EX-4.4



 Exhibit 4.4 

DIMENSION THERAPEUTICS, INC. 

2015 STOCK OPTION AND INCENTIVE PLAN 

SECTION 1. GENERAL PURPOSE OF THE PLAN; DEFINITIONS 

The name of the plan is the Dimension Therapeutics, Inc. 2015 Stock Option and Incentive Plan (the “Plan”). The purpose of
the Plan is to encourage and enable the officers, employees, Non-Employee Directors and Consultants of Dimension Therapeutics, Inc. (the “Company”) and its Subsidiaries upon whose judgment, initiative and efforts the Company largely
depends for the successful conduct of its business to acquire a proprietary interest in the Company. It is anticipated that providing such persons with a direct stake in the Company’s welfare will assure a closer identification of their
interests with those of the Company and its stockholders, thereby stimulating their efforts on the Company’s behalf and strengthening their desire to remain with the Company. 

The following terms shall be defined as set forth below: 

“Act” means the Securities Act of 1933, as amended, and the rules and regulations thereunder. 

“Administrator” means either the Board or the compensation committee of the Board or a similar committee performing the
functions of the compensation committee and which is comprised of not less than two Non-Employee Directors who are independent. 

“Award” or “Awards,” except where referring to a particular category of grant under the Plan, shall include
Incentive Stock Options, Non-Qualified Stock Options, Stock Appreciation Rights, Restricted Stock Units, Restricted Stock Awards, Unrestricted Stock Awards, Cash-Based Awards, Performance Share Awards and Dividend Equivalent Rights. 

“Award Certificate” means a written or electronic document setting forth the terms and provisions applicable to an Award
granted under the Plan. Each Award Certificate is subject to the terms and conditions of the Plan. 
 “Board”
means the Board of Directors of the Company. 
 “Cash-Based Award” means an Award entitling the recipient to receive a
cash-denominated payment. 
 “Code” means the Internal Revenue Code of 1986, as amended, and any successor Code, and
related rules, regulations and interpretations. 
 “Consultant” means any natural person that provides bona fide services
to the Company, and such services are not in connection with the offer or sale of securities in a capital-raising transaction and do not directly or indirectly promote or maintain a market for the Company’s securities. 

“Covered Employee” means an employee who is a “Covered Employee” within the meaning of Section 162(m) of the
Code. 
 “Dividend Equivalent Right” means an Award entitling the grantee to receive credits based on cash dividends that
would have been paid on the shares of Stock specified in the Dividend Equivalent Right (or other award to which it relates) if such shares had been issued to and held by the grantee. 

“Effective Date” means the date on which the Plan becomes effective as set forth in Section 22. 

“Exchange Act” means the Securities Exchange Act of 1934, as amended, and the rules and regulations thereunder. 

  
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 “Fair Market Value” of the Stock on any given date means the fair market value
of the Stock determined in good faith by the Administrator; provided, however, that if the Stock is admitted to quotation on the National Association of Securities Dealers Automated Quotation System (“NASDAQ”), NASDAQ Global Market or
another national securities exchange, the determination shall be made by reference to market quotations. If there are no market quotations for such date, the determination shall be made by reference to the last date preceding such date for
which there are market quotations; provided further, however, that if the date for which Fair Market Value is determined is the first day when trading prices for the Stock are reported on a national securities exchange, the Fair Market Value shall
be the “Price to the Public” (or equivalent) set forth on the cover page for the final prospectus relating to the Company’s Initial Public Offering. 

“Incentive Stock Option” means any Stock Option designated and qualified as an “incentive stock option” as defined
in Section 422 of the Code. 
 “Initial Public Offering” means the first underwritten, firm commitment public
offering pursuant to an effective registration statement under the Act covering the offer and sale by the Company of its equity securities, or such other event as a result of or following which the Stock shall be publicly held. 

“Non-Employee Director” means a member of the Board who is not also an employee of the Company or any Subsidiary. 

“Non-Qualified Stock Option” means any Stock Option that is not an Incentive Stock Option. 

“Option” or “Stock Option” means any option to purchase shares of Stock granted pursuant to Section 6. 

“Performance-Based Award” means any Restricted Stock Award, Restricted Stock Units, Performance Share Award or Cash-Based
Award granted to a Covered Employee that is intended to qualify as “performance-based compensation” under Section 162(m) of the Code and the regulations promulgated thereunder. 

“Performance Criteria” means the criteria that the Administrator selects for purposes of establishing the Performance Goal or
Performance Goals for an individual for a Performance Cycle. The Performance Criteria (which shall be applicable to the organizational level specified by the Administrator, including, but not limited to, the Company or a unit, division, group,
or Subsidiary of the Company) that will be used to establish Performance Goals are limited to the following: total shareholder return; earnings before interest, taxes, depreciation and amortization; net income (loss) (either before or after
interest, taxes, depreciation and/or amortization); changes in the market price of the Stock; economic value-added; development, clinical, regulatory or commercial milestones; funds from operations or similar measure; sales or revenue; acquisitions
or strategic transactions; operating income (loss); cash flow (including, but not limited to, operating cash flow and free cash flow); return on capital, assets, equity, or investment; return on sales; gross or net profit levels; productivity;
expense; margins; operating efficiency; customer satisfaction; working capital; earnings (loss) per share of Stock; sales or market shares and number of customers, any of which may be measured either in absolute terms or as compared to any
incremental increase or as compared to results of a peer group. The Committee may appropriately adjust any evaluation performance under a Performance Criterion to exclude any of the following events that occurs during a Performance Cycle:
(i) asset write-downs or impairments, (ii) litigation or claim judgments or settlements, (iii) the effect of changes in tax law, accounting principles or other such laws or provisions affecting reporting results, (iv) accruals
for reorganizations and restructuring programs, and (v) any item of an unusual nature or of a type that indicates infrequency of occurrence, or both, including those described in the Financial Accounting Standards Board’s authoritative
guidance and/or in management’s discussion and analysis of financial condition of operations appearing the Company’s annual report to stockholders for the applicable year. 

“Performance Cycle” means one or more periods of time, which may be of varying and overlapping durations, as the
Administrator may select, over which the attainment of one or more Performance Criteria will be measured for the purpose of determining a grantee’s right to and the payment of a Restricted Stock Award, Restricted Stock Units, Performance Share
Award or Cash-Based Award, the vesting and/or payment of which is subject to the attainment of one or more Performance Goals. Each such period shall not be less than 12 months. 

  
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 “Performance Goals” means, for a Performance Cycle, the specific goals
established in writing by the Administrator for a Performance Cycle based upon the Performance Criteria. 
 “Performance
Share Award” means an Award entitling the recipient to acquire shares of Stock upon the attainment of specified performance goals. 

“Restricted Shares” means the shares of Stock underlying a Restricted Stock Award that remain subject to a risk of forfeiture
or the Company’s right of repurchase. 
 “Restricted Stock Award” means an Award of Restricted Shares subject
to such restrictions and conditions as the Administrator may determine at the time of grant. 
 “Restricted Stock
Units” means an Award of stock units subject to such restrictions and conditions as the Administrator may determine at the time of grant. 

“Sale Event” means (i) the sale of all or substantially all of the assets of the Company on a consolidated basis to an
unrelated person or entity, (ii) a merger, reorganization or consolidation pursuant to which the holders of the Company’s outstanding voting power and outstanding stock immediately prior to such transaction do not own a majority of the
outstanding voting power and outstanding stock or other equity interests of the resulting or successor entity (or its ultimate parent, if applicable) immediately upon completion of such transaction, (iii) the sale of all of the Stock of the
Company to an unrelated person, entity or group thereof acting in concert, or (iv) any other transaction in which the owners of the Company’s outstanding voting power immediately prior to such transaction do not own at least a majority of
the outstanding voting power of the Company or any successor entity immediately upon completion of the transaction other than as a result of the acquisition of securities directly from the Company. 

“Sale Price” means the value as determined by the Administrator of the consideration payable, or otherwise to be received by
stockholders, per share of Stock pursuant to a Sale Event. 
 “Section 409A” means Section 409A of the Code and the
regulations and other guidance promulgated thereunder. 
 “Stock” means the Common Stock, par value $0.0001 per share, of
the Company, subject to adjustments pursuant to Section 3. 
 “Stock Appreciation Right” means an Award entitling the
recipient to receive shares of Stock having a value equal to the excess of the Fair Market Value of the Stock on the date of exercise over the exercise price of the Stock Appreciation Right multiplied by the number of shares of Stock with respect to
which the Stock Appreciation Right shall have been exercised. 
 “Subsidiary” means any corporation or other entity
(other than the Company) in which the Company has at least a 50 percent interest, either directly or indirectly. 
 “Ten
Percent Owner” means an employee who owns or is deemed to own (by reason of the attribution rules of Section 424(d) of the Code) more than 10 percent of the combined voting power of all classes of stock of the Company or any
parent or subsidiary corporation. 
 “Unrestricted Stock Award” means an Award of shares of Stock free of any
restrictions. 
 SECTION 2. ADMINISTRATION OF PLAN; ADMINISTRATOR AUTHORITY TO SELECT GRANTEES AND DETERMINE AWARDS 

(a) Administration of Plan. The Plan shall be administered by the Administrator. 

(b) Powers of Administrator. The Administrator shall have the power and authority to grant Awards consistent with the terms of the
Plan, including the power and authority: 

  
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 (i) to select the individuals to whom Awards may from time to time be granted; 

(ii) to determine the time or times of grant, and the extent, if any, of Incentive Stock Options, Non-Qualified Stock Options, Stock
Appreciation Rights, Restricted Stock Awards, Restricted Stock Units, Unrestricted Stock Awards, Cash-Based Awards, Performance Share Awards and Dividend Equivalent Rights, or any combination of the foregoing, granted to any one or more
grantees; 
 (iii) to determine the number of shares of Stock to be covered by any Award; 

(iv) to determine and modify from time to time the terms and conditions, including restrictions, not inconsistent with the terms of the Plan,
of any Award, which terms and conditions may differ among individual Awards and grantees, and to approve the forms of Award Certificates; 

(v) to accelerate at any time the exercisability or vesting of all or any portion of any Award; 

(vi) subject to the provisions of Section 6(c), to extend at any time the period in which Stock Options may be exercised; and 

(vii) at any time to adopt, alter and repeal such rules, guidelines and practices for administration of the Plan and for its own acts and
proceedings as it shall deem advisable; 
 (viii) to interpret the terms and provisions of the Plan and any Award (including related written
instruments); 
 (ix) to make all determinations it deems advisable for the administration of the Plan; to decide all disputes arising in
connection with the Plan; and 
 (x) to otherwise supervise the administration of the Plan. 

All decisions and interpretations of the Administrator shall be binding on all persons, including the Company and Plan grantees. 

(c) Delegation of Authority to Grant Awards. Subject to applicable law, the Administrator, in its discretion, may delegate to the
Chief Executive Officer of the Company all or part of the Administrator’s authority and duties with respect to the granting of Awards to individuals who are (i) not subject to the reporting and other provisions of Section 16 of the
Exchange Act and (ii) not Covered Employees. Any such delegation by the Administrator shall include a limitation as to the amount of Stock underlying Awards that may be granted during the period of the delegation and shall contain
guidelines as to the determination of the exercise price and the vesting criteria. The Administrator may revoke or amend the terms of a delegation at any time but such action shall not invalidate any prior actions of the Administrator’s
delegate or delegates that were consistent with the terms of the Plan. 
 (d) Award Certificate. Awards under the Plan shall be
evidenced by Award Certificates that set forth the terms, conditions and limitations for each Award which may include, without limitation, the term of an Award and the provisions applicable in the event employment or service terminates. 

(e) Indemnification. Neither the Board nor the Administrator, nor any member of either or any delegate thereof, shall be liable
for any act, omission, interpretation, construction or determination made in good faith in connection with the Plan, and the members of the Board and the Administrator (and any delegate thereof) shall be entitled in all cases to indemnification and
reimbursement by the Company in respect of any claim, loss, damage or expense (including, without limitation, reasonable attorneys’ fees) arising or resulting therefrom to the fullest extent permitted by law and/or under the Company’s
articles or bylaws or any directors’ and officers’ liability insurance coverage which may be in effect from time to time and/or any indemnification agreement between such individual and the Company. 

  
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 (f) Foreign Award Recipients. Notwithstanding any provision of the Plan to the
contrary, in order to comply with the laws in other countries in which the Company and its Subsidiaries operate or have employees or other individuals eligible for Awards, the Administrator, in its sole discretion, shall have the power and authority
to: (i) determine which Subsidiaries shall be covered by the Plan; (ii) determine which individuals outside the United States are eligible to participate in the Plan; (iii) modify the terms and conditions of any Award granted to
individuals outside the United States to comply with applicable foreign laws; (iv) establish subplans and modify exercise procedures and other terms and procedures, to the extent the Administrator determines such actions to be necessary or
advisable (and such subplans and/or modifications shall be attached to this Plan as appendices); provided, however, that no such subplans and/or modifications shall increase the share limitations contained in Section 3(a) hereof; and
(v) take any action, before or after an Award is made, that the Administrator determines to be necessary or advisable to obtain approval or comply with any local governmental regulatory exemptions or approvals. Notwithstanding the
foregoing, the Administrator may not take any actions hereunder, and no Awards shall be granted, that would violate the Exchange Act or any other applicable United States securities law, the Code, or any other applicable United States governing
statute or law. 
 SECTION 3. STOCK ISSUABLE UNDER THE PLAN; MERGERS; SUBSTITUTION 

(a) Stock Issuable. The maximum number of shares of Stock reserved and available for issuance under the Plan shall be the sum of
1,368,376 shares outstanding plus the shares of Stock remaining available for issuance under the Dimension Therapeutics, Inc. 2013 Stock Plan (the “2013 Plan”) as of the Effective Date (the “Initial Limit”), subject to
adjustment as provided in this Section 3, plus on January 1, 2016 and each January 1 thereafter through January 1 2020, the number of shares of Stock reserved and available for issuance under the Plan shall be cumulatively
increased by 4.0% percent of the number of shares of Stock issued and outstanding on the immediately preceding December 31 or such lesser number of shares determined by the Administrator (the “Annual Increase”). For purposes of
this limitation, the shares of Stock underlying any Awards that are forfeited, canceled, held back upon exercise of an Option or settlement of an Award to cover the exercise price or tax withholding, reacquired by the Company prior to vesting,
satisfied without the issuance of Stock or otherwise terminated under this Plan or the 2013 Plan (other than by exercise) shall be added back to the shares of Stock available for issuance under the Plan. In the event the Company repurchases
shares of Stock on the open market, such shares shall not be added to the shares of Stock available for issuance under the Plan. Subject to such overall limitation, (a) the maximum aggregate number of shares of Stock that may be issued in
the form of Incentive Stock Options shall not exceed the Initial Limit cumulatively increased on January 1, 2016 and on each January 1 thereafter by the lesser of the Annual Increase for such year or 4,500,000 shares of Stock and
(b) shares of Stock may be issued up to such maximum number pursuant to any type or types of Award; provided, however, that Stock Options or Stock Appreciation Rights with respect to no more than 4,000,000 shares of Stock may be granted to any
one individual grantee during any one calendar year period. The shares available for issuance under the Plan may be authorized but unissued shares of Stock or shares of Stock reacquired by the Company. 

(b) Changes in Stock. Subject to Section 3(c) hereof, if, as a result of any reorganization, recapitalization,
reclassification, stock dividend, stock split, reverse stock split or other similar change in the Company’s capital stock, the outstanding shares of Stock are increased or decreased or are exchanged for a different number or kind of shares or
other securities of the Company, or additional shares or new or different shares or other securities of the Company or other non-cash assets are distributed with respect to such shares of Stock or other securities, or, if, as a result of any merger
or consolidation, sale of all or substantially all of the assets of the Company, the outstanding shares of Stock are converted into or exchanged for securities of the Company or any successor entity (or a parent or subsidiary thereof), the
Administrator shall make an appropriate or proportionate adjustment in (i) the maximum number of shares reserved for issuance under the Plan, including the maximum number of shares that may be issued in the form of Incentive Stock Options,
(ii) the number of Stock Options or Stock Appreciation Rights that can be granted to any one individual grantee and the maximum number of shares that may be granted under a Performance-Based Award, (iii) the number and kind of shares or
other securities subject to any then outstanding Awards under the Plan, (iv) the repurchase price, if any, per share subject to each outstanding Restricted Stock Award, and (v) the exercise price for each share subject to any then
outstanding Stock Options and Stock Appreciation Rights under the Plan, without changing the aggregate exercise price (i.e., the exercise price multiplied by the number of Stock Options and Stock Appreciation Rights) as to which such Stock Options
and Stock Appreciation Rights remain exercisable. The Administrator shall also make equitable or proportionate adjustments in the number of shares subject to outstanding Awards and the exercise price and the terms of


  
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outstanding Awards to take into consideration cash dividends paid other than in the ordinary course or any other extraordinary corporate event. The adjustment by the Administrator shall be
final, binding and conclusive. No fractional shares of Stock shall be issued under the Plan resulting from any such adjustment, but the Administrator in its discretion may make a cash payment in lieu of fractional shares. 

(c) Mergers and Other Transactions. In the case of and subject to the consummation of a Sale Event, the parties thereto may cause
the assumption or continuation of Awards theretofore granted by the successor entity, or the substitution of such Awards with new Awards of the successor entity or parent thereof, with appropriate adjustment as to the number and kind of shares and,
if appropriate, the per share exercise prices, as such parties shall agree. To the extent the parties to such Sale Event do not provide for the assumption, continuation or substitution of Awards, upon the effective time of the Sale Event, the
Plan and all outstanding Awards granted hereunder shall terminate. In such case, except as may be otherwise provided in the relevant Award Certificate, all Options and Stock Appreciation Rights that are not exercisable immediately prior to the
effective time of the Sale Event shall become fully exercisable as of the effective time of the Sale Event, all other Awards with time-based vesting, conditions or restrictions shall become fully vested and nonforfeitable as of the effective time of
the Sale Event, and all Awards with conditions and restrictions relating to the attainment of performance goals may become vested and nonforfeitable in connection with a Sale Event in the Administrator’s discretion or to the extent specified in
the relevant Award Certificate. In the event of such termination, (i) the Company shall have the option (in its sole discretion) to make or provide for a cash payment to the grantees holding Options and Stock Appreciation Rights, in
exchange for the cancellation thereof, in an amount equal to the difference between (A) the Sale Price multiplied by the number of shares of Stock subject to outstanding Options and Stock Appreciation Rights (to the extent then exercisable at
prices not in excess of the Sale Price) and (B) the aggregate exercise price of all such outstanding Options and Stock Appreciation Rights; or (ii) each grantee shall be permitted, within a specified period of time prior to the
consummation of the Sale Event as determined by the Administrator, to exercise all outstanding Options and Stock Appreciation Rights (to the extent then exercisable) held by such grantee. 

SECTION 4. MAXIMUM AWARDS TO NON-EMPLOYEE DIRECTORS 

Notwithstanding anything to the contrary in this Plan, the value of all Awards awarded under this Plan and all other cash compensation paid by
the Company to any Non-Employee Director in any calendar year shall not exceed $1,000,000. For the purpose of this limitation, the value of any Award shall be its grant date fair value, as determined in accordance with Financial Accounting
Standards Board Accounting Standards Codification 718 but excluding the impact of estimated forfeitures related to service-based vesting provisions. 

SECTION 5. ELIGIBILITY 

Grantees under the Plan will be such full or part-time officers and other employees, Non-Employee Directors and Consultants of the Company and
its Subsidiaries as are selected from time to time by the Administrator in its sole discretion. 
 SECTION 6. STOCK OPTIONS 

(a) Award of Stock Options. The Administrator may grant Stock Options under the Plan. Any Stock Option granted under the Plan
shall be in such form as the Administrator may from time to time approve. 
 Stock Options granted under the Plan may be either Incentive
Stock Options or Non-Qualified Stock Options. Incentive Stock Options may be granted only to employees of the Company or any Subsidiary that is a “subsidiary corporation” within the meaning of Section 424(f) of the
Code. To the extent that any Option does not qualify as an Incentive Stock Option, it shall be deemed a Non-Qualified Stock Option. 

Stock Options granted pursuant to this Section 5 shall be subject to the following terms and conditions and shall contain such additional
terms and conditions, not inconsistent with the terms of the Plan, as the Administrator shall deem desirable. If the Administrator so determines, Stock Options may be granted in lieu of cash compensation at the optionee’s election, subject
to such terms and conditions as the Administrator may establish. 

  
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 (b) Exercise Price. The exercise price per share for the Stock covered by a Stock
Option granted pursuant to this Section 6 shall be determined by the Administrator at the time of grant but shall not be less than 100 percent of the Fair Market Value on the date of grant. In the case of an Incentive Stock Option that is
granted to a Ten Percent Owner, the option price of such Incentive Stock Option shall be not less than 110 percent of the Fair Market Value on the grant date. 

(c) Option Term. The term of each Stock Option shall be fixed by the Administrator, but no Stock Option shall be exercisable more
than ten years after the date the Stock Option is granted. In the case of an Incentive Stock Option that is granted to a Ten Percent Owner, the term of such Stock Option shall be no more than five years from the date of grant. 

(d) Exercisability; Rights of a Stockholder. Stock Options shall become exercisable at such time or times, whether or not in
installments, as shall be determined by the Administrator at or after the grant date. The Administrator may at any time accelerate the exercisability of all or any portion of any Stock Option. An optionee shall have the rights of a
stockholder only as to shares acquired upon the exercise of a Stock Option and not as to unexercised Stock Options. 
 (e) Method of
Exercise. Stock Options may be exercised in whole or in part, by giving written or electronic notice of exercise to the Company, specifying the number of shares to be purchased. Payment of the purchase price may be made by one or more
of the following methods except to the extent otherwise provided in the Option Award Certificate: 
 (i) In cash, by certified or bank check
or other instrument acceptable to the Administrator; 
 (ii) Through the delivery (or attestation to the ownership following such procedures
as the Company may prescribe) of shares of Stock that are not then subject to restrictions under any Company plan. Such surrendered shares shall be valued at Fair Market Value on the exercise date; 

(iii) By the optionee delivering to the Company a properly executed exercise notice together with irrevocable instructions to a broker to
promptly deliver to the Company cash or a check payable and acceptable to the Company for the purchase price; provided that in the event the optionee chooses to pay the purchase price as so provided, the optionee and the broker shall comply with
such procedures and enter into such agreements of indemnity and other agreements as the Company shall prescribe as a condition of such payment procedure; or 

(iv) With respect to Stock Options that are not Incentive Stock Options, by a “net exercise” arrangement pursuant to which the
Company will reduce the number of shares of Stock issuable upon exercise by the largest whole number of shares with a Fair Market Value that does not exceed the aggregate exercise price. 

Payment instruments will be received subject to collection. The transfer to the optionee on the records of the Company or of the transfer agent of the
shares of Stock to be purchased pursuant to the exercise of a Stock Option will be contingent upon receipt from the optionee (or a purchaser acting in his stead in accordance with the provisions of the Stock Option) by the Company of the full
purchase price for such shares and the fulfillment of any other requirements contained in the Option Award Certificate or applicable provisions of laws (including the satisfaction of any withholding taxes that the Company is obligated to withhold
with respect to the optionee). In the event an optionee chooses to pay the purchase price by previously-owned shares of Stock through the attestation method, the number of shares of Stock transferred to the optionee upon the exercise of the
Stock Option shall be net of the number of attested shares. In the event that the Company establishes, for itself or using the services of a third party, an automated system for the exercise of Stock Options, such as a system using an internet
website or interactive voice response, then the paperless exercise of Stock Options may be permitted through the use of such an automated system. 

(f) Annual Limit on Incentive Stock Options. To the extent required for “incentive stock option” treatment under
Section 422 of the Code, the aggregate Fair Market Value (determined as of the time of grant) of the shares of Stock with respect to which Incentive Stock Options granted under this Plan and any other plan of the Company or its parent and
subsidiary corporations become exercisable for the first time by an optionee during any calendar year shall not exceed $100,000. To the extent that any Stock Option exceeds this limit, it shall constitute a Non-Qualified Stock Option. 

  
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 SECTION 7. STOCK APPRECIATION RIGHTS 

(a) Award of Stock Appreciation Rights. The Administrator may grant Stock Appreciation Rights under the Plan. A Stock Appreciation
Right is an Award entitling the recipient to receive shares of Stock having a value equal to the excess of the Fair Market Value of a share of Stock on the date of exercise over the exercise price of the Stock Appreciation Right multiplied by the
number of shares of Stock with respect to which the Stock Appreciation Right shall have been exercised. 
 (b) Exercise Price of Stock
Appreciation Rights. The exercise price of a Stock Appreciation Right shall not be less than 100 percent of the Fair Market Value of the Stock on the date of grant. 

(c) Grant and Exercise of Stock Appreciation Rights. Stock Appreciation Rights may be granted by the Administrator independently
of any Stock Option granted pursuant to Section 5 of the Plan. 
 (d) Terms and Conditions of Stock Appreciation
Rights. Stock Appreciation Rights shall be subject to such terms and conditions as shall be determined from time to time by the Administrator. The term of a Stock Appreciation Right may not exceed ten years. 

SECTION 8. RESTRICTED STOCK AWARDS 

(a) Nature of Restricted Stock Awards. The Administrator may grant Restricted Stock Awards under the Plan. A Restricted Stock
Award is any Award of Restricted Shares subject to such restrictions and conditions as the Administrator may determine at the time of grant. Conditions may be based on continuing employment (or other service relationship) and/or achievement of
pre-established performance goals and objectives. The terms and conditions of each such Award shall be determined by the Administrator, and such terms and conditions may differ among individual Awards and grantees. 

(b) Rights as a Stockholder. Upon the grant of the Restricted Stock Award and payment of any applicable purchase price, a grantee
shall have the rights of a stockholder with respect to the voting of the Restricted Shares and receipt of dividends; provided that if the lapse of restrictions with respect to the Restricted Stock Award is tied to the attainment of performance
goals, any dividends paid by the Company during the performance period shall accrue and shall not be paid to the grantee until and to the extent the performance goals are met with respect to the Restricted Stock Award. Unless the Administrator
shall otherwise determine, (i) uncertificated Restricted Shares shall be accompanied by a notation on the records of the Company or the transfer agent to the effect that they are subject to forfeiture until such Restricted Shares are vested as
provided in Section 8(d) below, and (ii) certificated Restricted Shares shall remain in the possession of the Company until such Restricted Shares are vested as provided in Section 8(d) below, and the grantee shall be
required, as a condition of the grant, to deliver to the Company such instruments of transfer as the Administrator may prescribe. 
 (c)
Restrictions. Restricted Shares may not be sold, assigned, transferred, pledged or otherwise encumbered or disposed of except as specifically provided herein or in the Restricted Stock Award Certificate. Except as may otherwise be
provided by the Administrator either in the Award Certificate or, subject to Section 19 below, in writing after the Award is issued, if a grantee’s employment (or other service relationship) with the Company and its Subsidiaries terminates
for any reason, any Restricted Shares that have not vested at the time of termination shall automatically and without any requirement of notice to such grantee from or other action by or on behalf of, the Company be deemed to have been reacquired by
the Company at its original purchase price (if any) from such grantee or such grantee’s legal representative simultaneously with such termination of employment (or other service relationship), and thereafter shall cease to represent any
ownership of the Company by the grantee or rights of the grantee as a stockholder. Following such deemed reacquisition of Restricted Shares that are represented by physical certificates, a grantee shall surrender such certificates to the
Company upon request without consideration. 

  
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 (d) Vesting of Restricted Shares. The Administrator at the time of grant shall
specify the date or dates and/or the attainment of pre-established performance goals, objectives and other conditions on which the non-transferability of the Restricted Shares and the Company’s right of repurchase or forfeiture shall
lapse. Subsequent to such date or dates and/or the attainment of such pre-established performance goals, objectives and other conditions, the shares on which all restrictions have lapsed shall no longer be Restricted Shares and shall be deemed
“vested.” 
 SECTION 9. RESTRICTED STOCK UNITS 

(a) Nature of Restricted Stock Units. The Administrator may grant Restricted Stock Units under the Plan. A Restricted Stock
Unit is an Award of stock units that may be settled in shares of Stock upon the satisfaction of such restrictions and conditions at the time of grant. Conditions may be based on continuing employment (or other service relationship) and/or
achievement of pre-established performance goals and objectives. The terms and conditions of each such Award shall be determined by the Administrator, and such terms and conditions may differ among individual Awards and grantees. Except in
the case of Restricted Stock Units with a deferred settlement date that complies with Section 409A, at the end of the vesting period, the Restricted Stock Units, to the extent vested, shall be settled in the form of shares of Stock. Restricted
Stock Units with deferred settlement dates are subject to Section 409A and shall contain such additional terms and conditions as the Administrator shall determine in its sole discretion in order to comply with the requirements of
Section 409A. 
 (b) Election to Receive Restricted Stock Units in Lieu of Compensation. The Administrator may, in its sole
discretion, permit a grantee to elect to receive a portion of future cash compensation otherwise due to such grantee in the form of an award of Restricted Stock Units. Any such election shall be made in writing and shall be delivered to the
Company no later than the date specified by the Administrator and in accordance with Section 409A and such other rules and procedures established by the Administrator. Any such future cash compensation that the grantee elects to defer
shall be converted to a fixed number of Restricted Stock Units based on the Fair Market Value of Stock on the date the compensation would otherwise have been paid to the grantee if such payment had not been deferred as provided herein. The
Administrator shall have the sole right to determine whether and under what circumstances to permit such elections and to impose such limitations and other terms and conditions thereon as the Administrator deems appropriate. Any Restricted
Stock Units that are elected to be received in lieu of cash compensation shall be fully vested, unless otherwise provided in the Award Certificate. 

(c) Rights as a Stockholder. A grantee shall have the rights as a stockholder only as to shares of Stock acquired by the grantee
upon settlement of Restricted Stock Units; provided, however, that the grantee may be credited with Dividend Equivalent Rights with respect to the stock units underlying his Restricted Stock Units, subject to the provisions of Section 14 and
such terms and conditions as the Administrator may determine. 
 (d) Termination. Except as may otherwise be provided by the
Administrator either in the Award Certificate or, subject to Section 19 below, in writing after the Award is issued, a grantee’s right in all Restricted Stock Units that have not vested shall automatically terminate upon the grantee’s
termination of employment (or cessation of service relationship) with the Company and its Subsidiaries for any reason. 

SECTION 10. UNRESTRICTED STOCK AWARDS 

Grant or Sale of Unrestricted Stock. The Administrator may grant (or sell at par value or such higher purchase price determined by
the Administrator) an Unrestricted Stock Award under the Plan. An Unrestricted Stock Award is an Award pursuant to which the grantee may receive shares of Stock free of any restrictions under the Plan. Unrestricted Stock Awards may be
granted in respect of past services or other valid consideration, or in lieu of cash compensation due to such grantee. 

SECTION 11. CASH-BASED AWARDS 

Grant of Cash-Based Awards. The Administrator may grant Cash-Based Awards under the Plan. A Cash-Based Award is an Award that
entitles the grantee to a payment in cash upon the attainment of specified Performance Goals. The Administrator shall determine the maximum duration of the Cash-Based Award, the amount of cash to which the Cash-Based Award pertains, the
conditions upon which the Cash-Based Award shall 

  
 9 








 
become vested or payable, and such other provisions as the Administrator shall determine. Each Cash-Based Award shall specify a cash-denominated payment amount, formula or payment ranges as
determined by the Administrator. Payment, if any, with respect to a Cash-Based Award shall be made in accordance with the terms of the Award and may be made in cash. 

SECTION 12. PERFORMANCE SHARE AWARDS 

(a) Nature of Performance Share Awards. The Administrator may grant Performance Share Awards under the Plan. A Performance
Share Award is an Award entitling the grantee to receive shares of Stock upon the attainment of performance goals. The Administrator shall determine whether and to whom Performance Share Awards shall be granted, the performance goals, the
periods during which performance is to be measured, which may not be less than one year except in the case of a Sale Event, and such other limitations and conditions as the Administrator shall determine. 

(b) Rights as a Stockholder. A grantee receiving a Performance Share Award shall have the rights of a stockholder only as to
shares of Stock actually received by the grantee under the Plan and not with respect to shares subject to the Award but not actually received by the grantee. A grantee shall be entitled to receive shares of Stock under a Performance Share Award
only upon satisfaction of all conditions specified in the Performance Share Award Certificate (or in a performance plan adopted by the Administrator). 

(c) Termination. Except as may otherwise be provided by the Administrator either in the Award agreement or, subject to
Section 19 below, in writing after the Award is issued, a grantee’s rights in all Performance Share Awards shall automatically terminate upon the grantee’s termination of employment (or cessation of service relationship) with the
Company and its Subsidiaries for any reason. 
 SECTION 13. PERFORMANCE-BASED AWARDS TO COVERED EMPLOYEES 

(a) Performance-Based Awards. The Administrator may grant one or more Performance-Based Awards in the form of a Restricted Stock
Award, Restricted Stock Units, Performance Share Awards or Cash-Based Award payable upon the attainment of Performance Goals that are established by the Administrator and relate to one or more of the Performance Criteria, in each case on a specified
date or dates or over any period or periods determined by the Administrator. The Administrator shall define in an objective fashion the manner of calculating the Performance Criteria it selects to use for any Performance Cycle. Depending
on the Performance Criteria used to establish such Performance Goals, the Performance Goals may be expressed in terms of overall Company performance or the performance of a division, business unit, or an individual. Each Performance-Based Award
shall comply with the provisions set forth below. 
 (b) Grant of Performance-Based Awards. With respect to each
Performance-Based Award granted to a Covered Employee, the Administrator shall select, within the first 90 days of a Performance Cycle (or, if shorter, within the maximum period allowed under Section 162(m) of the Code) the Performance
Criteria for such grant, and the Performance Goals with respect to each Performance Criterion (including a threshold level of performance below which no amount will become payable with respect to such Award). Each Performance-Based Award will
specify the amount payable, or the formula for determining the amount payable, upon achievement of the various applicable performance targets. The Performance Criteria established by the Administrator may be (but need not be) different for each
Performance Cycle and different Performance Goals may be applicable to Performance-Based Awards to different Covered Employees. 
 (c)
Payment of Performance-Based Awards. Following the completion of a Performance Cycle, the Administrator shall meet to review and certify in writing whether, and to what extent, the Performance Goals for the Performance Cycle have been
achieved and, if so, to also calculate and certify in writing the amount of the Performance-Based Awards earned for the Performance Cycle. The Administrator shall then determine the actual size of each Covered Employee’s Performance-Based
Award. 
 (d) Maximum Award Payable. The maximum Performance-Based Award payable to any one Covered Employee under the Plan for
a Performance Cycle is 4,000,000 shares of Stock (subject to adjustment as provided in Section 3(b) hereof) or $5,000,000 in the case of a Performance-Based Award that is a Cash-Based Award. 

  
 10 








 SECTION 14. DIVIDEND EQUIVALENT RIGHTS 

(a) Dividend Equivalent Rights. The Administrator may grant Dividend Equivalent Rights under the Plan. A Dividend Equivalent
Right is an Award entitling the grantee to receive credits based on cash dividends that would have been paid on the shares of Stock specified in the Dividend Equivalent Right (or other Award to which it relates) if such shares had been issued to the
grantee. A Dividend Equivalent Right may be granted hereunder to any grantee as a component of an award of Restricted Stock Units or Performance Share Award or as a freestanding award. The terms and conditions of Dividend Equivalent Rights
shall be specified in the Award Certificate. Dividend equivalents credited to the holder of a Dividend Equivalent Right may be paid currently or may be deemed to be reinvested in additional shares of Stock, which may thereafter accrue
additional equivalents. Any such reinvestment shall be at Fair Market Value on the date of reinvestment or such other price as may then apply under a dividend reinvestment plan sponsored by the Company, if any. Dividend Equivalent Rights
may be settled in cash or shares of Stock or a combination thereof, in a single installment or installments. A Dividend Equivalent Right granted as a component of an Award of Restricted Stock Units or Performance Share Award shall provide that
such Dividend Equivalent Right shall be settled only upon settlement or payment of, or lapse of restrictions on, such other Award, and that such Dividend Equivalent Right shall expire or be forfeited or annulled under the same conditions as such
other Award. 
 (b) Termination. Except as may otherwise be provided by the Administrator either in the Award Certificate or,
subject to Section 19 below, in writing after the Award is issued, a grantee’s rights in all Dividend Equivalent Rights shall automatically terminate upon the grantee’s termination of employment (or cessation of service relationship)
with the Company and its Subsidiaries for any reason. 
 SECTION 15. TRANSFERABILITY OF AWARDS 

(a) Transferability. Except as provided in Section 15(b) below, during a grantee’s lifetime, his or her Awards
shall be exercisable only by the grantee, or by the grantee’s legal representative or guardian in the event of the grantee’s incapacity. No Awards shall be sold, assigned, transferred or otherwise encumbered or disposed of by a
grantee other than by will or by the laws of descent and distribution or pursuant to a domestic relations order. No Awards shall be subject, in whole or in part, to attachment, execution, or levy of any kind, and any purported transfer in
violation hereof shall be null and void. 
 (b) Administrator Action. Notwithstanding Section 15(a), the Administrator, in
its discretion, may provide either in the Award Certificate regarding a given Award or by subsequent written approval that the grantee (who is an employee or director) may transfer his or her Non-Qualified Stock Options to his or her immediate
family members, to trusts for the benefit of such family members, or to partnerships in which such family members are the only partners, provided that the transferee agrees in writing with the Company to be bound by all of the terms and conditions
of this Plan and the applicable Award. In no event may an Award be transferred by a grantee for value. 
 (c) Family
Member. For purposes of Section 15(b), “family member” shall mean a grantee’s child, stepchild, grandchild, parent, stepparent, grandparent, spouse, former spouse, sibling, niece, nephew, mother-in-law, father-in-law,
son-in-law, daughter-in-law, brother-in-law, or sister-in-law, including adoptive relationships, any person sharing the grantee’s household (other than a tenant of the grantee), a trust in which these persons (or the grantee) have more than 50
percent of the beneficial interest, a foundation in which these persons (or the grantee) control the management of assets, and any other entity in which these persons (or the grantee) own more than 50 percent of the voting interests. 

(d) Designation of Beneficiary. To the extent permitted by the Company, each grantee to whom an Award has been made under the Plan
may designate a beneficiary or beneficiaries to exercise any Award or receive any payment under any Award payable on or after the grantee’s death. Any such designation shall be on a form provided for that purpose by the Administrator and
shall not be effective until received by the Administrator. If no beneficiary has been designated by a deceased grantee, or if the designated beneficiaries have predeceased the grantee, the beneficiary shall be the grantee’s estate. 

  
 11 








 SECTION 16. TAX WITHHOLDING 

(a) Payment by Grantee. Each grantee shall, no later than the date as of which the value of an Award or of any Stock or other
amounts received thereunder first becomes includable in the gross income of the grantee for Federal income tax purposes, pay to the Company, or make arrangements satisfactory to the Administrator regarding payment of, any Federal, state, or local
taxes of any kind required by law to be withheld by the Company with respect to such income. The Company and its Subsidiaries shall, to the extent permitted by law, have the right to deduct any such taxes from any payment of any kind otherwise
due to the grantee. The Company’s obligation to deliver evidence of book entry (or stock certificates) to any grantee is subject to and conditioned on tax withholding obligations being satisfied by the grantee. 

(b) Payment in Stock. Subject to approval by the Administrator, a grantee may elect to have the Company’s minimum required
tax withholding obligation satisfied, in whole or in part, by authorizing the Company to withhold from shares of Stock to be issued pursuant to any Award a number of shares with an aggregate Fair Market Value (as of the date the withholding is
effected) that would satisfy the withholding amount due. The Administrator may also require Awards to be subject to mandatory share withholding up to the required withholding amount. For purposes of share withholding, the Fair Market Value
of withheld shares shall be determined in the same manner as the value of Stock includible in income of the Participants. 

SECTION 17. SECTION 409A AWARDS 

To the extent that any Award is determined to constitute “nonqualified deferred compensation” within the meaning of
Section 409A (a “409A Award”), the Award shall be subject to such additional rules and requirements as specified by the Administrator from time to time in order to comply with Section 409A. In this regard, if any amount
under a 409A Award is payable upon a “separation from service” (within the meaning of Section 409A) to a grantee who is then considered a “specified employee” (within the meaning of Section 409A), then no such payment
shall be made prior to the date that is the earlier of (i) six months and one day after the grantee’s separation from service, or (ii) the grantee’s death, but only to the extent such delay is necessary to prevent such payment
from being subject to interest, penalties and/or additional tax imposed pursuant to Section 409A. Further, the settlement of any such Award may not be accelerated except to the extent permitted by Section 409A. 

SECTION 18. TERMINATION OF EMPLOYMENT, TRANSFER, LEAVE OF ABSENCE, ETC. 

(a) Termination of Employment. If the grantee’s employer ceases to be a Subsidiary, the grantee shall be deemed to have
terminated employment for purposes of the Plan. 
 (b) For purposes of the Plan, the following events shall not be deemed a termination of
employment: 
 (i) a transfer to the employment of the Company from a Subsidiary or from the Company to a Subsidiary, or from one Subsidiary
to another; or 
 (ii) an approved leave of absence for military service or sickness, or for any other purpose approved by the Company, if
the employee’s right to re-employment is guaranteed either by a statute or by contract or under the policy pursuant to which the leave of absence was granted or if the Administrator otherwise so provides in writing. 

SECTION 19. AMENDMENTS AND TERMINATION 

The Board may, at any time, amend or discontinue the Plan and the Administrator may, at any time, amend or cancel any outstanding Award for
the purpose of satisfying changes in law or for any other lawful purpose, but no such action shall adversely affect rights under any outstanding Award without the holder’s consent. Except as provided in Section 3(b) or 3(c),
without prior stockholder approval, in no event may the Administrator exercise its discretion to reduce the exercise price of outstanding Stock Options or Stock Appreciation Rights or effect repricing through cancellation and re-grants or
cancellation of Stock Options or Stock Appreciation Rights in exchange for cash or other Awards. To the extent required under the rules of any securities exchange or market system on which 


  
 12 








 
the Stock is listed, to the extent determined by the Administrator to be required by the Code to ensure that Incentive Stock Options granted under the Plan are qualified under Section 422 of
the Code, or to ensure that compensation earned under Awards qualifies as performance-based compensation under Section 162(m) of the Code, Plan amendments shall be subject to approval by the Company stockholders entitled to vote at a
meeting of stockholders. Nothing in this Section 19 shall limit the Administrator’s authority to take any action permitted pursuant to Section 3(b) or 3(c). 

SECTION 20. STATUS OF PLAN 

With respect to the portion of any Award that has not been exercised and any payments in cash, Stock or other consideration not received by a
grantee, a grantee shall have no rights greater than those of a general creditor of the Company unless the Administrator shall otherwise expressly determine in connection with any Award or Awards. In its sole discretion, the Administrator may
authorize the creation of trusts or other arrangements to meet the Company’s obligations to deliver Stock or make payments with respect to Awards hereunder, provided that the existence of such trusts or other arrangements is consistent with the
foregoing sentence. 
 SECTION 21. GENERAL PROVISIONS 

(a) No Distribution. The Administrator may require each person acquiring Stock pursuant to an Award to represent to and agree with
the Company in writing that such person is acquiring the shares without a view to distribution thereof. 
 (b) Delivery of Stock
Certificates. Stock certificates to grantees under this Plan shall be deemed delivered for all purposes when the Company or a stock transfer agent of the Company shall have mailed such certificates in the United States mail, addressed to
the grantee, at the grantee’s last known address on file with the Company. Uncertificated Stock shall be deemed delivered for all purposes when the Company or a Stock transfer agent of the Company shall have given to the grantee by
electronic mail (with proof of receipt) or by United States mail, addressed to the grantee, at the grantee’s last known address on file with the Company, notice of issuance and recorded the issuance in its records (which may include electronic
“book entry” records). Notwithstanding anything herein to the contrary, the Company shall not be required to issue or deliver any certificates evidencing shares of Stock pursuant to the exercise of any Award, unless and until the
Administrator has determined, with advice of counsel (to the extent the Administrator deems such advice necessary or advisable), that the issuance and delivery of such certificates is in compliance with all applicable laws, regulations of
governmental authorities and, if applicable, the requirements of any exchange on which the shares of Stock are listed, quoted or traded. All Stock certificates delivered pursuant to the Plan shall be subject to any stop-transfer orders and
other restrictions as the Administrator deems necessary or advisable to comply with federal, state or foreign jurisdiction, securities or other laws, rules and quotation system on which the Stock is listed, quoted or traded. The
Administrator may place legends on any Stock certificate to reference restrictions applicable to the Stock. In addition to the terms and conditions provided herein, the Administrator may require that an individual make such reasonable
covenants, agreements, and representations as the Administrator, in its discretion, deems necessary or advisable in order to comply with any such laws, regulations, or requirements. The Administrator shall have the right to require any
individual to comply with any timing or other restrictions with respect to the settlement or exercise of any Award, including a window-period limitation, as may be imposed in the discretion of the Administrator. 

(c) Stockholder Rights. Until Stock is deemed delivered in accordance with Section 21(b), no right to vote or receive
dividends or any other rights of a stockholder will exist with respect to shares of Stock to be issued in connection with an Award, notwithstanding the exercise of a Stock Option or any other action by the grantee with respect to an Award. 

(d) Other Compensation Arrangements; No Employment Rights. Nothing contained in this Plan shall prevent the Board from adopting
other or additional compensation arrangements, including trusts, and such arrangements may be either generally applicable or applicable only in specific cases. The adoption of this Plan and the grant of Awards do not confer upon any employee
any right to continued employment with the Company or any Subsidiary. 

  
 13 








 (e) Trading Policy Restrictions. Option exercises and other Awards under the Plan
shall be subject to the Company’s insider trading policies and procedures, as in effect from time to time. 
 (f) Clawback
Policy. Awards under the Plan shall be subject to the Company’s clawback policy, as in effect from time to time. 

SECTION 22. EFFECTIVE DATE OF PLAN 

This Plan shall become effective on the date immediately preceding the date of the Company’s Initial Public Offering following
stockholder approval in accordance with applicable state law, the Company’s bylaws and articles of incorporation, and applicable stock exchange rules or pursuant to written consent. No grants of Stock Options and other Awards may be
made hereunder after the tenth anniversary of the Effective Date and no grants of Incentive Stock Options may be made hereunder after the tenth anniversary of the date the Plan is approved by the Board. 

SECTION 23. GOVERNING LAW 

This Plan and all Awards and actions taken thereunder shall be governed by, and construed in accordance with, the laws of the State of
Delaware, applied without regard to conflict of law principles. 
 DATE APPROVED BY BOARD OF DIRECTORS: 

DATE APPROVED BY STOCKHOLDERS: 

  
 14 








 INCENTIVE STOCK OPTION AGREEMENT 

UNDER THE DIMENSION THERAPEUTICS, INC. 

2015 STOCK OPTION AND INCENTIVE PLAN 
 Name
of Optionee: 
 No. of Option Shares: 
 Option Exercise
Price per Share:             $             

Grant Date: 
 Expiration Date: 

Pursuant to the Dimension Therapeutics, Inc. 2015 Stock Option and Incentive Plan as amended through the date hereof (the
“Plan”), Dimension Therapeutics, Inc. (the “Company”) hereby grants to the Optionee named above an option (the “Stock Option”) to purchase on or prior to the Expiration Date specified above all or part of the
number of shares of Common Stock, par value $0.0001 per share (the “Stock”), of the Company specified above at the Option Exercise Price per Share specified above subject to the terms and conditions set forth herein and in the Plan. 

1. Exercisability Schedule. No portion of this Stock Option may be exercised until such portion shall have become
exercisable. Except as set forth below, and subject to the discretion of the Administrator (as defined in Section 2 of the Plan) to accelerate the exercisability schedule hereunder, this Stock Option shall be exercisable with respect to
the following number of Option Shares on the dates indicated so long as the Optionee remains an employee of the Company or a Subsidiary on such dates: 
  

					
	 Incremental Number of

Option Shares Exercisable*
	  	Exercisability Date	 
	
(            %)
	  			
	
(            %)
	  			
	
(            %)
	  			
	
(            %)
	  			
	
(            %)
	  			

  

	*	Max. of $100,000 per yr. 

 Once exercisable, this Stock Option shall continue to be exercisable
at any time or times prior to the close of business on the Expiration Date, subject to the provisions hereof and of the Plan. 
 2.
Manner of Exercise. 
 (a) The Optionee may exercise this Stock Option only in the following manner: from time to time on or
prior to the Expiration Date of this Stock Option, the Optionee may give written notice to the Administrator of his or her election to purchase some or all of the Option Shares purchasable at the time of such notice. This notice shall specify
the number of Option Shares to be purchased. 
 Payment of the purchase price for the Option Shares may be made by one or more of the
following methods: (i) in cash, by certified or bank check or other instrument acceptable to the Administrator; (ii) through the delivery (or attestation to the ownership) of shares of Stock that have been purchased by the Optionee on
the open market or that are beneficially owned by the Optionee and are not then subject to any restrictions under any Company plan and that otherwise satisfy any holding periods as may be required by the Administrator; or (iii) by the Optionee
delivering to the Company a properly executed exercise notice together with irrevocable instructions to a 

  
 15 








 
broker to promptly deliver to the Company cash or a check payable and acceptable to the Company to pay the option purchase price, provided that in the event the Optionee chooses to pay the option
purchase price as so provided, the Optionee and the broker shall comply with such procedures and enter into such agreements of indemnity and other agreements as the Administrator shall prescribe as a condition of such payment procedure; or
(iv) a combination of (i), (ii) and (iii) above. Payment instruments will be received subject to collection. 
 The
transfer to the Optionee on the records of the Company or of the transfer agent of the Option Shares will be contingent upon (i) the Company’s receipt from the Optionee of the full purchase price for the Option Shares, as set forth above,
(ii) the fulfillment of any other requirements contained herein or in the Plan or in any other agreement or provision of laws, and (iii) the receipt by the Company of any agreement, statement or other evidence that the Company may require
to satisfy itself that the issuance of Stock to be purchased pursuant to the exercise of Stock Options under the Plan and any subsequent resale of the shares of Stock will be in compliance with applicable laws and regulations. In the event the
Optionee chooses to pay the purchase price by previously-owned shares of Stock through the attestation method, the number of shares of Stock transferred to the Optionee upon the exercise of the Stock Option shall be net of the shares attested to.

 (b) The shares of Stock purchased upon exercise of this Stock Option shall be transferred to the Optionee on the records of the Company
or of the transfer agent upon compliance to the satisfaction of the Administrator with all requirements under applicable laws or regulations in connection with such transfer and with the requirements hereof and of the Plan. The determination of
the Administrator as to such compliance shall be final and binding on the Optionee. The Optionee shall not be deemed to be the holder of, or to have any of the rights of a holder with respect to, any shares of Stock subject to this Stock Option
unless and until this Stock Option shall have been exercised pursuant to the terms hereof, the Company or the transfer agent shall have transferred the shares to the Optionee, and the Optionee’s name shall have been entered as the stockholder
of record on the books of the Company. Thereupon, the Optionee shall have full voting, dividend and other ownership rights with respect to such shares of Stock. 

(c) The minimum number of shares with respect to which this Stock Option may be exercised at any one time shall be 100 shares, unless the
number of shares with respect to which this Stock Option is being exercised is the total number of shares subject to exercise under this Stock Option at the time. 

(d) Notwithstanding any other provision hereof or of the Plan, no portion of this Stock Option shall be exercisable after the Expiration Date
hereof. 
 3. Termination of Employment. If the Optionee’s employment by the Company or a Subsidiary (as defined in the
Plan) is terminated, the period within which to exercise the Stock Option may be subject to earlier termination as set forth below. 
 (a)
Termination Due to Death. If the Optionee’s employment terminates by reason of the Optionee’s death, any portion of this Stock Option outstanding on such date, to the extent exercisable on the date of death, may thereafter be
exercised by the Optionee’s legal representative or legatee for a period of 12 months from the date of death or until the Expiration Date, if earlier. Any portion of this Stock Option that is not exercisable on the date of death shall
terminate immediately and be of no further force or effect. 
 (b) Termination Due to Disability. If the Optionee’s
employment terminates by reason of the Optionee’s disability (as determined by the Administrator), any portion of this Stock Option outstanding on such date, to the extent exercisable on the date of such termination of employment, may
thereafter be exercised by the Optionee for a period of 12 months from the date of disability or until the Expiration Date, if earlier. Any portion of this Stock Option that is not exercisable on the date of disability shall terminate
immediately and be of no further force or effect. 
 (c) Termination for Cause. If the Optionee’s employment terminates for
Cause, any portion of this Stock Option outstanding on such date shall terminate immediately and be of no further force and effect. For purposes hereof, “Cause” shall mean, unless otherwise provided in an employment agreement between
the Company and the Optionee, a determination by the Administrator that the Optionee shall be dismissed as a result of (i) any material breach by the Optionee of any agreement between the Optionee and the Company; (ii) the conviction of,
indictment for or plea of nolo contendere by the Optionee to a felony or a crime involving moral turpitude; or (iii) any material misconduct or willful and deliberate non-performance (other than by reason of disability) by the Optionee of the
Optionee’s duties to the Company. 

  
 16 








 (d) Other Termination. If the Optionee’s employment terminates for any reason
other than the Optionee’s death, the Optionee’s disability, or Cause, and unless otherwise determined by the Administrator, any portion of this Stock Option outstanding on such date may be exercised, to the extent exercisable on the date
of termination, for a period of three months from the date of termination or until the Expiration Date, if earlier. Any portion of this Stock Option that is not exercisable on the date of termination shall terminate immediately and be of no
further force or effect. 
 The Administrator’s determination of the reason for termination of the Optionee’s employment shall be
conclusive and binding on the Optionee and his or her representatives or legatees. 
 4. Incorporation of Plan. Notwithstanding
anything herein to the contrary, this Stock Option shall be subject to and governed by all the terms and conditions of the Plan, including the powers of the Administrator set forth in Section 2(b) of the Plan. Capitalized terms in
this Agreement shall have the meaning specified in the Plan, unless a different meaning is specified herein. 
 5.
Transferability. This Agreement is personal to the Optionee, is non-assignable and is not transferable in any manner, by operation of law or otherwise, other than by will or the laws of descent and distribution. This Stock Option is
exercisable, during the Optionee’s lifetime, only by the Optionee, and thereafter, only by the Optionee’s legal representative or legatee. 

6. Status of the Stock Option. This Stock Option is intended to qualify as an “incentive stock option” under
Section 422 of the Internal Revenue Code of 1986, as amended (the “Code”), but the Company does not represent or warrant that this Stock Option qualifies as such. The Optionee should consult with his or her own tax advisors
regarding the tax effects of this Stock Option and the requirements necessary to obtain favorable income tax treatment under Section 422 of the Code, including, but not limited to, holding period requirements. To the extent any portion of
this Stock Option does not so qualify as an “incentive stock option,” such portion shall be deemed to be a non-qualified stock option. If the Optionee intends to dispose or does dispose (whether by sale, gift, transfer or otherwise)
of any Option Shares within the one-year period beginning on the date after the transfer of such shares to him or her, or within the two-year period beginning on the day after the grant of this Stock Option, he or she will so notify the Company
within 30 days after such disposition. 
 7. Tax Withholding. The Optionee shall, not later than the date as of which the
exercise of this Stock Option becomes a taxable event for Federal income tax purposes, pay to the Company or make arrangements satisfactory to the Administrator for payment of any Federal, state, and local taxes required by law to be withheld on
account of such taxable event. The Company shall have the authority to cause the minimum required tax withholding obligation to be satisfied, in whole or in part, by withholding from shares of Stock to be issued to the Optionee a number of
shares of Stock with an aggregate Fair Market Value that would satisfy the minimum withholding amount due. 
 8. No Obligation to
Continue Employment. Neither the Company nor any Subsidiary is obligated by or as a result of the Plan or this Agreement to continue the Optionee in employment and neither the Plan nor this Agreement shall interfere in any way with the
right of the Company or any Subsidiary to terminate the employment of the Optionee at any time. 
 9. Integration. This
Agreement constitutes the entire agreement between the parties with respect to this Stock Option and supersedes all prior agreements and discussions between the parties concerning such subject matter. 

10. Data Privacy Consent. In order to administer the Plan and this Agreement and to implement or structure future equity grants,
the Company, its subsidiaries and affiliates and certain agents thereof (together, the “Relevant Companies”) may process any and all personal or professional data, including but not limited to Social Security or other identification
number, home address and telephone number, date of birth and other information that is necessary or desirable for the administration of the Plan and/or this Agreement (the “Relevant Information”).


  
 17 








 
By entering into this Agreement, the Optionee (i) authorizes the Company to collect, process, register and transfer to the Relevant Companies all Relevant Information; (ii) waives any
privacy rights the Optionee may have with respect to the Relevant Information; (iii) authorizes the Relevant Companies to store and transmit such information in electronic form; and (iv) authorizes the transfer of the Relevant Information
to any jurisdiction in which the Relevant Companies consider appropriate. The Optionee shall have access to, and the right to change, the Relevant Information. Relevant Information will only be used in accordance with applicable law. 

11. Notices. Notices hereunder shall be mailed or delivered to the Company at its principal place of business and shall be mailed
or delivered to the Optionee at the address on file with the Company or, in either case, at such other address as one party may subsequently furnish to the other party in writing. 

 

			
	DIMENSION THERAPEUTICS, INC.


 
			
		
	By: 	 	  

		 	Title:


 The foregoing Agreement is hereby accepted and the terms and conditions thereof hereby agreed to by the
undersigned. Electronic acceptance of this Agreement pursuant to the Company’s instructions to the Optionee (including through an online acceptance process) is acceptable. 

 

							
	Dated: 	 	  
	 		 	  

		 		 		 	Optionee’s Signature
				
		 		 		 	Optionee’s name and address:
				
		 		 		 	  

		 		 		 	  

		 		 		 	  



  
 18 








 NON-QUALIFIED STOCK OPTION AGREEMENT 

FOR COMPANY EMPLOYEES 

UNDER THE DIMENSION THERAPEUTICS, INC. 

2015 STOCK OPTION AND INCENTIVE PLAN 
 Name
of Optionee: 
 No. of Option Shares: 
 Option Exercise
Price per Share:             $             

Grant Date: 
 Expiration Date: 

Pursuant to the Dimension Therapeutics, Inc. 2015 Stock Option and Incentive Plan as amended through the date hereof (the
“Plan”), Dimension Therapeutics, Inc. (the “Company”) hereby grants to the Optionee named above an option (the “Stock Option”) to purchase on or prior to the Expiration Date specified above all or part of the
number of shares of Common Stock, par value $0.0001 per share (the “Stock”), of the Company specified above at the Option Exercise Price per Share specified above subject to the terms and conditions set forth herein and in the
Plan. This Stock Option is not intended to be an “incentive stock option” under Section 422 of the Internal Revenue Code of 1986, as amended. 

1. Exercisability Schedule. No portion of this Stock Option may be exercised until such portion shall have become
exercisable. Except as set forth below, and subject to the discretion of the Administrator (as defined in Section 2 of the Plan) to accelerate the exercisability schedule hereunder, this Stock Option shall be exercisable with respect to
the following number of Option Shares on the dates indicated so long as Optionee remains an employee of the Company or a Subsidiary on such dates: 
  

					
	 Incremental Number of

Option Shares Exercisable
	  	Exercisability Date	 
	
(            %)
	  			
	
(            %)
	  			
	
(            %)
	  			
	
(            %)
	  			
	
(            %)
	  			

 Once exercisable, this Stock Option shall continue to be exercisable at any time or times prior to the close
of business on the Expiration Date, subject to the provisions hereof and of the Plan. 
 2. Manner of Exercise. 

(a) The Optionee may exercise this Stock Option only in the following manner: from time to time on or prior to the Expiration Date of
this Stock Option, the Optionee may give written notice to the Administrator of his or her election to purchase some or all of the Option Shares purchasable at the time of such notice. This notice shall specify the number of Option Shares to be
purchased. 
 Payment of the purchase price for the Option Shares may be made by one or more of the following methods: (i) in
cash, by certified or bank check or other instrument acceptable to the Administrator; (ii) through the delivery (or attestation to the ownership) of shares of Stock that have been purchased by the Optionee on the open market or that are
beneficially owned by the Optionee and are not then subject to any restrictions under any Company plan and that otherwise satisfy any holding periods as may be required by the Administrator; (iii) by the Optionee delivering to the Company a
properly executed exercise notice together with irrevocable instructions to a broker to promptly deliver to the Company cash or a check payable and acceptable to the Company to pay the option purchase price, provided that in the event the Optionee
chooses to pay the option purchase price as so provided, the Optionee and the broker shall comply with such procedures and enter into such agreements of indemnity and other 

  
 19 








 
agreements as the Administrator shall prescribe as a condition of such payment procedure; (iv) by a “net exercise” arrangement pursuant to which the Company will reduce the number
of shares of Stock issuable upon exercise by the largest whole number of shares with a Fair Market Value that does not exceed the aggregate exercise price; or (v) a combination of (i), (ii), (iii) and (iv) above. Payment
instruments will be received subject to collection. 
 The transfer to the Optionee on the records of the Company or of the transfer agent
of the Option Shares will be contingent upon (i) the Company’s receipt from the Optionee of the full purchase price for the Option Shares, as set forth above, (ii) the fulfillment of any other requirements contained herein or in the
Plan or in any other agreement or provision of laws, and (iii) the receipt by the Company of any agreement, statement or other evidence that the Company may require to satisfy itself that the issuance of Stock to be purchased pursuant to the
exercise of Stock Options under the Plan and any subsequent resale of the shares of Stock will be in compliance with applicable laws and regulations. In the event the Optionee chooses to pay the purchase price by previously-owned shares of
Stock through the attestation method, the number of shares of Stock transferred to the Optionee upon the exercise of the Stock Option shall be net of the shares attested to. 

(b) The shares of Stock purchased upon exercise of this Stock Option shall be transferred to the Optionee on the records of the Company or of
the transfer agent upon compliance to the satisfaction of the Administrator with all requirements under applicable laws or regulations in connection with such transfer and with the requirements hereof and of the Plan. The determination of the
Administrator as to such compliance shall be final and binding on the Optionee. The Optionee shall not be deemed to be the holder of, or to have any of the rights of a holder with respect to, any shares of Stock subject to this Stock Option
unless and until this Stock Option shall have been exercised pursuant to the terms hereof, the Company or the transfer agent shall have transferred the shares to the Optionee, and the Optionee’s name shall have been entered as the stockholder
of record on the books of the Company. Thereupon, the Optionee shall have full voting, dividend and other ownership rights with respect to such shares of Stock. 

(c) The minimum number of shares with respect to which this Stock Option may be exercised at any one time shall be 100 shares, unless the
number of shares with respect to which this Stock Option is being exercised is the total number of shares subject to exercise under this Stock Option at the time. 

(d) Notwithstanding any other provision hereof or of the Plan, no portion of this Stock Option shall be exercisable after the Expiration Date
hereof. 
 3. Termination of Employment. If the Optionee’s employment by the Company or a Subsidiary (as defined in the
Plan) is terminated, the period within which to exercise the Stock Option may be subject to earlier termination as set forth below. 
 (a)
Termination Due to Death. If the Optionee’s employment terminates by reason of the Optionee’s death, any portion of this Stock Option outstanding on such date, to the extent exercisable on the date of death, may thereafter be
exercised by the Optionee’s legal representative or legatee for a period of 12 months from the date of death or until the Expiration Date, if earlier. Any portion of this Stock Option that is not exercisable on the date of death shall
terminate immediately and be of no further force or effect. 
 (b) Termination Due to Disability. If the Optionee’s
employment terminates by reason of the Optionee’s disability (as determined by the Administrator), any portion of this Stock Option outstanding on such date, to the extent exercisable on the date of such termination of employment, may
thereafter be exercised by the Optionee for a period of 12 months from the date of disability or until the Expiration Date, if earlier. Any portion of this Stock Option that is not exercisable on the date of disability shall terminate
immediately and be of no further force or effect. 
 (c) Termination for Cause. If the Optionee’s employment terminates for
Cause, any portion of this Stock Option outstanding on such date shall terminate immediately and be of no further force and effect. For purposes hereof, “Cause” shall mean, unless otherwise provided in an employment agreement between
the Company and the Optionee, a determination by the Administrator that the Optionee shall be dismissed as a result of (i) any material breach by the Optionee of any agreement between the Optionee and the Company; (ii) the conviction of,
indictment for or plea of nolo contendere by the Optionee to a felony or a crime involving moral turpitude; or (iii) any material misconduct or willful and deliberate non-performance (other than by reason of disability) by the Optionee of the
Optionee’s duties to the Company. 

  
 20 








 (d) Other Termination. If the Optionee’s employment terminates for any reason
other than the Optionee’s death, the Optionee’s disability or Cause, and unless otherwise determined by the Administrator, any portion of this Stock Option outstanding on such date may be exercised, to the extent exercisable on the date of
termination, for a period of three months from the date of termination or until the Expiration Date, if earlier. Any portion of this Stock Option that is not exercisable on the date of termination shall terminate immediately and be of no
further force or effect. 
 The Administrator’s determination of the reason for termination of the Optionee’s employment shall be
conclusive and binding on the Optionee and his or her representatives or legatees. 
 4. Incorporation of Plan. Notwithstanding
anything herein to the contrary, this Stock Option shall be subject to and governed by all the terms and conditions of the Plan, including the powers of the Administrator set forth in Section 2(b) of the Plan. Capitalized terms
in this Agreement shall have the meaning specified in the Plan, unless a different meaning is specified herein. 
 5.
Transferability. This Agreement is personal to the Optionee, is non-assignable and is not transferable in any manner, by operation of law or otherwise, other than by will or the laws of descent and distribution. This Stock Option is
exercisable, during the Optionee’s lifetime, only by the Optionee, and thereafter, only by the Optionee’s legal representative or legatee. 

6. Tax Withholding. The Optionee shall, not later than the date as of which the exercise of this Stock Option becomes a taxable
event for Federal income tax purposes, pay to the Company or make arrangements satisfactory to the Administrator for payment of any Federal, state, and local taxes required by law to be withheld on account of such taxable event. The Company
shall have the authority to cause the minimum required tax withholding obligation to be satisfied, in whole or in part, by withholding from shares of Stock to be issued to the Optionee a number of shares of Stock with an aggregate Fair Market Value
that would satisfy the minimum withholding amount due. 
 7. No Obligation to Continue Employment. Neither the Company nor any
Subsidiary is obligated by or as a result of the Plan or this Agreement to continue the Optionee in employment and neither the Plan nor this Agreement shall interfere in any way with the right of the Company or any Subsidiary to terminate the
employment of the Optionee at any time. 
 8. Integration. This Agreement constitutes the entire agreement between the parties
with respect to this Stock Option and supersedes all prior agreements and discussions between the parties concerning such subject matter. 

9. Data Privacy Consent. In order to administer the Plan and this Agreement and to implement or structure future equity grants,
the Company, its subsidiaries and affiliates and certain agents thereof (together, the “Relevant Companies”) may process any and all personal or professional data, including but not limited to Social Security or other identification
number, home address and telephone number, date of birth and other information that is necessary or desirable for the administration of the Plan and/or this Agreement (the “Relevant Information”). By entering into this Agreement, the
Optionee (i) authorizes the Company to collect, process, register and transfer to the Relevant Companies all Relevant Information; (ii) waives any privacy rights the Optionee may have with respect to the Relevant Information;
(iii) authorizes the Relevant Companies to store and transmit such information in electronic form; and (iv) authorizes the transfer of the Relevant Information to any jurisdiction in which the Relevant Companies consider
appropriate. The Optionee shall have access to, and the right to change, the Relevant Information. Relevant Information will only be used in accordance with applicable law. 

10. Notices. Notices hereunder shall be mailed or delivered to the Company at its principal place of business and shall be mailed
or delivered to the Optionee at the address on file with the Company or, in either case, at such other address as one party may subsequently furnish to the other party in writing. 

  
 21 








 
			
	DIMENSION THERAPEUTICS, INC.


 
			
		
	By: 	 	  

		 	Title:


 The foregoing Agreement is hereby accepted and the terms and conditions thereof hereby agreed to by the
undersigned. Electronic acceptance of this Agreement pursuant to the Company’s instructions to the Optionee (including through an online acceptance process) is acceptable. 

 

							
	Dated: 	 	  
	 		 	  

		 		 		 	Optionee’s Signature
				
		 		 		 	Optionee’s name and address:
				
		 		 		 	  

		 		 		 	  

		 		 		 	  



  
 22 








 NON-QUALIFIED STOCK OPTION AGREEMENT 

FOR NON-EMPLOYEE DIRECTORS 

UNDER THE DIMENSION THERAPEUTICS, INC. 

2015 STOCK OPTION AND INCENTIVE PLAN 
 Name
of Optionee: 
 No. of Option Shares: 
 Option Exercise
Price per Share:             $             

Grant Date: 
 Expiration Date: 

Pursuant to the Dimension Therapeutics, Inc. 2015 Stock Option and Incentive Plan as amended through the date hereof (the
“Plan”), Dimension Therapeutics, Inc. (the “Company”) hereby grants to the Optionee named above, who is a member of the Board (a “Director”) but is not an employee of the Company, an option (the “Stock
Option”) to purchase on or prior to the Expiration Date specified above all or part of the number of shares of Common Stock, par value $0.0001 per share (the “Stock”), of the Company specified above at the Option Exercise Price per
Share specified above subject to the terms and conditions set forth herein and in the Plan. This Stock Option is not intended to be an “incentive stock option” under Section 422 of the Internal Revenue Code of 1986, as amended.

 1. Exercisability Schedule. No portion of this Stock Option may be exercised until such portion shall have become
exercisable. Except as set forth below, and subject to the discretion of the Administrator (as defined in Section 2 of the Plan) to accelerate the exercisability schedule hereunder, this Stock Option shall be exercisable with respect to
the following number of Option Shares on the dates indicated so long as the Optionee remains in service as a member of the Board on such dates: 
  

					
	 Incremental Number of

Option Shares Exercisable
	  	Exercisability Date	 
	
(            %)
	  			
	
(            %)
	  			
	
(            %)
	  			
	
(            %)
	  			
	
(            %)
	  			

 Once exercisable, this Stock Option shall continue to be exercisable at any time or times prior to the close
of business on the Expiration Date, subject to the provisions hereof and of the Plan. 
 2. Manner of Exercise. 

(a) The Optionee may exercise this Stock Option only in the following manner: from time to time on or prior to the Expiration Date of
this Stock Option, the Optionee may give written notice to the Administrator of his or her election to purchase some or all of the Option Shares purchasable at the time of such notice. This notice shall specify the number of Option Shares to be
purchased. 
 Payment of the purchase price for the Option Shares may be made by one or more of the following methods: (i) in
cash, by certified or bank check or other instrument acceptable to the Administrator; (ii) through the delivery (or attestation to the ownership) of shares of Stock that have been purchased by the Optionee on the open market or that are
beneficially owned by the Optionee and are not then subject to any restrictions under any Company plan and that otherwise satisfy any holding periods as may be required by the Administrator; (iii) by the Optionee delivering to the Company a
properly executed exercise notice together with irrevocable instructions to a broker to promptly deliver to the Company cash or a check payable and acceptable to the Company to pay the option purchase price, provided that in the event the Optionee
chooses to pay the option purchase price as so provided, the 

  
 23 








 
Optionee and the broker shall comply with such procedures and enter into such agreements of indemnity and other agreements as the Administrator shall prescribe as a condition of such payment
procedure; (iv) by a “net exercise” arrangement pursuant to which the Company will reduce the number of shares of Stock issuable upon exercise by the largest whole number of shares with a Fair Market Value that does not exceed the
aggregate exercise price; or (v) a combination of (i), (ii), (iii) and (iv) above. Payment instruments will be received subject to collection. 

The transfer to the Optionee on the records of the Company or of the transfer agent of the Option Shares will be contingent upon (i) the
Company’s receipt from the Optionee of the full purchase price for the Option Shares, as set forth above, (ii) the fulfillment of any other requirements contained herein or in the Plan or in any other agreement or provision of laws, and
(iii) the receipt by the Company of any agreement, statement or other evidence that the Company may require to satisfy itself that the issuance of Stock to be purchased pursuant to the exercise of Stock Options under the Plan and any subsequent
resale of the shares of Stock will be in compliance with applicable laws and regulations. In the event the Optionee chooses to pay the purchase price by previously-owned shares of Stock through the attestation method, the number of shares of
Stock transferred to the Optionee upon the exercise of the Stock Option shall be net of the shares attested to. 
 (b) The shares of Stock
purchased upon exercise of this Stock Option shall be transferred to the Optionee on the records of the Company or of the transfer agent upon compliance to the satisfaction of the Administrator with all requirements under applicable laws or
regulations in connection with such transfer and with the requirements hereof and of the Plan. The determination of the Administrator as to such compliance shall be final and binding on the Optionee. The Optionee shall not be deemed to be
the holder of, or to have any of the rights of a holder with respect to, any shares of Stock subject to this Stock Option unless and until this Stock Option shall have been exercised pursuant to the terms hereof, the Company or the transfer agent
shall have transferred the shares to the Optionee, and the Optionee’s name shall have been entered as the stockholder of record on the books of the Company. Thereupon, the Optionee shall have full voting, dividend and other ownership
rights with respect to such shares of Stock. 
 (c) The minimum number of shares with respect to which this Stock Option may be exercised at
any one time shall be 100 shares, unless the number of shares with respect to which this Stock Option is being exercised is the total number of shares subject to exercise under this Stock Option at the time. 

(d) Notwithstanding any other provision hereof or of the Plan, no portion of this Stock Option shall be exercisable after the Expiration Date
hereof. 
 3. Termination as Director. If the Optionee ceases to be a Director of the Company, the period within which to exercise
the Stock Option may be subject to earlier termination as set forth below. 
 (a) Termination Due to Death. If the
Optionee’s service as a Director terminates by reason of the Optionee’s death, any portion of this Stock Option outstanding on such date, to the extent exercisable on the date of death, may thereafter be exercised by the Optionee’s
legal representative or legatee for a period of 12 months from the date of death or until the Expiration Date, if earlier. Any portion of this Stock Option that is not exercisable on the date of death shall terminate immediately and be of no
further force or effect. 
 (b) Other Termination. If the Optionee ceases to be a Director for any reason other than the
Optionee’s death, any portion of this Stock Option outstanding on such date may be exercised, to the extent exercisable on the date the Optionee ceased to be a Director, for a period of six months from the date the Optionee ceased to be a
Director or until the Expiration Date, if earlier. Any portion of this Stock Option that is not exercisable on the date the Optionee ceases to be a Director shall terminate immediately and be of no further force or effect. 

4. Incorporation of Plan. Notwithstanding anything herein to the contrary, this Stock Option shall be subject to and governed by
all the terms and conditions of the Plan, including the powers of the Administrator set forth in Section 2(b) of the Plan. Capitalized terms in this Agreement shall have the meaning specified in the Plan, unless a different meaning is
specified herein. 

  
 24 








 5. Transferability. This Agreement is personal to the Optionee, is non-assignable and
is not transferable in any manner, by operation of law or otherwise, other than by will or the laws of descent and distribution. This Stock Option is exercisable, during the Optionee’s lifetime, only by the Optionee, and thereafter, only
by the Optionee’s legal representative or legatee. 
 6. No Obligation to Continue as a Director. Neither the Plan nor this
Stock Option confers upon the Optionee any rights with respect to continuance as a Director. 
 7. Integration. This Agreement
constitutes the entire agreement between the parties with respect to this Stock Option and supersedes all prior agreements and discussions between the parties concerning such subject matter. 

8. Data Privacy Consent. In order to administer the Plan and this Agreement and to implement or structure future equity grants,
the Company, its subsidiaries and affiliates and certain agents thereof (together, the “Relevant Companies”) may process any and all personal or professional data, including but not limited to Social Security or other identification
number, home address and telephone number, date of birth and other information that is necessary or desirable for the administration of the Plan and/or this Agreement (the “Relevant Information”). By entering into this Agreement, the
Optionee (i) authorizes the Company to collect, process, register and transfer to the Relevant Companies all Relevant Information; (ii) waives any privacy rights the Optionee may have with respect to the Relevant Information;
(iii) authorizes the Relevant Companies to store and transmit such information in electronic form; and (iv) authorizes the transfer of the Relevant Information to any jurisdiction in which the Relevant Companies consider
appropriate. The Optionee shall have access to, and the right to change, the Relevant Information. Relevant Information will only be used in accordance with applicable law. 

9. Notices. Notices hereunder shall be mailed or delivered to the Company at its principal place of business and shall be mailed
or delivered to the Optionee at the address on file with the Company or, in either case, at such other address as one party may subsequently furnish to the other party in writing. 

 

			
	DIMENSION THERAPEUTICS, INC.


 
			
		
	By: 	 	  

		 	Title:


 The foregoing Agreement is hereby accepted and the terms and conditions thereof hereby agreed to by the
undersigned. Electronic acceptance of this Agreement pursuant to the Company’s instructions to the Optionee (including through an online acceptance process) is acceptable. 

 

							
	Dated: 	 	  
	 		 	  

		 		 		 	Optionee’s Signature
				
		 		 		 	Optionee’s name and address:
				
		 		 		 	  

		 		 		 	  

		 		 		 	  



  
 25 








 RESTRICTED STOCK AWARD AGREEMENT 

UNDER THE DIMENSION THERAPEUTICS, INC. 

2015 STOCK OPTION AND INCENTIVE PLAN 
 Name
of Grantee: 
 No. of Shares: 
 Grant Date: 

Pursuant to the Dimension Therapeutics, Inc. 2015 Stock Option and Incentive Plan (the “Plan”) as amended through the date
hereof, Dimension Therapeutics, Inc. (the “Company”) hereby grants a Restricted Stock Award (an “Award”) to the Grantee named above. Upon acceptance of this Award, the Grantee shall receive the number of shares of
Common Stock, par value $0.0001 per share (the “Stock”), of the Company specified above, subject to the restrictions and conditions set forth herein and in the Plan. The Company acknowledges the receipt from the Grantee of
consideration with respect to the par value of the Stock in the form of cash, past or future services rendered to the Company by the Grantee or such other form of consideration as is acceptable to the Administrator. 

1. Award. The shares of Restricted Stock awarded hereunder shall be issued and held by the Company’s transfer agent in book
entry form, and the Grantee’s name shall be entered as the stockholder of record on the books of the Company. Thereupon, the Grantee shall have all the rights of a stockholder with respect to such shares, including voting and dividend
rights, subject, however, to the restrictions and conditions specified in Paragraph 2 below. The Grantee shall (i) sign and deliver to the Company a copy of this Award Agreement and (ii) deliver to the Company a stock power
endorsed in blank. 
 2. Restrictions and Conditions. 

(a) Any book entries for the shares of Restricted Stock granted herein shall bear an appropriate legend, as determined by the Administrator in
its sole discretion, to the effect that such shares are subject to restrictions as set forth herein and in the Plan. 
 (b) Shares of
Restricted Stock granted herein may not be sold, assigned, transferred, pledged or otherwise encumbered or disposed of by the Grantee prior to vesting. 

(c) If the Grantee’s employment with the Company and its Subsidiaries is voluntarily or involuntarily terminated for any reason
(including death) prior to vesting of shares of Restricted Stock granted herein, all shares of Restricted Stock shall immediately and automatically be forfeited and returned to the Company. 

3. Vesting of Restricted Stock. The restrictions and conditions in Paragraph 2 of this Agreement shall lapse on the Vesting
Date or Dates specified in the following schedule so long as the Grantee remains an employee of the Company or a Subsidiary on such Dates. If a series of Vesting Dates is specified, then the restrictions and conditions in Paragraph 2 shall
lapse only with respect to the number of shares of Restricted Stock specified as vested on such date. 
  

					
	 Incremental Number

of Shares Vested
	  	Vesting Date	 
	
(            %)
	  			
	
(            %)
	  			
	
(            %)
	  			
	
(            %)
	  			
	
(            %)
	  			

 Subsequent to such Vesting Date or Dates, the shares of Stock on which all restrictions and conditions have
lapsed shall no longer be deemed Restricted Stock. The Administrator may at any time accelerate the vesting schedule specified in this Paragraph 3. 

  
 26 








 4. Dividends. Dividends on shares of Restricted Stock shall be paid currently to the
Grantee. 
 5. Incorporation of Plan. Notwithstanding anything herein to the contrary, this Award shall be subject to and
governed by all the terms and conditions of the Plan, including the powers of the Administrator set forth in Section 2(b) of the Plan. Capitalized terms in this Agreement shall have the meaning specified in the Plan, unless a
different meaning is specified herein. 
 6. Transferability. This Agreement is personal to the Grantee, is non-assignable and
is not transferable in any manner, by operation of law or otherwise, other than by will or the laws of descent and distribution. 
 7.
Tax Withholding. The Grantee shall, not later than the date as of which the receipt of this Award becomes a taxable event for Federal income tax purposes, pay to the Company or make arrangements satisfactory to the Administrator for
payment of any Federal, state, and local taxes required by law to be withheld on account of such taxable event. Except in the case where an election is made pursuant to Paragraph 8 below, the Company shall have the authority to cause the
required minimum tax withholding obligation to be satisfied, in whole or in part, by withholding from shares of Stock to be issued or released by the transfer agent a number of shares of Stock with an aggregate Fair Market Value that would satisfy
the minimum withholding amount due. 
 8. Election Under Section 83(b). The Grantee and the Company hereby agree that the
Grantee may, within 30 days following the Grant Date of this Award, file with the Internal Revenue Service and the Company an election under Section 83(b) of the Internal Revenue Code. In the event the Grantee makes such an election,
he or she agrees to provide a copy of the election to the Company. The Grantee acknowledges that he or she is responsible for obtaining the advice of his or her tax advisors with regard to the Section 83(b) election and that he or she
is relying solely on such advisors and not on any statements or representations of the Company or any of its agents with regard to such election. 

9. No Obligation to Continue Employment. Neither the Company nor any Subsidiary is obligated by or as a result of the Plan or this
Agreement to continue the Grantee in employment and neither the Plan nor this Agreement shall interfere in any way with the right of the Company or any Subsidiary to terminate the employment of the Grantee at any time. 

10. Integration. This Agreement constitutes the entire agreement between the parties with respect to this Award and supersedes all
prior agreements and discussions between the parties concerning such subject matter. 
 11. Data Privacy Consent. In order to
administer the Plan and this Agreement and to implement or structure future equity grants, the Company, its subsidiaries and affiliates and certain agents thereof (together, the “Relevant Companies”) may process any and all personal or
professional data, including but not limited to Social Security or other identification number, home address and telephone number, date of birth and other information that is necessary or desirable for the administration of the Plan and/or this
Agreement (the “Relevant Information”). By entering into this Agreement, the Grantee (i) authorizes the Company to collect, process, register and transfer to the Relevant Companies all Relevant Information; (ii) waives any
privacy rights the Grantee may have with respect to the Relevant Information; (iii) authorizes the Relevant Companies to store and transmit such information in electronic form; and (iv) authorizes the transfer of the Relevant Information
to any jurisdiction in which the Relevant Companies consider appropriate. The Grantee shall have access to, and the right to change, the Relevant Information. Relevant Information will only be used in accordance with applicable law. 

12. Notices. Notices hereunder shall be mailed or delivered to the Company at its principal place of business and shall be mailed
or delivered to the Grantee at the address on file with the Company or, in either case, at such other address as one party may subsequently furnish to the other party in writing. 

 

			
	DIMENSION THERAPEUTICS, INC.


 
			
		
	By: 	 	  

		 	Title:



  
 27 








 The foregoing Agreement is hereby accepted and the terms and conditions thereof hereby agreed to by the
undersigned. Electronic acceptance of this Agreement pursuant to the Company’s instructions to the Grantee (including through an online acceptance process) is acceptable. 

 

							
	Dated: 	 	  
	 		 	  

		 		 		 	Grantee’s Signature
				
		 		 		 	Grantee’s name and address:
				
		 		 		 	  

		 		 		 	  

		 		 		 	  



  
 28 








 RESTRICTED STOCK UNIT AWARD AGREEMENT 

FOR COMPANY EMPLOYEES 

UNDER THE DIMENSION THERAPEUTICS, INC. 

2015 STOCK OPTION AND INCENTIVE PLAN 
 Name
of Grantee: 
 No. of Restricted Stock Units: 
 Grant
Date: 
 Pursuant to the Dimension Therapeutics, Inc. 2015 Stock Option and Incentive Plan as amended through the date hereof (the
“Plan”), Dimension Therapeutics, Inc. (the “Company”) hereby grants an award of the number of Restricted Stock Units listed above (an “Award”) to the Grantee named above. Each Restricted Stock Unit shall
relate to one share of Common Stock, par value $0.0001 per share (the “Stock”), of the Company. 
 1. Restrictions on Transfer
of Award. This Award may not be sold, transferred, pledged, assigned or otherwise encumbered or disposed of by the Grantee, and any shares of Stock issuable with respect to the Award may not be sold, transferred, pledged, assigned or
otherwise encumbered or disposed of until (i) the Restricted Stock Units have vested as provided in Paragraph 2 of this Agreement and (ii) shares of Stock have been issued to the Grantee in accordance with the terms of the Plan and this
Agreement. 
 2. Vesting of Restricted Stock Units. The restrictions and conditions of Paragraph 1 of this Agreement shall
lapse on the Vesting Date or Dates specified in the following schedule so long as the Grantee remains an employee of the Company or a Subsidiary on such Dates. If a series of Vesting Dates is specified, then the restrictions and conditions in
Paragraph 1 shall lapse only with respect to the number of Restricted Stock Units specified as vested on such date. 
  

					
	 Incremental Number of

Restricted Stock Units Vested
	  	Vesting Date	 
	
(            %)
	  			
	
(            %)
	  			
	
(            %)
	  			
	
(            %)
	  			
	
(            %)
	  			

 The Administrator may at any time accelerate the vesting schedule specified in this Paragraph 2. 

3. Termination of Employment. If the Grantee’s employment with the Company and its Subsidiaries terminates for any reason
(including death or disability) prior to the satisfaction of the vesting conditions set forth in Paragraph 2 above, any Restricted Stock Units that have not vested as of such date shall automatically and without notice terminate and be forfeited,
and neither the Grantee nor any of his or her successors, heirs, assigns, or personal representatives will thereafter have any further rights or interests in such unvested Restricted Stock Units. 

4. Issuance of Shares of Stock. As soon as practicable following each Vesting Date (but in no event later than two and one-half
months after the end of the year in which the Vesting Date occurs), the Company shall issue to the Grantee the number of shares of Stock equal to the aggregate number of Restricted Stock Units that have vested pursuant to Paragraph 2 of this
Agreement on such date and the Grantee shall thereafter have all the rights of a stockholder of the Company with respect to such shares. 

5. Incorporation of Plan. Notwithstanding anything herein to the contrary, this Agreement shall be subject to and governed by all
the terms and conditions of the Plan, including the powers of the Administrator set forth in Section 2(b) of the Plan. Capitalized terms in this Agreement shall have the meaning specified in the Plan, unless a different meaning is
specified herein. 

  
 29 








 6. Tax Withholding. The Grantee shall, not later than the date as of which the receipt of
this Award becomes a taxable event for Federal income tax purposes, pay to the Company or make arrangements satisfactory to the Administrator for payment of any Federal, state, and local taxes required by law to be withheld on account of such
taxable event. The Company shall have the authority to cause the required minimum tax withholding obligation to be satisfied, in whole or in part, by withholding from shares of Stock to be issued to the Grantee a number of shares of Stock with
an aggregate Fair Market Value that would satisfy the withholding amount due. 
 7. Section 409A of the Code. This
Agreement shall be interpreted in such a manner that all provisions relating to the settlement of the Award are exempt from the requirements of Section 409A of the Code as “short-term deferrals” as described in Section 409A of
the Code. 
 8. No Obligation to Continue Employment. Neither the Company nor any Subsidiary is obligated by or as a result of
the Plan or this Agreement to continue the Grantee in employment and neither the Plan nor this Agreement shall interfere in any way with the right of the Company or any Subsidiary to terminate the employment of the Grantee at any time. 

9. Integration. This Agreement constitutes the entire agreement between the parties with respect to this Award and supersedes all
prior agreements and discussions between the parties concerning such subject matter. 
 10. Data Privacy Consent. In order to
administer the Plan and this Agreement and to implement or structure future equity grants, the Company, its subsidiaries and affiliates and certain agents thereof (together, the “Relevant Companies”) may process any and all personal or
professional data, including but not limited to Social Security or other identification number, home address and telephone number, date of birth and other information that is necessary or desirable for the administration of the Plan and/or this
Agreement (the “Relevant Information”). By entering into this Agreement, the Grantee (i) authorizes the Company to collect, process, register and transfer to the Relevant Companies all Relevant Information; (ii) waives any
privacy rights the Grantee may have with respect to the Relevant Information; (iii) authorizes the Relevant Companies to store and transmit such information in electronic form; and (iv) authorizes the transfer of the Relevant Information
to any jurisdiction in which the Relevant Companies consider appropriate. The Grantee shall have access to, and the right to change, the Relevant Information. Relevant Information will only be used in accordance with applicable law. 

11. Notices. Notices hereunder shall be mailed or delivered to the Company at its principal place of business and shall be mailed
or delivered to the Grantee at the address on file with the Company or, in either case, at such other address as one party may subsequently furnish to the other party in writing. 

 

			
	DIMENSION THERAPEUTICS, INC.


 
			
		
	By: 	 	  

		 	Title:


 The foregoing Agreement is hereby accepted and the terms and conditions thereof hereby agreed to by the
undersigned. Electronic acceptance of this Agreement pursuant to the Company’s instructions to the Grantee (including through an online acceptance process) is acceptable. 

 

							
	Dated: 	 	  
	 		 	  

		 		 		 	Grantee’s Signature
				
		 		 		 	Grantee’s name and address:
				
		 		 		 	  

		 		 		 	  

		 		 		 	  



  
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 RESTRICTED STOCK UNIT AWARD AGREEMENT 

FOR NON-EMPLOYEE DIRECTORS 

UNDER THE DIMENSION THERAPEUTICS, INC. 

2015 STOCK OPTION AND INCENTIVE PLAN 
 Name
of Grantee: 
 No. of Restricted Stock Units: 
 Grant
Date: 
 Pursuant to the Dimension Therapeutics, Inc. 2015 Stock Option and Incentive Plan as amended through the date hereof (the
“Plan”), Dimension Therapeutics, Inc. (the “Company”) hereby grants an award of the number of Restricted Stock Units listed above (an “Award”) to the Grantee named above. Each Restricted Stock Unit shall
relate to one share of Common Stock, par value $0.0001 per share (the “Stock”), of the Company. 
 1. Restrictions on Transfer
of Award. This Award may not be sold, transferred, pledged, assigned or otherwise encumbered or disposed of by the Grantee, and any shares of Stock issuable with respect to the Award may not be sold, transferred, pledged, assigned or
otherwise encumbered or disposed of until (i) the Restricted Stock Units have vested as provided in Paragraph 2 of this Agreement and (ii) shares of Stock have been issued to the Grantee in accordance with the terms of the Plan and this
Agreement. 
 2. Vesting of Restricted Stock Units. The restrictions and conditions of Paragraph 1 of this Agreement shall
lapse on the Vesting Date or Dates specified in the following schedule so long as the Grantee remains in service as a member of the Board on such Dates. If a series of Vesting Dates is specified, then the restrictions and conditions in
Paragraph 1 shall lapse only with respect to the number of Restricted Stock Units specified as vested on such date. 
  

					
	 Incremental Number of

Restricted Stock Units Vested
	  	Vesting Date	 
	
(            %)
	  			
	
(            %)
	  			
	
(            %)
	  			
	
(            %)
	  			
	
(            %)
	  			

 The Administrator may at any time accelerate the vesting schedule specified in this Paragraph 2. 

3. Termination of Service. If the Grantee’s service as a member of the Board terminates for any reason (including death or
disability) prior to the satisfaction of the vesting conditions set forth in Paragraph 2 above, any Restricted Stock Units that have not vested as of such date shall automatically and without notice terminate and be forfeited, and neither the
Grantee nor any of his or her successors, heirs, assigns, or personal representatives will thereafter have any further rights or interests in such unvested Restricted Stock Units. 

4. Issuance of Shares of Stock. As soon as practicable following each Vesting Date (but in no event later than two and one-half
months after the end of the year in which the Vesting Date occurs), the Company shall issue to the Grantee the number of shares of Stock equal to the aggregate number of Restricted Stock Units that have vested pursuant to Paragraph 2 of this
Agreement on such date and the Grantee shall thereafter have all the rights of a stockholder of the Company with respect to such shares. 

5. Incorporation of Plan. Notwithstanding anything herein to the contrary, this Agreement shall be subject to and governed by all
the terms and conditions of the Plan, including the powers of the Administrator set forth in Section 2(b) of the Plan. Capitalized terms in this Agreement shall have the meaning specified in the Plan, unless a different meaning is
specified herein. 

  
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 6. Section 409A of the Code. This Agreement shall be interpreted in such a
manner that all provisions relating to the settlement of the Award are exempt from the requirements of Section 409A of the Code as “short-term deferrals” as described in Section 409A of the Code. 

7. No Obligation to Continue as a Director. Neither the Plan nor this Award confers upon the Grantee any rights with respect to
continuance as a member of the Board. 
 8. Integration. This Agreement constitutes the entire agreement between the parties
with respect to this Award and supersedes all prior agreements and discussions between the parties concerning such subject matter. 
 9.
Data Privacy Consent. In order to administer the Plan and this Agreement and to implement or structure future equity grants, the Company, its subsidiaries and affiliates and certain agents thereof (together, the “Relevant
Companies”) may process any and all personal or professional data, including but not limited to Social Security or other identification number, home address and telephone number, date of birth and other information that is necessary or
desirable for the administration of the Plan and/or this Agreement (the “Relevant Information”). By entering into this Agreement, the Grantee (i) authorizes the Company to collect, process, register and transfer to the Relevant
Companies all Relevant Information; (ii) waives any privacy rights the Grantee may have with respect to the Relevant Information; (iii) authorizes the Relevant Companies to store and transmit such information in electronic form; and
(iv) authorizes the transfer of the Relevant Information to any jurisdiction in which the Relevant Companies consider appropriate. The Grantee shall have access to, and the right to change, the Relevant Information. Relevant
Information will only be used in accordance with applicable law. 
 10. Notices. Notices hereunder shall be mailed or delivered
to the Company at its principal place of business and shall be mailed or delivered to the Grantee at the address on file with the Company or, in either case, at such other address as one party may subsequently furnish to the other party in writing.

  

			
	DIMENSION THERAPEUTICS, INC.


 
			
		
	By: 	 	  

		 	Title:


 The foregoing Agreement is hereby accepted and the terms and conditions thereof hereby agreed to by the
undersigned. Electronic acceptance of this Agreement pursuant to the Company’s instructions to the Grantee (including through an online acceptance process) is acceptable. 

 

							
	Dated: 	 	  
	 		 	  

		 		 		 	Grantee’s Signature
				
		 		 		 	Grantee’s name and address:
				
		 		 		 	  

		 		 		 	  

		 		 		 	  



  
 32