect to other rights with respect to the stock of the Company issued or issuable)
pursuant to any other stock award granted to Executive pursuant to any equity incentive plan of the Company shall lapse and (iii) the time period that Executive has to exercise any outstanding options to purchase the Company’s common stock
that are held by Executive on the Termination Date shall be extended for a period equal to the shorter of (A) twelve (12) months or (B) the remaining term of the outstanding option. 

ARTICLE 4 
 COVERED
TERMINATION SEVERANCE BENEFITS 
 4.1. Severance Benefits. Upon a Covered Termination, and subject to the limitations and
conditions set forth in this Agreement, Executive shall be eligible to receive the benefits set forth in this Article 4. The receipt of any severance payments or benefits pursuant to this Agreement is subject to Executive signing and not revoking
the appropriate Release, which Release must become effective and irrevocable no later than the Release Deadline Date. If the Release does not become effective and irrevocable by the Release Deadline Date, Executive will forfeit any right to any
severance payments or benefits under this Agreement. In no event will severance payments or benefits be paid or provided until the Release actually becomes effective and irrevocable. 

4.2. Salary Payment. In consideration of Executive’s timely execution and non-revocation of a full release of all claims, in a form
provided by the Company and in accordance with Article 5, the Company shall pay Executive a severance payment equal to the sum of (i) Executive’s Monthly Base Salary multiplied by the number of months in the Covered Termination Severance
Period and (ii) the Target Performance Bonus as in effect on the date of a Covered Termination multiplied by the number of days Executive was employed in the year of the Covered Termination divided by the total number of days in such year, less
applicable withholdings. The severance payment shall be payable (except as set forth in Article 5) in a lump sum on the first regularly-scheduled payroll date occurring on or after the Release Deadline Date. 

  
 4 

 4.3. Health Continuation Coverage. 

a) Provided that Executive is eligible and has made the necessary elections for continuation coverage pursuant to COBRA under a health, dental
or vision plan sponsored by the Company, the Company shall pay for the applicable premiums (inclusive of premiums for Executive’s dependents for such health, dental or vision plan coverage as in effect immediately prior to the date of the
Covered Termination) for such continued health, dental or vision plan coverage following the date of the Covered Termination for up to the number of months equal to the Covered Termination Benefits Period (but in no event after such time as
Executive is eligible for coverage under a health, dental or vision insurance plan of a subsequent employer or as Executive and Executive’s dependents are no longer eligible for COBRA coverage); provided that if continued payment by the Company
of the applicable premiums would result in a violation of the nondiscrimination rules of Section 105(h)(2) of the Internal Revenue Code of 1986, as amended, or any statute or regulation of similar effect (including, without limitation, the 2010
Patient Protection and Affordable Care Act, as amended by the 2010 Health Care and Education Reconciliation Act), then in lieu of providing such continued payment, the Company will instead pay Executive on the first day of each month a fully taxable
cash payment equal to the applicable premiums for that month, subject to applicable tax withholdings, for the remainder of the Covered Termination Benefits Period. Such coverage shall be counted as coverage pursuant to COBRA. The Company shall have
no obligation in respect of any premium payments (or any other payments in respect of health, dental or vision coverage from the Company) following the effective date of Executive’s coverage by a health, dental or vision insurance plan of a
subsequent employer. Executive shall be required to notify the Company immediately if Executive becomes covered by a health, dental or vision insurance plan of a subsequent employer. If Executive and Executive’s dependents continue coverage
pursuant to COBRA following the conclusion of the Covered Termination Benefits Period, Executive will be responsible for the entire payment of such premiums required under COBRA for the duration of the COBRA period. 

b) For purposes of this Section 4.3, (i) references to COBRA shall be deemed to refer also to analogous provisions of state law, and
(ii) any applicable insurance premiums that are paid by the Company shall not include any amounts payable by Executive under a Code Section 125 health care reimbursement plan, which amounts, if any, are the sole responsibility of
Executive. 
 4.4. Stock Awards. Upon a Covered Termination: 

a) the vesting and exercisability of all outstanding options to purchase the Company’s common stock (or stock appreciation rights or other
rights with respect to the stock of the Company issued pursuant to any equity incentive plan of the Company) that are held by Executive on the Termination Date shall be accelerated as to the number of shares of common stock issuable upon exercise of
such option (“Option Shares”) as equals the number of Option Shares as would otherwise vest during the twelve (12) month period following the Termination Date in accordance with the applicable options’ vesting
schedule were the Executive to remain an employee of the Company during such twelve (12) month period (disregarding any other basis for acceleration of vesting of Option Shares during such twelve (12) month period); 

b) any reacquisition or repurchase rights held by the Company with respect to common stock issued or issuable (or with respect to other rights
with respect to the stock of the Company issued or issuable) pursuant to any option to purchase the Company’s common stock (or stock appreciation rights or other rights with respect to the stock of the Company) (“Restricted
Shares”) held by the Executive as of the Termination Date shall lapse as to the number of Restricted Shares as equals the number of Restricted Shares as to which such reacquisition or repurchase rights would otherwise lapse during the
twelve (12) month period following the Termination Date in accordance with the option’s vesting schedule were the Executive to remain an employee of the Company during such twelve (12) month period (disregarding any other basis for
acceleration of the lapsing of such reacquisition or repurchase rights on Restricted Shares during such twelve (12) month period); and 

  
 5 

 c) the time period that Executive has to exercise any outstanding options to purchase the
Company’s common stock that are held by Executive on the Termination Date shall be extended for a period equal to the shorter of (A) twelve (12) months or (B) the remaining term of the outstanding option. 

ARTICLE 5 
 LIMITATIONS
AND CONDITIONS ON BENEFITS 
 5.1. Rights Conditioned on Compliance. Executive’s rights to receive all severance benefits
described in Article 3 and Article 4 shall be conditioned upon and subject to Executive’s compliance with the limitations and conditions on benefits as described in this Article 5. 

5.2. Continuation of Service Until Date of Termination. Executive shall continue to provide service to the Company in good faith until
the Termination Date, unless such performance is otherwise excused in writing by the Company. 
 5.3. Release Prior to Payment of
Benefits. Upon the occurrence of a Change in Control Termination or a Covered Termination, as applicable, and prior to Executive earning any entitlement to any severance or separation benefits under this Agreement on account of such Change in
Control Termination or Covered Termination, as applicable, Executive must execute the appropriate Release, and such Release must become effective in accordance with its terms, but in no event later than the Release Deadline Date. No amount shall be
paid prior to such date. Instead, on the first regularly-scheduled payroll date occurring on or after the Release Deadline Date, the Company will pay Executive the severance amount that Executive would otherwise have received on or prior to such
date but for the delay in payment related to the effectiveness of the Release, with the balance of the severance amount being paid as originally scheduled. The Company may modify the Release in its discretion to comply with changes in applicable law
at any time prior to Executive’s execution of such Release. Such Release shall specifically relate to all of Executive’s rights and claims in existence at the time of such execution and shall confirm Executive’s obligations under the
Confidentiality Agreement and any similar obligations under applicable law. It is understood that, as specified in the applicable Release, Executive has a certain number of calendar days to consider whether to execute such Release. If Executive does
not execute and deliver such Release within the applicable period, no benefits shall be provided or payable under this Agreement, and Executive shall have no further rights, title or interests in or to any severance benefits or payments pursuant to
this Agreement. It is further understood that if Executive is age 40 or older at the time of a Change in Control Termination or a Covered Termination, as applicable, Executive may revoke the applicable Release within seven (7) calendar days
after its execution by Executive. If Executive revokes such Release within such subsequent seven (7) day period, no benefits shall be provided or payable under this Agreement pursuant to such Change in Control Termination or Covered
Termination, as applicable. 
 5.4. Return of Company Property. Not later than the Termination Date, Executive shall return to the
Company all documents (and all copies thereof) and other property belonging to the Company that Executive has in his or her possession or control. The documents and property to be returned include, but are not limited to, all files, correspondence,
email, memoranda, notes, notebooks, records, plans, forecasts, reports, studies, analyses, compilations of data, proposals, agreements, financial information, research and development information, marketing information, operational and personnel
information, databases, computer-recorded information, tangible property and equipment (including, but not limited to, computers, facsimile machines, mobile telephones and servers), credit cards, entry cards, identification badges and keys, and any
materials of any kind which contain or embody any proprietary or confidential information of the Company (and all reproductions thereof in whole or in part). Executive agrees to make a diligent search to locate any such documents, property and
information. If Executive has used any personally owned computer, server or e-mail system to receive, store, review, prepare or 

  
 6 

 
transmit any Company confidential or proprietary data, materials or information, then within ten (10) business days after the Termination Date, Executive shall provide the Company with a
computer-useable copy of all such information and then permanently delete and expunge such confidential or proprietary information from those systems. Executive agrees to provide the Company access to Executive’s system as requested to verify
that the necessary copying and/or deletion is done. 
 5.5. Cooperation and Continued Compliance with Restrictive Covenants.

 a) From and after the Termination Date, Executive shall cooperate fully with the Company in connection with its actual or contemplated
defense, prosecution or investigation of any existing or future litigation, arbitrations, mediations, claims, demands, audits, government or regulatory inquiries, or other matters arising from events, acts or failures to act that occurred during the
time period in which Executive was employed by the Company (including any period of employment with an entity acquired by the Company). Such cooperation includes, without limitation, being available upon reasonable notice, without subpoena, to
provide accurate and complete advice, assistance and information to the Company, including offering and explaining evidence, providing truthful and accurate sworn statements, and participating in discovery and trial preparation and testimony.
Executive also agrees to promptly send the Company copies of all correspondence (for example, but not limited to, subpoenas) received by Executive in connection with any such legal proceedings, unless Executive is expressly prohibited by law from so
doing. The Company will reimburse Executive for reasonable out-of-pocket expenses incurred in connection with any such cooperation (excluding foregone wages, salary or other compensation) within thirty (30) days of Executive’s timely
presentation of appropriate documentation thereof, in accordance with the Company’s standard reimbursement policies and procedures, and will make reasonable efforts to accommodate Executive’s scheduling needs. 

b) From and after the Termination Date, Executive shall continue to abide by all of the terms and provisions of the Confidentiality Agreement
(and any other comparable agreement signed by Executive), in accordance with its terms. 
 c) Executive agrees that the choice of law and
choice of forum provisions in Section 10.10 of the Confidentiality Agreement shall be amended to conform to the choice of law and choice of forum provisions in Section 8.11 of this Agreement. No other terms of the Confidentiality Agreement
are amended by this Agreement, and the Confidentiality Agreement remains in full force and effect. 
 d) Executive acknowledges and agrees
that Executive’s obligations under this Section 5.5 are an essential part of the consideration Executive is providing hereunder in exchange for which and in reliance upon which the Company has agreed to provide the payments and benefits
under this Agreement. Executive further acknowledges and agrees that Executive’s violation of this Section 5.5 inevitably would involve use or disclosure of the Company’s proprietary and confidential information. Accordingly,
Executive agrees that Executive will forfeit, effective as of the date of any breach, any right, entitlement, claim or interest in or to any unpaid portion of the severance payments or benefits provided in Article 3 or Article 4. If it is determined
by a court of competent jurisdiction in any state that any restriction in this Section 5.5 is excessive in duration or scope or is unreasonable or unenforceable under the laws of that state, it is the intention of the parties that such
restriction may be modified or amended by the court to render it enforceable to the maximum extent permitted by the law of that state. 

5.6. Parachute Payments. 

a) Parachute Payment Limitation. If any payment or benefit (including payments and benefits pursuant to this Agreement) Executive
would receive in connection with a Change in Control from the Company or otherwise (“Payment”) would (i) constitute a “parachute payment” within the meaning of Section 280G of the Code, and (ii) but
for this paragraph, be subject to the excise tax imposed by Section 4999 of the Code (the “Excise Tax”), then the Company shall cause to be determined, before 

  
 7 

 
any amounts of the Payment are paid to Executive, which of the following two alternative forms of payment shall be paid to Executive: (A) payment in full of the entire amount of the Payment
(a “Full Payment”), or (B) payment of only a part of the Payment so that Executive receives the largest payment possible without the imposition of the Excise Tax (a “Reduced Payment”). A Full
Payment shall be made in the event that the amount received by Executive on a net after-tax basis is greater than what would be received by Executive on a net after-tax basis if the Reduced Payment were made, otherwise a Reduced Payment shall be
made. If a Reduced Payment is made, (i) the Payment shall be paid only to the extent permitted under the Reduced Payment alternative, and Executive shall have no rights to any additional payments and/or benefits constituting the Payment, and
(ii) reduction in payments and/or benefits shall occur in the following order: (A) reduction of cash payments; (B) cancellation of accelerated vesting of equity awards other than stock options; (C) cancellation of accelerated
vesting of stock options; and (D) reduction of other benefits paid to Executive. In the event that acceleration of compensation from Executive’s equity awards is to be reduced, such acceleration of vesting shall be canceled in the reverse
order of the date of grant. 
 b) The independent registered public accounting firm engaged by the Company for general audit purposes as of
the day prior to the effective date of the Change in Control shall make all determinations required to be made under this Section 5.6. If the independent registered public accounting firm so engaged by the Company is serving as accountant or
auditor for the individual, entity or group effecting the Change in Control, the Company shall appoint a nationally recognized independent registered public accounting firm to make the determinations required hereunder. The Company shall bear all
expenses with respect to the determinations by such independent registered public accounting firm required to be made hereunder. 
 c) The
independent registered public accounting firm engaged to make the determinations hereunder shall provide its calculations, together with detailed supporting documentation, to the Company and Executive within fifteen (15) calendar days after the
date on which Executive’s right to a Payment is triggered (if requested at that time by the Company or Executive) or such other time as requested by the Company or Executive. If the independent registered public accounting firm determines that
no Excise Tax is payable with respect to a Payment, either before or after the application of the Reduced Amount, it shall furnish the Company and Executive with an opinion reasonably acceptable to Executive that no Excise Tax will be imposed with
respect to such Payment. Any good faith determinations of the accounting firm made hereunder shall be final, binding and conclusive upon the Company and Executive. 

5.7. Certain Reductions and Offsets. To the extent that any federal, state or local laws, including, without limitation, the Worker
Adjustment and Retraining Notification Act or any other so-called “plant closing” laws, require the Company to give advance notice or make a payment of any kind to Executive because of Executive’s involuntary termination due to a
layoff, reduction in force, plant or facility closing, sale of business, change in control or any other similar event or reason, the benefits payable under this Agreement shall be correspondingly reduced. The benefits provided under this Agreement
are intended to satisfy any and all statutory obligations that may arise out of Executive’s involuntary termination of employment for the foregoing reasons, and the parties shall construe and enforce the terms of this Agreement accordingly.

 5.8. Mitigation. Except as otherwise specifically provided herein, Executive shall not be required to mitigate damages or the
amount of any payment provided under this Agreement by seeking other employment or otherwise, nor shall the amount of any payment provided for under this Agreement be reduced by any compensation earned by Executive as a result of employment by
another employer or by any retirement benefits received by Executive after the date of a Change in Control Termination or Covered Termination (except as expressly provided in Sections 3.3 and 4.3 above). 

  
 8 

 5.9. Indebtedness of Executive. If Executive is indebted to the Company on the effective
date of a Change in Control Termination or Covered Termination, the Company reserves the right to offset any severance payments and benefits under this Agreement by the amount of such indebtedness. 

5.10. Application of Section 409A. 

a) Separation from Service. Notwithstanding any provision to the contrary in this Agreement, no amount deemed deferred compensation
subject to Section 409A of the Code shall be payable pursuant to Article 3 or Article 4 unless Executive’s termination of employment constitutes a “separation from service” with the Company within the meaning of Section 409A
of the Code and the Department of Treasury Regulations and other guidance promulgated thereunder and, except as provided under Section 5.10(b) hereof, any such amount shall not be paid, or in the case of installments, commence payment, until
the first regularly-scheduled payroll date occurring on or after the sixtieth (60th) day following Executive’s separation from service. Any installment payments that would have been made
to Executive during the sixty (60) day period immediately following Executive’s separation from service but for the preceding sentence shall be paid to Executive on the first regularly-scheduled payroll date occurring on or after the
sixtieth (60th) day after Executive’s separation from service and the remaining payments shall be made as provided in this Agreement. 

b) Specified Executive. Notwithstanding any provision to the contrary in this Agreement, if Executive is deemed at the time of his or
her separation from service to be a “specified employee” for purposes of Section 409A(a)(2)(B)(i) of the Code, to the extent delayed commencement of any portion of the benefits to which Executive is entitled under this Agreement is
required in order to avoid a prohibited distribution under Section 409A(a)(2)(B)(i) of the Code, such portion of Executive’s benefits shall not be provided to Executive prior to the earlier of (i) the expiration of the six (6)-month
period measured from the date of Executive’s “separation from service” with the Company (as such term is defined in the Treasury Regulations issued under Section 409A of the Code) or (ii) the date of Executive’s death.
Upon the first business day following the expiration of the applicable Code Section 409A(a)(2)(B)(i) period, all payments deferred pursuant to this Section 5.10(b) shall be paid in a lump sum to Executive, and any remaining payments due
under this Agreement shall be paid as otherwise provided herein. 
 c) Expense Reimbursements. To the extent that any reimbursement
payable pursuant to this Agreement is subject to the provisions of Section 409A of the Code, any such reimbursement payable to Executive pursuant to this Agreement shall be paid to Executive no later than December 31 of the year following
the year in which the expense was incurred; the amount of expenses reimbursed in one year shall not affect the amount eligible for reimbursement in any subsequent year; and Executive’s right to reimbursement under this Agreement will not be
subject to liquidation or exchange for another benefit. 
 d) Installments. For purposes of Section 409A of the Code (including,
without limitation, for purposes of Treasury Regulation Section 1.409A-2(b)(2)(iii)), Executive’s right to receive any installment payments under this Agreement shall be treated as a right to receive a series of separate payments and,
accordingly, each such installment payment shall at all times be considered a separate and distinct payment. 
 5.11. Tax Withholding.
All payments under this Agreement shall be subject to applicable withholding for federal, state and local income and employment taxes. 

5.12. No Duplication of Severance Benefits. The severance and other benefits provided in Article 3 and Article 4 are mutually exclusive
of each other, and in no event shall Executive receive any severance or other benefits pursuant to both Article 3 and Article 4. 

  
 9 

 ARTICLE 6 

TERMINATION WITH CAUSE OR BY VOLUNTARY RESIGNATION; 

OTHER RIGHTS AND BENEFITS 

6.1. Termination for Cause by the Company. If the Company shall terminate the Executive’s employment with the Company for Cause,
then upon such termination, the Company shall have no further obligation to Executive hereunder except for the payment or provision, as applicable, of (i) the portion of the Annual Base Salary for the period prior to the effective date of
termination earned but unpaid (if any), (ii) all unreimbursed expenses (if any), subject to Sections 2.4 and 5.10(c), and (iii) other payments, entitlements or benefits, if any, in accordance with terms of the applicable plans, programs,
arrangements or other agreements of the Company (other than any severance plan or policy) as to which the Executive held rights to such payments, entitlements or benefits, whether as a participant, beneficiary or otherwise on the date of termination
(“Other Benefits”). For the avoidance of doubt, Executive shall have no right to receive (and Other Benefits shall not include) any amounts under any Company severance plan or policy or pursuant to Article 3 or Article 4 upon
Executive’s termination for Cause. 
 6.2. Termination by Voluntary Resignation by the Executive (other than Resignation for Good
Reason). Upon any voluntary resignation by Executive that is not a Resignation for Good Reason, the Company shall have no further obligation to the Executive hereunder except for the payment of (i) the portion of the Annual Base Salary for
the period prior to the effective date of termination earned but unpaid (if any), (ii) all unreimbursed expenses (if any), subject to Section 2.4 and Section 5.10(c), and (iii) the payment or provision of any Other Benefits. For
the avoidance of doubt, Executive shall have no right to receive (and Other Benefits shall not include) any amounts under any Company severance plan or policy or pursuant to Article 3 or Article 4 upon any voluntary resignation by Executive that is
not a Resignation for Good Reason. 
 6.3. Other Rights and Benefits. Nothing in this Agreement shall prevent or limit
Executive’s continuing or future participation in any benefit, bonus, incentive or other plans, programs, policies or practices provided by the Company and for which Executive may otherwise qualify, nor shall anything herein limit or otherwise
affect such rights as Executive may have under other agreements with the Company except as provided in Article 1, Article 5, Section 6.1 and Section 6.2 above. Except as otherwise expressly provided herein, amounts that are vested benefits
or that Executive is otherwise entitled to receive under any plan, policy, practice or program of the Company at or subsequent to the date of a Change in Control shall be payable in accordance with such plan, policy, practice or program. 

ARTICLE 7 
 DEFINITIONS

 Unless otherwise provided, for purposes of this Agreement, the following definitions shall apply: 

7.1. “Board” means the Board of Directors of the Company. 

7.2. “Cause” means, upon a reasonable determination by the Company, Executive’s: (i) dishonest
statements or acts with respect to the Company, any subsidiary or any affiliate of the Company, which has the effect of materially injuring (whether financially or otherwise) the business or reputation of the Company; (ii) conviction of or
indictment for (A) a felony or (B) any misdemeanor (excluding minor traffic violations) involving moral turpitude, deceit, dishonesty or fraud (“indictment,” for these purposes, meaning an indictment, probable cause hearing or
any other procedure pursuant to which an initial determination of probable or reasonable cause with respect to such offense is made); (iii) gross negligence, willful misconduct or insubordination with respect to the Company or any

  
 10 

 
subsidiary or any affiliate of the Company; or (iv) material breach of any of Executive’s obligations under any agreement to which Executive and the Company or any subsidiary are a
party. With respect to clause (iv), Executive will be given notice and a 30-day period in which to cure such breach, only to the extent such breach can be reasonably expected to be able to be cured within such period. Executive agrees that the
breach of any confidentiality obligation to the Company or any subsidiary shall not be curable to any extent. 
 7.3.
“Change in Control” means the occurrence, in a single transaction or in a series of related transactions, of any one or more of the following events: 

a) Any natural person, entity or group within the meaning of Section 13(d) or 14(d) of the Securities Exchange Act of 1934, as amended
(“Exchange Act Person”), becomes the owner, directly or indirectly, of securities of the Company representing more than fifty percent (50%) of the combined voting power of the Company’s then outstanding securities
other than by virtue of a merger, consolidation or similar transaction. Notwithstanding the foregoing, a Change in Control shall not be deemed to occur (i) on account of the acquisition of securities of the Company by any institutional
investor, any affiliate thereof or any other Exchange Act Person that acquires the Company’s securities in a transaction or series of related transactions that are primarily a private financing transaction for the Company or (ii) solely
because the level of ownership held by any Exchange Act Person (the “Subject Person”) exceeds the designated percentage threshold of the outstanding voting securities as a result of a repurchase or other acquisition of voting
securities by the Company reducing the number of shares outstanding, provided that if a Change in Control would occur (but for the operation of this sentence) as a result of the acquisition of voting securities by the Company, and after such share
acquisition, the Subject Person becomes the owner of any additional voting securities that, assuming the repurchase or other acquisition had not occurred, increases the percentage of the then outstanding voting securities owned by the Subject Person
over the designated percentage threshold, then a Change in Control shall be deemed to occur; 
 b) There is consummated a merger,
consolidation or similar transaction involving, directly or indirectly, the Company if, immediately after the consummation of such merger, consolidation or similar transaction, the stockholders of the Company immediately prior thereto do not own,
directly or indirectly, either (i) outstanding voting securities representing more than fifty percent (50%) of the combined outstanding voting power of the surviving entity in such merger, consolidation or similar transaction or
(ii) more than fifty percent (50%) of the combined outstanding voting power of the parent of the surviving entity in such merger, consolidation or similar transaction; 

c) The stockholders of the Company approve or the Board approves a plan of complete dissolution or liquidation of the Company, or a complete
dissolution or liquidation of the Company shall otherwise occur; or 
 d) There is consummated a sale, lease, license or other disposition of
all or substantially all of the consolidated assets of the Company and its subsidiaries, other than a sale, lease, license or other disposition of all or substantially all of the consolidated assets of the Company and its subsidiaries to an entity,
more than fifty percent (50%) of the combined voting power of the voting securities of which are owned by stockholders of the Company in substantially the same proportion as their ownership of the Company immediately prior to such sale, lease,
license or other disposition. 
 The term Change in Control shall not include a sale of assets, merger or other transaction effected
exclusively for the purpose of changing the domicile of the Company. Notwithstanding the foregoing or any other provision of this Agreement, the definition of Change in Control (or any analogous term) in an individual written agreement between the
Company or any affiliate and the participant shall supersede the foregoing definition with respect to stock awards subject to such agreement (it being understood, however, that if no definition of Change in Control or any analogous term is set forth
in such an individual written agreement, the foregoing definition shall apply). 

  
 11 

 7.4. “Change in Control Benefits Period” means the period of
eighteen (18) months commencing on the Termination Date. 
 7.5. “Change in Control Severance Period”
means the period of twelve (12) months commencing on the Termination Date. 
 7.6. “Change in Control
Termination” means an “Involuntary Termination Without Cause” or “Resignation for Good Reason,” either of which occurs on, or within three (3) months prior to, or within twelve
(12) months following, the effective date of a Change in Control, provided that any such termination is a “separation from service” within the meaning of Treasury Regulation Section 1.409A-1(h). Death and disability shall not be
deemed Change in Control Terminations. 
 7.7. “COBRA” means the Consolidated Omnibus Budget Reconciliation
Act of 1985, as amended. 
 7.8. “Code” means the Internal Revenue Code of 1986, as amended. 

7.9. “Company” means Syndax Pharmaceuticals, Inc. or, following a Change in Control, the surviving entity
resulting from such transaction, or any subsequent surviving entity resulting from any subsequent Change in Control. 
 7.10.
“Confidentiality Agreement” means Executive’s Assignment of Developments, Non-Disclosure and Non-Solicitation Agreement with the Company, dated June 22, 2015 (or any successor agreement thereto). 

7.11. “Covered Termination” means an “Involuntary Termination Without Cause” or
“Resignation for Good Reason,” provided that any such termination is a “separation from service” within the meaning of Treasury Regulation Section 1.409A-1(h). Death and disability shall not be deemed
Covered Terminations. If an Involuntary Termination Without Cause or Resignation for Good Reason qualifies as a Change in Control Termination, it shall not constitute a Covered Termination. 

7.12. “Covered Termination Benefits Period” means the period of eighteen (18) months commencing on the
Termination Date. 
 7.13. “Covered Termination Severance Period” means the period of twelve (12) months
commencing on the Termination Date. 
 7.14. “Effective Date” means the effective date of the first
registration statement filed by the Company to register shares of its common stock for sale to the public through one or more underwriters. 

7.15. “Involuntary Termination Without Cause” means Executive’s dismissal or discharge by the Company for
reasons other than Cause and other than as a result of death or disability. 
 7.16. “IPO” means the
consummation of the Company’s first underwritten initial public offering of the Company’s common stock under the Securities Act of 1933, as amended, that results in such common stock being lasted for trading on a national securities
exchange or an over the counter market. 
 7.17. “Monthly Base Salary” means 1/12th of the greater of (i) Executive’s annual base salary (excluding incentive pay, premium pay, commissions, overtime, bonuses and other forms of variable compensation) as in effect on the
date of a Change in Control Termination or a Covered Termination, as applicable, or (ii) in the case of a Change in Control Termination, Executive’s annual base salary (excluding incentive pay, premium pay, commissions, overtime, bonuses
and other forms of variable compensation) as in effect on the date of a Change in Control. 

  
 12 

 7.18. “Prior Employment Agreement” means that certain offer letter
agreement, between the Company and Executive, dated June 5, 2015. 
 7.19. “Pro-Rata Bonus” means 1/12th of the greater of (i) the average Target Performance Bonus paid to Executive for the three years preceding the date of a Change in Control Termination (or such lesser number of years during
which Executive has been employed by the Company), or (ii) the Target Performance Bonus, as in effect on the date of a Change in Control Termination. 

7.20. “Resignation for Good Reason” means Executive’s resignation from all employee positions Executive
then holds with the Company within sixty (60) days following any of the following events taken without Executive’s consent, provided Executive has given the Company written notice of such event within thirty (30) days after the first
occurrence of such event and the Company has not cured such event within thirty (30) days thereafter: 
 a) A decrease in
Executive’s total target cash compensation (base and bonus) of more than 10% (i.e., a material reduction in Executive’s base compensation and a material breach by the Company of Executive’s employment terms with the Company), other
than in connection with a comparable decrease in compensation for all comparable executives of the Company; 
 b) Executive’s duties,
authority or responsibilities are materially diminished (not simply a change in title or reporting relationships); 
 c) A material breach by
the Company of the terms of the Agreement; 
 d) Either (i) Executive is required to establish residence in a location more than 50
miles from Executive’s current principal personal residence or (ii) there is an increase in Executive’s round-trip driving distance of more than fifty (50) miles from Executive’s current principal personal residence to the
principal office or business location at which Executive is required to perform services (except for required business travel to the extent consistent with Executive’s prior business travel obligations) (“Executive’s Principal
Place of Business”) as a result of a change in location by the Company of Executive’s Principal Place of Business; provided however, that the foregoing shall not include the establishment of a secondary residence within fifty
(50) miles from the Company’s Waltham headquarters with Executive’s consent or any commute between Executive’s current principal personal residence and the Company’s Waltham headquarters; or 

e) The failure of the Company to obtain a satisfactory agreement from any successor to materially assume and materially agree to perform under
the terms of this Agreement. 
 7.21. “Termination Date” means the effective date of the Change in Control
Termination, the Covered Termination or a termination for Cause, as applicable. 
 ARTICLE 8 

GENERAL PROVISIONS 

8.1. Employment Status. This Agreement does not constitute a contract of employment or impose upon Executive any obligation to remain
as an employee, or impose on the Company any obligation (i) to retain Executive as an employee, (ii) to change the status of Executive as an at-will employee or (iii) to change the Company’s policies regarding termination of
employment. 

  
 13 

 8.2. Notices. Any notices provided hereunder must be in writing, and such notices or any
other written communication shall be deemed effective upon the earlier of personal delivery (including personal delivery by facsimile) or the third day after mailing by first class mail, to the Company at its primary office location and to Executive
at Executive’s address as listed in the Company’s payroll records. Any payments made by the Company to Executive under the terms of this Agreement shall be delivered to Executive either in person or at the address as listed in the
Company’s payroll records. 
 8.3. Severability. Whenever possible, each provision of this Agreement will be interpreted in such
manner as to be effective and valid under applicable law, but if any provision of this Agreement is held to be invalid, illegal or unenforceable in any respect under any applicable law or rule in any jurisdiction, such invalidity, illegality or
unenforceability will not affect any other provision or any other jurisdiction, but this Agreement will be reformed, construed and enforced in such jurisdiction as if such invalid, illegal or unenforceable provisions had never been contained herein.

 8.4. Waiver. If either party should waive any breach of any provisions of this Agreement, he, she or it shall not thereby be deemed
to have waived any preceding or succeeding breach of the same or any other provision of this Agreement. 
 8.5. Complete Agreement.
This Agreement, including Exhibit A, Exhibit B and Exhibit C, and the Confidentiality Agreement constitute the entire agreement between Executive and the Company and is the complete, final and exclusive embodiment of their
agreement with regard to this subject matter, wholly superseding all written and oral agreements with respect to payments and benefits to Executive in the event of employment termination. It is entered into without reliance on any promise or
representation other than those expressly contained herein. 
 8.6. Amendment or Termination of Agreement; Continuation of Agreement.
This Agreement may be changed or terminated only upon the mutual written consent of the Company and Executive. The written consent of the Company to a change or termination of this Agreement must be signed by an executive officer of the Company
(other than Executive) after such change or termination has been approved by the Board. Unless so terminated, this Agreement shall continue in effect for as long as Executive continues to be employed by the Company or by any surviving entity
following any Change in Control. In other words, if, following a Change in Control, Executive continues to be employed by the surviving entity without a Change in Control Termination and the surviving entity then undergoes a Change in Control,
following which Executive is terminated by the subsequent surviving entity in a Change in Control Termination, then Executive shall receive the benefits described in Article 3 hereof. 

8.7. Counterparts. This Agreement may be executed in separate counterparts, any one of which need not contain signatures of more than
one party, but all of which taken together will constitute one and the same Agreement. 
 8.8. Headings. The headings of the Articles
and Sections hereof are inserted for convenience only and shall not be deemed to constitute a part hereof nor to affect the meaning thereof. 

8.9. Successors and Assigns. This Agreement is intended to bind and inure to the benefit of and be enforceable by Executive, and the
Company, and any surviving entity resulting from a Change in Control and upon any other person who is a successor by merger, acquisition, consolidation or otherwise to the business formerly carried on by the Company, and their respective successors,
assigns, heirs, executors and administrators, without regard to whether or not such person actively assumes any rights or duties hereunder; provided, however, that Executive may not assign any duties hereunder and may not assign any rights hereunder
without the written consent of the Company, which consent shall not be withheld unreasonably. 

  
 14 

 8.10. Choice of Law. Because of the Company’s and Executive’s interests in
ensuring that disputes regarding this Agreement are resolved on a uniform basis, the parties agree that all questions concerning the construction, validity and interpretation of this Agreement will be governed by the law of the State of New York,
without regard for any conflict of law principles. Further, the parties consent to the jurisdiction of the state and federal courts of the State of New York for all purposes in connection with this Agreement. The parties hereby irrevocably waive, to
the fullest extent permitted by applicable law, any objection which Executive or the Company may now or hereafter have to the laying of venue of any such dispute brought in such court or any defense of inconvenient forum for the maintenance of such
dispute.
 8.11. Arbitration. To ensure the rapid and economical resolution of any disputes that may arise under or relate to this
Agreement or Executive’s employment relationship, Executive and the Company agree that any and all disputes, claims, or causes of action, in law or equity, arising from or relating to the performance, enforcement, execution, or interpretation
of this Agreement, Executive’s employment with the Company, or the termination of Executive’s employment (collectively, “Claims”), shall be resolved to the fullest extent permitted by law, by final, binding, and (to the extent
permitted by law) confidential arbitration before a single arbitrator in the state where Executive is employed. The arbitration shall be governed by the Federal Arbitration Act, 9 U.S.C. Section 1 et seq., as amended, and shall be
administered by the Judicial Arbitration & Mediation Services, Inc. (“JAMS”), in accordance with its then-current Employment Arbitration Rules & Procedures (the “JAMS Rules”). The JAMS Rules are also
available online at http://www.jamsadr.com/rules-employment-arbitration/. The parties or their representatives may also call JAMS at 800.352.5267 if they have questions about the arbitration process. If the JAMS Rules are
inconsistent with the terms of this Agreement, the terms of this Agreement shall govern. Notwithstanding the foregoing, this provision shall exclude Claims that by law are not subject to arbitration. The arbitrator
shall: (a) have the authority to compel adequate discovery for the resolution of all Claims and to award such relief as would otherwise be permitted by law; and (b) issue a written arbitration decision including the arbitrator’s
essential findings and conclusions and a statement of the award. The Company shall pay all JAMS fees in excess of the amount of filing and other court-related fees Executive would have been required to pay if the Claims were asserted in a court
of law. EXECUTIVE AND THE COMPANY UNDERSTAND AND FULLY AGREE THAT BY ENTERING INTO THIS AGREEMENT, BOTH EXECUTIVE AND THE COMPANY ARE GIVING UP THE CONSTITUTIONAL RIGHT TO HAVE A TRIAL BY JURY, AND ARE GIVING UP THE NORMAL RIGHTS OF APPEAL
FOLLOWING THE RENDERING OF A DECISION, EXCEPT AS THE FEDERAL ARBITRATION ACT AND APPLICABLE FEDERAL LAW ALLOW FOR JUDICIAL REVIEW OF ARBITRATION PROCEEDINGS. Nothing in this Agreement shall prevent either Executive or the Company from obtaining
injunctive relief in court to prevent irreparable harm pending the conclusion of any such arbitration. Any awards or final orders in such arbitrations may be entered and enforced as judgments or orders in the federal and state courts of any
competent jurisdiction in compliance with Section 8.11 of this Agreement. 
 8.12. Construction of Agreement. In the event of a
conflict between the text of this Agreement and any summary, description or other information regarding this Agreement, the text of this Agreement shall control. 

8.13. Circular 230 Disclaimer. THE FOLLOWING DISCLAIMER IS PROVIDED IN ACCORDANCE WITH THE INTERNAL REVENUE SERVICE’S CIRCULAR
230 (21 C.F.R. PART 10). ANY TAX ADVICE CONTAINED IN THIS AGREEMENT IS INTENDED TO BE PRELIMINARY, FOR DISCUSSION PURPOSES ONLY AND NOT FINAL. ANY SUCH ADVICE IS NOT INTENDED TO BE USED FOR MARKETING, PROMOTING OR RECOMMENDING ANY TRANSACTION OR FOR
THE USE OF ANY PERSON IN CONNECTION WITH THE PREPARATION OF ANY TAX RETURN. ACCORDINGLY, THIS ADVICE IS NOT INTENDED OR WRITTEN TO BE USED, AND IT CANNOT BE USED, BY ANY PERSON FOR THE PURPOSE OF AVOIDING TAX PENALTIES THAT MAY BE IMPOSED ON SUCH
PERSON. 

  
 15 

 IN WITNESS WHEREOF, the parties have executed this Agreement on the Execution Date written
above. 
  

									
	SYNDAX PHARMACEUTICALS, INC.	  		  	        EXECUTIVE
					
	By:	 	 /s/ Dennis G. Podlesak
	  	        	  	By:	 	 /s/ Briggs W. Morrison

	Name:	 	Dennis G. Podlesak	  		  	Name:	 	Briggs W. Morrison, M.D.
	Title:	 	Chairman of the Board	  		  		 	

  

			
	Exhibit A:	  	Release (Individual Termination – Age 40 or Older)
		
	Exhibit B:	  	Release (Individual and Group Termination – Under Age 40)
		
	Exhibit C:	  	Release (Group Termination – Age 40 or Older)

 EXHIBIT A 

RELEASE 
 (INDIVIDUAL
TERMINATION – AGE 40 OR OLDER) 
 Certain capitalized terms used in this Release are defined in the Executive Employment Agreement
(the “Agreement”) which I have executed and of which this Release is a part. 
 I hereby confirm my obligations
under the Confidentiality Agreement (or other comparable agreement that I have signed, if any). 
 Except as otherwise set forth in this
Release, I hereby release, acquit and forever discharge the Company, its parents and subsidiaries, and their officers, directors, agents, servants, employees, shareholders, successors, assigns and affiliates, of and from any and all claims,
liabilities, demands, causes of action, costs, expenses, attorneys’ fees, damages, indemnities and obligations of every kind and nature, in law, equity or otherwise, known and unknown, suspected and unsuspected, disclosed and undisclosed (other
than any claim for indemnification I may have as a result of any third party action against me based on my employment with the Company), arising out of or in any way related to agreements, events, acts or conduct at any time prior to the date I
execute this Release, including, but not limited to: all such claims and demands directly or indirectly arising out of or in any way connected with my employment with the Company or the termination of that employment, including, but not limited to,
claims of intentional and negligent infliction of emotional distress, any and all tort claims for personal injury, claims or demands related to salary, bonuses, commissions, stock, stock options, or any other ownership interests in the Company,
vacation pay, fringe benefits, expense reimbursements, severance pay, or any other form of compensation; and claims pursuant to any federal, state or local law or cause of action including, but not limited to, the federal Civil Rights Act of 1964,
as amended, the federal Age Discrimination in Employment Act of 1967, as amended (“ADEA”), the federal Employee Retirement Income Security Act of 1974, as amended, the federal Americans with Disabilities Act of 1990, the
California Fair Employment and Housing Act, as amended, the New York City Human Rights Law, as amended, the Massachusetts Fair Employment Practices Law, as amended, the South Carolina Human Affairs Law, as amended, tort law, contract law, wrongful
discharge, discrimination, fraud, defamation, emotional distress, and breach of the implied covenant of good faith and fair dealing; provided, however, that nothing in this paragraph shall be construed in any way to (1) release the Company from
its obligation to indemnify me pursuant to the Company’s indemnification obligation pursuant to written agreement or applicable law; (2) release any claim by me against the Company relating to the validity or enforceability of this release
or the Agreement; (3) prohibit me from exercising any non-waivable right to file a charge with the United States Equal Employment Opportunity Commission (“EEOC”), the National Labor Relations Board (“NLRB”), or any other
government agency (provided, however, that I shall not be entitled to recover any monetary damages or to obtain non-monetary relief if the agency were to pursue any claims relating to my employment with the Company). 

I acknowledge that I am knowingly and voluntarily waiving and releasing any rights I may have to assert claims for age discrimination under
applicable law, including under the ADEA. I also acknowledge that the consideration given under the Agreement for the waiver and release in the preceding paragraph hereof is in addition to anything of value to which I was already entitled. I further
acknowledge that I have been advised by this writing, as required by the ADEA, that: (A) my waiver and release do not apply to any rights or claims that may arise on or after the date I execute this Release; (B) I have the right to consult
with an attorney prior to executing this Release; (C) I have twenty-one (21) days to consider this Release (although I may choose to voluntarily execute this Release earlier); (D) I have seven (7) days following my execution of
this Release to revoke the Release by providing a written notice of revocation to the Company’s Chief Executive Officer; and (E) this Release shall not be effective until the date upon which the revocation period has expired, which shall
be the eighth (8th) day after I execute this Release (provided that I do not revoke it). 

  
 A-1 

 I hereby represent that I have been paid all compensation owed and for all hours worked, I have
received all the leave and leave benefits and protections for which I am eligible, pursuant to the federal Family and Medical Leave Act, any Company policy or applicable law, and I have not suffered any on-the-job injury or illness for which I have
not already filed a workers’ compensation claim. 
 I agree that I will not make any disparaging statements regarding the Company or
its officers, directors, shareholders, members, agents or products jointly or severally. The foregoing shall not be violated by truthful statements in response to legal process, required governmental testimony or filings, or administrative or
arbitral proceedings (including, without limitation, depositions in connection with such proceedings). 
  

	
	EXECUTIVE:
	   

	Signature
	   

	Printed Name
	
	Date:

  
 A-2 

 EXHIBIT B 

RELEASE 
 (INDIVIDUAL
AND GROUP TERMINATION – UNDER AGE 40) 
 Certain capitalized terms used in this Release are defined in the Executive Employment
Agreement (the “Agreement”) which I have executed and of which this Release is a part. 
 I hereby confirm my
obligations under the Confidentiality Agreement (or other comparable agreement that I have signed, if any). 
 Except as otherwise set forth
in this Release, I hereby release, acquit and forever discharge the Company, its parents and subsidiaries, and their officers, directors, agents, servants, employees, shareholders, successors, assigns and affiliates, of and from any and all claims,
liabilities, demands, causes of action, costs, expenses, attorneys’ fees, damages, indemnities and obligations of every kind and nature, in law, equity or otherwise, known and unknown, suspected and unsuspected, disclosed and undisclosed (other
than any claim for indemnification I may have as a result of any third party action against me based on my employment with the Company), arising out of or in any way related to agreements, events, acts or conduct at any time prior to the date I
execute this Release, including, but not limited to: all such claims and demands directly or indirectly arising out of or in any way connected with my employment with the Company or the termination of that employment, including, but not limited to,
claims of intentional and negligent infliction of emotional distress, any and all tort claims for personal injury, claims or demands related to salary, bonuses, commissions, stock, stock options, or any other ownership interests in the Company,
vacation pay, fringe benefits, expense reimbursements, severance pay, or any other form of compensation; and claims pursuant to any federal, state or local law or cause of action including, but not limited to, the federal Civil Rights Act of 1964,
as amended, the federal Employee Retirement Income Security Act of 1974, as amended, the federal Americans with Disabilities Act of 1990, the California Fair Employment and Housing Act, as amended, the New York City Human Rights Law, as amended, the
Massachusetts Fair Employment Practices Law, as amended, the South Carolina Human Affairs Law, as amended, tort law, contract law, wrongful discharge, discrimination, fraud, defamation, emotional distress, and breach of the implied covenant of good
faith and fair dealing; provided, however, that nothing in this paragraph shall be construed in any way to (1) release the Company from its obligation to indemnify me pursuant to the Company’s indemnification obligation pursuant to written
agreement or applicable law; (2) release any claim by me against the Company relating to the validity or enforceability of this release or the Agreement; (3) prohibit me from exercising any non-waivable right to file a charge with the
United States Equal Employment Opportunity Commission (“EEOC”), the National Labor Relations Board (“NLRB”), or any other government agency (provided, however, that I shall not be entitled to recover any monetary damages or to
obtain non-monetary relief if the agency were to pursue any claims relating to my employment with the Company). 
 I acknowledge that the
consideration given under the Agreement for the waiver and release in the preceding paragraph hereof is in addition to anything of value to which I was already entitled. I further acknowledge that I have been advised by this writing that:
(A) my waiver and release do not apply to any rights or claims that may arise on or after the date I execute this Release; (B) I have the right to consult with an attorney prior to executing this Release; and (C) I have twenty-one
(21) days to consider this Release (although I may choose to voluntarily execute this Release earlier). 
 I hereby represent that I
have been paid all compensation owed and for all hours worked, I have received all the leave and leave benefits and protections for which I am eligible, pursuant to the federal Family and Medical Leave Act, any Company policy or applicable law, and
I have not suffered any on-the-job injury or illness for which I have not already filed a workers’ compensation claim. 

  
 B-1 

 I agree that I will not make any disparaging statements regarding the Company or its officers,
directors, shareholders, members, agents or products jointly or severally. The foregoing shall not be violated by truthful statements in response to legal process, required governmental testimony or filings, or administrative or arbitral proceedings
(including, without limitation, depositions in connection with such proceedings). 
  

	
	EXECUTIVE:
	   

	Signature
	   

	Printed Name
	
	Date:

  
 B-2 

 EXHIBIT C 

RELEASE 
 (GROUP
TERMINATION – AGE 40 OR OLDER) 
 Certain capitalized terms used in this Release are defined in the Executive Employment Agreement
(the “Agreement”) which I have executed and of which this Release is a part. 
 I hereby confirm my obligations
under the Confidentiality Agreement (or other comparable agreement that I have signed, if any). 
 Except as otherwise set forth in this
Release, I hereby release, acquit and forever discharge the Company, its parents and subsidiaries, and their officers, directors, agents, servants, employees, shareholders, successors, assigns and affiliates, of and from any and all claims,
liabilities, demands, causes of action, costs, expenses, attorneys’ fees, damages, indemnities and obligations of every kind and nature, in law, equity or otherwise, known and unknown, suspected and unsuspected, disclosed and undisclosed (other
than any claim for indemnification I may have as a result of any third party action against me based on my employment with the Company), arising out of or in any way related to agreements, events, acts or conduct at any time prior to the date I
execute this Release, including, but not limited to: all such claims and demands directly or indirectly arising out of or in any way connected with my employment with the Company or the termination of that employment, including, but not limited to,
claims of intentional and negligent infliction of emotional distress, any and all tort claims for personal injury, claims or demands related to salary, bonuses, commissions, stock, stock options, or any other ownership interests in the Company,
vacation pay, fringe benefits, expense reimbursements, severance pay, or any other form of compensation; and claims pursuant to any federal, state or local law or cause of action including, but not limited to, the federal Civil Rights Act of 1964,
as amended, the federal Age Discrimination in Employment Act of 1967, as amended (“ADEA”), the federal Employee Retirement Income Security Act of 1974, as amended, the federal Americans with Disabilities Act of 1990, the
California Fair Employment and Housing Act, as amended, the New York City Human Rights Law, as amended, the Massachusetts Fair Employment Practices Law, as amended, the South Carolina Human Affairs Law, as amended, tort law, contract law, wrongful
discharge, discrimination, fraud, defamation, emotional distress, and breach of the implied covenant of good faith and fair dealing; provided, however, that nothing in this paragraph shall be construed in any way to (1) release the Company from
its obligation to indemnify me pursuant to the Company’s indemnification obligation pursuant to written agreement or applicable law; (2) release any claim by me against the Company relating to the validity or enforceability of this release
or the Agreement; (3) prohibit me from exercising any non-waivable right to file a charge with the United States Equal Employment Opportunity Commission (“EEOC”), the National Labor Relations Board (“NLRB”), or any other
government agency (provided, however, that I shall not be entitled to recover any monetary damages or to obtain non-monetary relief if the agency were to pursue any claims relating to my employment with the Company). 

I acknowledge that I am knowingly and voluntarily waiving and releasing any rights I may have to assert claims for age discrimination under
applicable law, including under the ADEA. I also acknowledge that the consideration given under the Agreement for the waiver and release in the preceding paragraph hereof is in addition to anything of value to which I was already entitled. I further
acknowledge that I have been advised by this writing, as required by the ADEA, that: (A) my waiver and release do not apply to any rights or claims that may arise on or after the date I execute this Release; (B) I have the right to consult
with an attorney prior to executing this Release; (C) I have forty-five (45) days to consider this Release (although I may choose to voluntarily execute this Release earlier); (D) I have seven

  
 C-1 

 
(7) days following my execution of this Release to revoke the Release by providing a written notice of revocation to the Company’s Chief Executive Officer; (E) this Release shall
not be effective until the date upon which the revocation period has expired, which shall be the eighth day (8th) after I execute this Release; and (F) I have received with this Release the required written disclosure for a “group
termination” under the ADEA, including a detailed list of the job titles and ages of all employees who were terminated in this group termination and the ages of all employees of the Company in the same job classification or organizational unit
who were not terminated. 
 I hereby represent that I have been paid all compensation owed and for all hours worked, I have received all the
leave and leave benefits and protections for which I am eligible, pursuant to the federal Family and Medical Leave Act, any Company policy or applicable law, and I have not suffered any on-the-job injury or illness for which I have not already filed
a workers’ compensation claim. 
 I agree that I will not engage in any conduct that is injurious to the reputation of the Company or
its parents, subsidiaries and affiliates, including but not limited to disparagement of the Company, its officers, Board members, employees and shareholders. The foregoing shall not be violated by a statement made in a deposition, trial or
administrative proceeding in response to legal process; by any statement made to a government agency; or whenever I make any statement to a court, administrative tribunal or government agency as required by law.

 

	
	EXECUTIVE:
	   

	Signature
	   

	Printed Name
	
	Date:

  
 C-2

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