EX-4.5



 Exhibit 4.5 

DIMENSION THERAPEUTICS, INC. 

2013 STOCK PLAN 

ADOPTED ON OCTOBER 29, 2013 







 TABLE OF CONTENTS 

 

									
	 	  	 	  	 	  	Page	 
			
	SECTION 1.	  	ESTABLISHMENT AND PURPOSE	  	 	1	 
			
	SECTION 2.	  	ADMINISTRATION	  	 	1	 
				
		  	(a)  	  	Committees of the Board of Directors	  	 	1	 
		  	(b)	  	Authority of the Board of Directors	  	 	1	 
			
	SECTION 3.	  	ELIGIBILITY	  	 	1	 
				
		  	(a)	  	General Rule	  	 	1	 
		  	(b)	  	Ten-Percent Stockholders	  	 	1	 
			
	SECTION 4.	  	STOCK SUBJECT TO PLAN	  	 	2	 
				
		  	(a)	  	Basic Limitation	  	 	2	 
		  	(b)	  	Additional Shares	  	 	2	 
			
	SECTION 5.	  	TERMS AND CONDITIONS OF AWARDS OR SALES	  	 	2	 
				
		  	(a)	  	Stock Grant or Purchase Agreement	  	 	2	 
		  	(b)	  	Duration of Offers and Nontransferability of Rights	  	 	2	 
		  	(c)	  	Purchase Price	  	 	3	 
			
	SECTION 6.	  	TERMS AND CONDITIONS OF OPTIONS	  	 	3	 
				
		  	(a)	  	Stock Option Agreement	  	 	3	 
		  	(b)	  	Number of Shares	  	 	3	 
		  	(c)	  	Exercise Price	  	 	3	 
		  	(d)	  	Exercisability	  	 	3	 
		  	(e)	  	Basic Term	  	 	3	 
		  	(f)	  	Termination of Service (Except by Death)	  	 	3	 
		  	(g)	  	Leaves of Absence	  	 	4	 
		  	(h)	  	Death of Optionee	  	 	4	 
		  	(i)	  	Restrictions on Transfer of Options	  	 	4	 
		  	(j)	  	No Rights as a Stockholder	  	 	5	 
		  	(k)	  	Modification, Extension and Assumption of Options	  	 	5	 
		  	(l)	  	Company’s Right to Cancel Certain Options	  	 	5	 
			
	SECTION 7.	  	PAYMENT FOR SHARES	  	 	5	 
				
		  	(a)	  	General Rule	  	 	5	 
		  	(b)	  	Services Rendered	  	 	5	 
		  	(c)	  	Promissory Note	  	 	5	 
		  	(d)	  	Surrender of Stock	  	 	6	 
		  	(e)	  	Exercise/Sale	  	 	6	 
		  	(f)	  	Net Exercise	  	 	6	 
		  	(g)	  	Other Forms of Payment	  	 	6	 
			
	SECTION 8.	  	ADJUSTMENT OF SHARES	  	 	6	 









									
	 	  	 	  	 	  	Page	 
				
		  	(a)	  	General	  	 	6	 
		  	(b)	  	Corporate Transactions	  	 	7	 
		  	(c)	  	Reservation of Rights	  	 	8	 
			
	SECTION 9.	  	MISCELLANEOUS PROVISIONS	  	 	8	 
				
		  	(a)	  	Securities Law Requirements	  	 	8	 
		  	(b)	  	No Retention Rights	  	 	8	 
		  	(c)	  	Treatment as Compensation	  	 	8	 
		  	(d)	  	Governing Law	  	 	9	 
		  	(e)	  	Conditions and Restrictions on Shares	  	 	9	 
		  	(f)	  	Tax Matters	  	 	9	 
			
	SECTION 10.	  	DURATION AND AMENDMENTS; STOCKHOLDER APPROVAL	  	 	10	 
				
		  	(a)	  	Term of the Plan	  	 	10	 
		  	(b)	  	Right to Amend or Terminate the Plan	  	 	10	 
		  	(c)	  	Effect of Amendment or Termination	  	 	10	 
		  	(d)	  	Stockholder Approval	  	 	10	 
			
	SECTION 11.	  	DEFINITIONS	  	 	10	 








 DIMENSION THERAPEUTICS, INC. 2013 STOCK PLAN 

 

	SECTION 1.	  ESTABLISHMENT AND PURPOSE. 

 The purpose of this Plan is to offer
persons selected by the Company an opportunity to acquire a proprietary interest in the success of the Company, or to increase such interest, by acquiring Shares of the Company’s Stock. The Plan provides both for the direct award or sale of
Shares and for the grant of Options to purchase Shares. Options granted under the Plan may be ISOs intended to qualify under Code Section 422 or NSOs which are not intended to so qualify. 

Capitalized terms are defined in Section 11. 
  

	SECTION 2.	  ADMINISTRATION. 

 (a) Committees of the Board of Directors. The
Plan may be administered by one or more Committees. Each Committee shall consist, as required by applicable law, of one or more members of the Board of Directors who have been appointed by the Board of Directors. Each Committee shall have such
authority and be responsible for such functions as the Board of Directors has assigned to it. If no Committee has been appointed, the entire Board of Directors shall administer the Plan. Any reference to the Board of Directors in the Plan shall be
construed as a reference to the Committee (if any) to whom the Board of Directors has assigned a particular function. 
 (b) Authority of
the Board of Directors. Subject to the provisions of the Plan, the Board of Directors shall have full authority and discretion to take any actions it deems necessary or advisable for the administration of the Plan. Notwithstanding anything to
the contrary in the Plan, with respect to the terms and conditions of awards granted to Participants outside the United States, the Board of Directors may vary from the provisions of the Plan to the extent it determines it necessary and appropriate
to do so; provided that it may not vary from those Plan terms requiring stockholder approval pursuant to Section 10(d) below. All decisions, interpretations and other actions of the Board of Directors shall be final and binding on all
Purchasers, all Optionees and all persons deriving their rights from a Purchaser or Optionee. 
  

	SECTION 3.	  ELIGIBILITY. 

 (a) General Rule. Only Employees, Outside
Directors and Consultants shall be eligible for the grant of NSOs or the direct award or sale of Shares1. Only Employees shall be eligible for the grant of ISOs. 

(b) Ten-Percent Stockholders. A person who owns more than 10% of the total combined voting power of all classes of outstanding stock of
the Company, its Parent or any of its Subsidiaries shall not be eligible for the grant of an ISO unless (i) the Exercise Price is 

 

	1 	Note that special considerations apply if the Company proposes to grant awards to an Employee or Consultant of a Parent company. 








 
at least 110% of the Fair Market Value of a Share on the Date of Grant and (ii) such ISO by its terms is not exercisable after the expiration of five years from the Date of Grant. For
purposes of this Subsection (b), in determining stock ownership, the attribution rules of Code Section 424(d) shall be applied. 
  

	SECTION 4.	  STOCK SUBJECT TO PLAN. 

 (a) Basic Limitation. Not more than
10,000,000 Shares may be issued under the Plan, subject to Subsection (b) below and Section 8(a).2 All of these Shares may be issued upon the exercise of ISOs. The number of Shares that
are subject to Options or other rights outstanding at any time under the Plan may not exceed the number of Shares that then remain available for issuance under the Plan. The Company, during the term of the Plan, shall at all times reserve and keep
available sufficient Shares to satisfy the requirements of the Plan. Shares offered under the Plan may be authorized but unissued Shares or treasury Shares. 

(b) Additional Shares. In the event that Shares previously issued under the Plan are reacquired by the Company, such Shares shall be
added to the number of Shares then available for issuance under the Plan. In the event that Shares that otherwise would have been issuable under the Plan are withheld by the Company in payment of the Purchase Price, Exercise Price or withholding
taxes, such Shares shall remain available for issuance under the Plan. In the event that an outstanding Option or other right for any reason expires or is canceled, the Shares allocable to the unexercised portion of such Option or other right shall
be added to the number of Shares then available for issuance under the Plan. 
  

	SECTION 5.	  TERMS AND CONDITIONS OF AWARDS OR SALES. 

 (a) Stock Grant or
Purchase Agreement. Each award of Shares under the Plan shall be evidenced by a Stock Grant Agreement between the Grantee and the Company. Each sale of Shares under the Plan (other than upon exercise of an Option) shall be evidenced by a Stock
Purchase Agreement between the Purchaser and the Company. Such award or sale shall be subject to all applicable terms and conditions of the Plan and may be subject to any other terms and conditions which are not inconsistent with the Plan and which
the Board of Directors deems appropriate for inclusion in a Stock Grant Agreement or Stock Purchase Agreement. The provisions of the various Stock Grant Agreements and Stock Purchase Agreements entered into under the Plan need not be identical. 

(b) Duration of Offers and Nontransferability of Rights. Any right to purchase Shares under the Plan (other than an Option) shall
automatically expire if not exercised by the Purchaser within 30 days (or such other period as may be specified in the Award Agreement) after the grant of such right was communicated to the Purchaser by the Company. Such right is not transferable
and may be exercised only by the Purchaser to whom such right was granted. 
  

	2 	Please refer to Exhibit A for a schedule of the initial share reserve and any subsequent increases in the reserve. 


  
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 (c) Purchase Price. The Board of Directors shall determine the Purchase Price of Shares to
be offered under the Plan at its sole discretion. The Purchase Price shall be payable in a form described in Section 7. 
  

	SECTION 6.	  TERMS AND CONDITIONS OF OPTIONS. 

 (a) Stock Option Agreement.
Each grant of an Option under the Plan shall be evidenced by a Stock Option Agreement between the Optionee and the Company. The Option shall be subject to all applicable terms and conditions of the Plan and may be subject to any other terms and
conditions that are not inconsistent with the Plan and that the Board of Directors deems appropriate for inclusion in a Stock Option Agreement. The provisions of the various Stock Option Agreements entered into under the Plan need not be identical.

 (b) Number of Shares. Each Stock Option Agreement shall specify the number of Shares that are subject to the Option and shall
provide for the adjustment of such number in accordance with Section 8. The Stock Option Agreement shall also specify whether the Option is an ISO or an NSO. 

(c) Exercise Price. Each Stock Option Agreement shall specify the Exercise Price. The Exercise Price of an Option shall not be less
than 100% of the Fair Market Value of a Share on the Date of Grant, and in the case of an ISO a higher percentage may be required by Section 3(b). Subject to the preceding sentence, the Exercise Price shall be determined by the Board of
Directors at its sole discretion. The Exercise Price shall be payable in a form described in Section 7. This Subsection (c) shall not apply to an Option granted pursuant to an assumption of, or substitution for, another option in a manner
that complies with Code Section 424(a) (whether or not the Option is an ISO). 
 (d) Exercisability. Each Stock Option Agreement
shall specify the date when all or any installment of the Option is to become exercisable. No Option shall be exercisable unless the Optionee (i) has delivered an executed copy of the Stock Option Agreement to the Company or (ii) otherwise
agrees to be bound by the terms of the Stock Option Agreement. The Board of Directors shall determine the exercisability provisions of the Stock Option Agreement at its sole discretion. 

(e) Basic Term. The Stock Option Agreement shall specify the term of the Option. The term shall not exceed 10 years from the Date of
Grant, and in the case of an ISO, a shorter term may be required by Section 3(b). Subject to the preceding sentence, the Board of Directors at its sole discretion shall determine when an Option is to expire. 

(f) Termination of Service (Except by Death). If an Optionee’s Service terminates for any reason other than the Optionee’s
death, then the Optionee’s Options shall expire on the earliest of the following dates: 
 (i) The expiration date
determined pursuant to Subsection (e) above; 
 (ii) The date three months after the termination of the Optionee’s
Service for any reason other than Disability, or such earlier or later date as the Board of Directors may determine (but in no event earlier than 30 days after the termination of the Optionee’s Service); or 

  
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 (iii) The date six months after the termination of the Optionee’s Service by
reason of Disability, or such later date as the Board of Directors may determine. 
 The Optionee may exercise all or part of the Optionee’s Options at
any time before the expiration of such Options under the preceding sentence, but only to the extent that such Options had become exercisable before the Optionee’s Service terminated (or became exercisable as a result of the termination) and the
underlying Shares had vested before the Optionee’s Service terminated (or vested as a result of the termination). The balance of such Options shall lapse when the Optionee’s Service terminates. In the event that the Optionee dies after the
termination of the Optionee’s Service but before the expiration of the Optionee’s Options, all or part of such Options may be exercised (prior to expiration) by the executors or administrators of the Optionee’s estate or by any person
who has acquired such Options directly from the Optionee by beneficiary designation, bequest or inheritance, but only to the extent that such Options had become exercisable before the Optionee’s Service terminated (or became exercisable as a
result of the termination) and the underlying Shares had vested before the Optionee’s Service terminated (or vested as a result of the termination). 

(g) Leaves of Absence. For purposes of Subsection (f) above, Service shall be deemed to continue while the Optionee is on a bona
fide leave of absence, if such leave was approved by the Company in writing and if continued crediting of Service for this purpose is expressly required by the terms of such leave or by applicable law (as determined by the Company). 

(h) Death of Optionee. If an Optionee dies while the Optionee is in Service, then the Optionee’s Options shall expire on the
earlier of the following dates: 
 (i) The expiration date determined pursuant to Subsection (e) above; or 

(ii) The date 12 months after the Optionee’s death, or such earlier or later date as the Board of Directors may determine
(but in no event earlier than six months after the Optionee’s death). 
 All or part of the Optionee’s Options may be exercised at any time before
the expiration of such Options under the preceding sentence by the executors or administrators of the Optionee’s estate or by any person who has acquired such Options directly from the Optionee by beneficiary designation, bequest or
inheritance, but only to the extent that such Options had become exercisable before the Optionee’s death (or became exercisable as a result of the death) and the underlying Shares had vested before the Optionee’s death (or vested as a
result of the Optionee’s death). The balance of such Options shall lapse when the Optionee dies. 
 (i) Restrictions on Transfer of
Options. An Option shall be transferable by the Optionee only by (i) a beneficiary designation, (ii) a will or (iii) the laws of descent and distribution, except as provided in the next sentence. If the applicable Stock Option
Agreement so provides, an NSO shall also be transferable by gift or domestic relations order to a Family Member of the Optionee. An ISO may be exercised during the lifetime of the Optionee only by the Optionee or by the Optionee’s guardian or
legal representative. 

  
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 (j) No Rights as a Stockholder. An Optionee, or a transferee of an Optionee, shall have no
rights as a stockholder with respect to any Shares covered by the Optionee’s Option until such person files a notice of exercise, pays the Exercise Price and satisfies all applicable withholding taxes pursuant to the terms of such Option. 

(k) Modification, Extension and Assumption of Options. Within the limitations of the Plan, the Board of Directors may modify, extend or
assume outstanding Options or may accept the cancellation of outstanding Options (whether granted by the Company or another issuer) in return for the grant of new Options or a different type of award for the same or a different number of Shares and
at the same or a different Exercise Price (if applicable). The foregoing notwithstanding, no modification of an Option shall, without the consent of the Optionee, impair the Optionee’s rights or increase the Optionee’s obligations under
such Option. 
 (l) Company’s Right to Cancel Certain Options. Any other provision of the Plan or a Stock Option Agreement
notwithstanding, the Company shall have the right at any time to cancel an Option that was not granted in compliance with Rule 701 under the Securities Act. Prior to canceling such Option, the Company shall give the Optionee not less than 30
days’ notice in writing. If the Company elects to cancel such Option, it shall deliver to the Optionee consideration with an aggregate Fair Market Value equal to the excess of (i) the Fair Market Value of the Shares subject to such Option
as of the time of the cancellation over (ii) the Exercise Price of such Option. The consideration may be delivered in the form of cash or cash equivalents, in the form of Shares, or a combination of both. If the consideration would be a
negative amount, such Option may be cancelled without the delivery of any consideration. 
  

	SECTION 7.	  PAYMENT FOR SHARES. 

 (a) General Rule. The entire Purchase
Price or Exercise Price of Shares issued under the Plan shall be payable in cash or cash equivalents at the time when such Shares are purchased, except as otherwise provided in this Section 7. In addition, the Board of Directors in its sole
discretion may also permit payment through any of the methods described in (b) through (g) below. 
 (b) Services Rendered.
Shares may be awarded under the Plan in consideration of services rendered to the Company, a Parent or a Subsidiary prior to the award. 

(c) Promissory Note. All or a portion of the Purchase Price or Exercise Price (as the case may be) of Shares issued under the Plan may
be paid with a full-recourse promissory note. The Shares shall be pledged as security for payment of the principal amount of the promissory note and interest thereon. The interest rate payable under the terms of the promissory note shall not be less
than the minimum rate (if any) required to avoid the imputation of additional interest under the Code. Subject to the foregoing, the Board of Directors (at its sole discretion) shall specify the term, interest rate, amortization requirements (if
any) and other provisions of such note. 

  
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 (d) Surrender of Stock. All or any part of the Exercise Price may be paid by surrendering,
or attesting to the ownership of, Shares that are already owned by the Optionee. Such Shares shall be surrendered to the Company in good form for transfer and shall be valued at their Fair Market Value as of the date when the Option is exercised.

 (e) Exercise/Sale. If the Stock is publicly traded, all or part of the Exercise Price and any withholding taxes may be paid by the
delivery (on a form prescribed by the Company) of an irrevocable direction to a securities broker approved by the Company to sell Shares and to deliver all or part of the sales proceeds to the Company. 

(f) Net Exercise. An Option may permit exercise through a “net exercise” arrangement pursuant to which the Company will
reduce the number of Shares issued upon exercise by the largest whole number of Shares having an aggregate Fair Market Value (determined by the Board of Directors as of the exercise date) that does not exceed the aggregate Exercise Price or the sum
of the aggregate Exercise Price plus all or a portion of the minimum amount required to be withheld under applicable tax law (with the Company accepting from the Optionee payment of cash or cash equivalents to satisfy any remaining balance of the
aggregate Exercise Price and, if applicable, any additional withholding obligation not satisfied through such reduction in Shares); provided that to the extent Shares subject to an Option are withheld in this manner, the number of Shares subject to
the Option following the net exercise will be reduced by the sum of the number of Shares withheld and the number of Shares delivered to the Optionee as a result of the exercise. 

(g) Other Forms of Payment. To the extent that an Award Agreement so provides, the Purchase Price or Exercise Price of Shares issued
under the Plan may be paid in any other form permitted by the Delaware General Corporation Law, as amended. 
  

	SECTION 8.	  ADJUSTMENT OF SHARES. 

 (a) General. In the event of a
subdivision of the outstanding Stock, a declaration of a dividend payable in Shares, a combination or consolidation of the outstanding Stock into a lesser number of Shares, a reclassification, or any other increase or decrease in the number of
issued shares of Stock effected without receipt of consideration by the Company, proportionate adjustments shall automatically be made in each of (i) the number and kind of Shares available for future grants under Section 4, (ii) the
number and kind of Shares covered by each outstanding Option and any outstanding and unexercised right to purchase Shares that has not yet expired pursuant to Section 5(b), (iii) the Exercise Price under each outstanding Option and the
Purchase Price applicable to any unexercised stock purchase right described in clause (ii) above, and (iv) any repurchase price that applies to Shares granted under the Plan pursuant to the terms of a Company repurchase right under the
applicable Award Agreement. In the event of a declaration of an extraordinary dividend payable in a form other than Shares in an amount that has a material effect on the Fair Market Value of the Stock, a recapitalization, a spin-off, or a similar
occurrence, the Board of Directors at its sole discretion may make appropriate adjustments in one or more of the items listed in clauses (i) through (iv) above; provided, however, that the Board of Directors shall in any event make such
adjustments as may be required by Section 25102(o) of the California Corporations Code. No fractional Shares shall be issued under the Plan as a result of an adjustment under this Section 8(a), although the Board of Directors in its sole
discretion may make a cash payment in lieu of fractional Shares. 

  
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 (b) Corporate Transactions. In the event that the Company is a party to a merger or
consolidation, or in the event of a sale of all or substantially all of the Company’s stock or assets, all Shares acquired under the Plan and all Options and other Plan awards outstanding on the effective date of the transaction shall be
treated in the manner described in the definitive transaction agreement (or, in the event the transaction does not entail a definitive agreement to which the Company is party, in the manner determined by the Board of Directors in its capacity as
administrator of the Plan, with such determination having final and binding effect on all parties), which agreement or determination need not treat all Options and awards (or all portions of an Option or an award) in an identical manner. The
treatment specified in the transaction agreement or as determined by the Board of Directors may include (without limitation) one or more of the following with respect to each outstanding Option or award: 

(i) Continuation of the Option or award by the Company (if the Company is the surviving corporation). 

(ii) Assumption of the Option by the surviving corporation or its parent in a manner that complies with Code
Section 424(a) (whether or not the Option is an ISO). 
 (iii) Substitution by the surviving corporation or its parent
of a new option for the Option in a manner that complies with Code Section 424(a) (whether or not the Option is an ISO). 

(iv) Cancellation of the Option and a payment to the Optionee with respect to each Share subject to the portion of the Option
that is vested as of the transaction date equal to the excess of (A) the value, as determined by the Board of Directors in its absolute discretion, of the property (including cash) received by the holder of a share of Stock as a result of the
transaction, over (B) the per-Share Exercise Price of the Option (such excess, the “Spread”). Such payment shall be made in the form of cash, cash equivalents, or securities of the surviving corporation or its parent having a
value equal to the Spread. In addition, any escrow, holdback, earn-out or similar provisions in the transaction agreement may apply to such payment to the same extent and in the same manner as such provisions apply to the holders of Stock. If the
Spread applicable to an Option is zero or a negative number, then the Option may be cancelled without making a payment to the Optionee. 

(v) Cancellation of the Option without the payment of any consideration; provided that the Optionee shall be notified of such
treatment and given an opportunity to exercise the Option (to the extent the Option is vested or becomes vested as of the effective date of the transaction) during a period of not less than five (5) business days preceding the effective date of
the transaction, unless (A) a shorter period is required to permit a timely closing of the transaction and (B) such shorter period still offers the Optionee a reasonable opportunity to exercise the Option. Any exercise of the Option during
such period may be contingent upon the closing of the transaction. 

  
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 (vi) Suspension of the Optionee’s right to exercise the Option during a
limited period of time preceding the closing of the transaction if such suspension is administratively necessary to permit the closing of the transaction. 

(vii) Termination of any right the Optionee has to exercise the Option prior to vesting in the Shares subject to the Option
(i.e., “early exercise”), such that following the closing of the transaction the Option may only be exercised to the extent it is vested. 
 For
the avoidance of doubt, the Board of Directors has discretion to accelerate, in whole or part, the vesting and exercisability of an Option or other Plan award in connection with a corporate transaction covered by this Section 8(b). 

(c) Reservation of Rights. Except as provided in this Section 8, a Participant shall have no rights by reason of (i) any
subdivision or consolidation of shares of stock of any class, (ii) the payment of any dividend or (iii) any other increase or decrease in the number of shares of stock of any class. Any issuance by the Company of shares of stock of any
class, or securities convertible into shares of stock of any class, shall not affect, and no adjustment by reason thereof shall be made with respect to, the number or Exercise Price of Shares subject to an Option. The grant of an Option pursuant to
the Plan shall not affect in any way the right or power of the Company to make adjustments, reclassifications, reorganizations or changes of its capital or business structure, to merge or consolidate or to dissolve, liquidate, sell or transfer all
or any part of its business or assets. 
  

	SECTION 9.	  MISCELLANEOUS PROVISIONS. 

 (a) Securities Law Requirements.
Shares shall not be issued under the Plan unless, in the opinion of counsel acceptable to the Board of Directors, the issuance and delivery of such Shares comply with (or are exempt from) all applicable requirements of law, including (without
limitation) the Securities Act, the rules and regulations promulgated thereunder, state securities laws and regulations, and the regulations of any stock exchange or other securities market on which the Company’s securities may then be traded.
The Company shall not be liable for a failure to issue Shares as a result of such requirements. 
 (b) No Retention Rights. Nothing
in the Plan or in any right or Option granted under the Plan shall confer upon the Participant any right to continue in Service for any period of specific duration or interfere with or otherwise restrict in any way the rights of the Company (or any
Parent or Subsidiary employing or retaining the Participant) or of the Participant, which rights are hereby expressly reserved by each, to terminate his or her Service at any time and for any reason, with or without cause. 

(c) Treatment as Compensation. Any compensation that an individual earns or is deemed to earn under this Plan shall not be considered a
part of his or her compensation for purposes of calculating contributions, accruals or benefits under any other plan or program that is maintained or funded by the Company, a Parent or a Subsidiary. 

  
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 (d) Governing Law. The Plan and all awards, sales and grants under the Plan shall be
governed by, and construed in accordance with, the laws of the State of Delaware, as such laws are applied to contracts entered into and performed in such State. 

(e) Conditions and Restrictions on Shares. Shares issued under the Plan shall be subject to such forfeiture conditions, rights of
repurchase, rights of first refusal, other transfer restrictions and such other terms and conditions as the Board of Directors may determine. Such conditions and restrictions shall be set forth in the applicable Award Agreement and shall apply in
addition to any restrictions that may apply to holders of Shares generally. In addition, Shares issued under the Plan shall be subject to conditions and restrictions imposed either by applicable law or by Company policy, as adopted from time to
time, designed to ensure compliance with applicable law or laws with which the Company determines in its sole discretion to comply including in order to maintain any statutory, regulatory or tax advantage. 

(f) Tax Matters. 

(i) As a condition to the award, grant, issuance, vesting, purchase, exercise or transfer of any award, or Shares issued
pursuant to any award, granted under this Plan, the Participant shall make such arrangements as the Board of Directors may require or permit for the satisfaction of any federal, state, local or foreign withholding tax obligations that may arise in
connection with such event. 
 (ii) Unless otherwise expressly set forth in an Award Agreement, it is intended that awards
granted under the Plan shall be exempt from Code Section 409A, and any ambiguity in the terms of an Award Agreement and the Plan shall be interpreted consistently with this intent. To the extent an award is not exempt from Code
Section 409A (any such award, a “409A Award”), any ambiguity in the terms of such award and the Plan shall be interpreted in a manner that to the maximum extent permissible supports the award’s compliance with the
requirements of that statute. Notwithstanding anything to the contrary permitted under the Plan, in no event shall a modification of an Award not already subject to Code Section 409A be given effect if such modification would cause the Award to
become subject to Code Section 409A unless the parties explicitly acknowledge and consent to the modification as one having that effect. A 409A Award shall be subject to such additional rules and requirements as specified by the Board of
Directors from time to time in order for it to comply with the requirements of Code Section 409A. In this regard, if any amount under a 409A Award is payable upon a “separation from service” to an individual who is considered a
“specified employee” (as each term is defined under Code 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 Participant’s separation from service
or (ii) the Participant’s death, but only to the extent such delay is necessary to prevent such payment from being subject to Section 409A(a)(1). In addition, if a transaction subject to Section 8(b) constitutes a payment event
with respect to any 409A Award, then the transaction with respect to such award must also constitute a “change in control event” as defined in Treasury Regulation Section 1.409A-3(i)(5) to the extent required by Code
Section 409A. 

  
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 (iii) Neither the Company nor any member of the Board of Directors shall have any
liability to a Participant in the event an award held by the Participant fails to achieve its intended characterization under applicable tax law. 
  

	SECTION 10.	  DURATION AND AMENDMENTS; STOCKHOLDER APPROVAL. 

 (a) Term of the
Plan. The Plan, as set forth herein, shall become effective on the date of its adoption by the Board of Directors, subject to approval of the Company’s stockholders under Subsection (d) below. The Plan shall terminate automatically 10
years after the later of (i) the date when the Board of Directors adopted the Plan or (ii) the date when the Board of Directors approved the most recent increase in the number of Shares reserved under Section 4 that was also approved
by the Company’s stockholders. The Plan may be terminated on any earlier date pursuant to Subsection (b) below. 
 (b) Right to
Amend or Terminate the Plan. Subject to Subsection (d) below, the Board of Directors may amend, suspend or terminate the Plan at any time and for any reason. 

(c) Effect of Amendment or Termination. No Shares shall be issued or sold and no Option granted under the Plan after the termination
thereof, except upon exercise of an Option (or any other right to purchase Shares) granted under the Plan prior to such termination. The termination of the Plan, or any amendment thereof, shall not affect any Share previously issued or any Option
previously granted under the Plan. 
 (d) Stockholder Approval. To the extent required by applicable law, the Plan will be subject to
approval of the Company’s stockholders within 12 months of its adoption date. To the extent required by applicable law, any amendment of the Plan will be subject to the approval of the Company’s stockholders within 12 months of the
amendment date if it (i) increases the number of Shares available for issuance under the Plan (except as provided in Section 8), or (ii) materially changes the class of persons who are eligible for the grant of ISOs. In addition, an
amendment effecting any other material change to the Plan terms will be subject to approval of the Company’s stockholder only if required by applicable law. Stockholder approval shall not be required for any other amendment of the Plan. 

SECTION 11.   DEFINITIONS. 

(a) “Award Agreement” means a Stock Grant Agreement, Stock Option Agreement or Stock Purchase Agreement. 

(b) “Board of Directors” means the Board of Directors of the Company, as constituted from time to time. 

(c) “Code” means the Internal Revenue Code of 1986, as amended. 

(d) “Committee” means a committee of the Board of Directors, as described in Section 2(a). 

(e) “Company” means Dimension Therapeutics, Inc., a Delaware corporation. 

  
 10 








 (f) “Consultant” means a person, excluding Employees and Outside Directors, who
performs bona fide services for the Company, a Parent3 or a Subsidiary as a consultant or advisor and who qualifies as a consultant or advisor under Rule 701(c)(1) of the Securities Act or under
Instruction A.1.(a)(1) of Form S-8 under the Securities Act. 
 (g) “Date of Grant” means the date of grant
specified in the applicable Stock Option Agreement, which date shall be the later of (i) the date on which the Board of Directors resolved to grant the Option or (ii) the first day of the Optionee’s Service. 

(h) “Disability” means that the Optionee is unable to engage in any substantial gainful activity by reason of any
medically determinable physical or mental impairment. 
 (i) “Employee” means any individual who is a common-law
employee of the Company, a Parent4 or a Subsidiary. 
 (j) “Exchange
Act” means the Securities Exchange Act of 1934, as amended. 
 (k) “Exercise Price” means the amount for which one
Share may be purchased upon exercise of an Option, as specified by the Board of Directors in the applicable Stock Option Agreement. 
 (l)
“Fair Market Value” means the fair market value of a Share, as determined by the Board of Directors in good faith. Such determination shall be conclusive and binding on all persons. 

(m) “Family Member” means (i) any 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, (ii) any person sharing the Optionee’s household (other than a tenant or employee),
(iii) a trust in which persons described in Clause (i) or (ii) have more than 50% of the beneficial interest, (iv) a foundation in which persons described in Clause (i) or (ii) or the Optionee control the management of
assets and (v) any other entity in which persons described in Clause (i) or (ii) or the Optionee own more than 50% of the voting interests. 

(n) “Grantee” means a person to whom the Board of Directors has awarded Shares under the Plan. 

(o) “ISO” means an Option that qualifies as an incentive stock option as described in Code Section 422(b).
Notwithstanding its designation as an ISO, an Option that does not qualify as an ISO under applicable law shall be treated for all purposes as an NSO. 

 

	3 	Note that special considerations apply if the Company proposes to grant awards to consultant or advisor of a Parent company. 


	4 	Note that special considerations apply if the Company proposes to grant awards to Employee of a Parent company. 


  
 11 








 (p) “NSO” means an Option that does not qualify as an incentive stock
option as described in Code Section 422(b) or 423(b). 
 (q) “Option” means an ISO or NSO granted under the
Plan and entitling the holder to purchase Shares. 
 (r) “Optionee” means a person who holds an Option. 

(s) “Outside Director” means a member of the Board of Directors who is not an Employee. 

(t) “Parent” means any corporation (other than the Company) in an unbroken chain of corporations ending with the
Company, if each of the corporations other than the Company owns stock possessing 50% or more of the total combined voting power of all classes of stock in one of the other corporations in such chain. A corporation that attains the status of a
Parent on a date after the adoption of the Plan shall be considered a Parent commencing as of such date. 
 (u)
“Participant” means a Grantee, Optionee or Purchaser. 
 (v) “Plan” means this Dimension Therapeutics,
Inc. 2013 Stock Plan. 
 (w) “Purchase Price” means the consideration for which one Share may be acquired under the
Plan (other than upon exercise of an Option), as specified by the Board of Directors. 
 (x) “Purchaser”
means a person to whom the Board of Directors has offered the right to purchase Shares under the Plan (other than upon exercise of an Option). 

(y) “Securities Act” means the Securities Act of 1933, as amended. 

(z) “Service” means service as an Employee, Outside Director or Consultant. 

(aa) “Share” means one share of Stock, as adjusted in accordance with Section 8 (if applicable). 

(bb) “Stock” means the Common Stock of the Company. 

(cc) “Stock Grant Agreement” means the agreement between the Company and a Grantee who is awarded Shares under the
Plan that contains the terms, conditions and restrictions pertaining to the award of such Shares. 
 (dd) “Stock
Option Agreement” means the agreement between the Company and an Optionee that contains the terms, conditions and restrictions pertaining to the Optionee’s Option. 

(ee) “Stock Purchase Agreement” means the agreement between the Company and a Purchaser who purchases Shares under the
Plan that contains the terms, conditions and restrictions pertaining to the purchase of such Shares. 
 (ff)
“Subsidiary” means any corporation (other than the Company) in an unbroken chain of corporations beginning with the Company, if each of the corporations other than the last 

  
 12 








 
corporation in the unbroken chain owns stock possessing 50% or more of the total combined voting power of all classes of stock in one of the other corporations in such chain. A corporation that
attains the status of a Subsidiary on a date after the adoption of the Plan shall be considered a Subsidiary commencing as of such date. 

  
 13 








 EXHIBIT A 

SCHEDULE OF SHARES RESERVED FOR ISSUANCE UNDER THE PLAN 
  

									
	 Date of Board

Approval
	  	Date of Stockholder
Approval	  	Number of
Shares Added	 	  	Cumulative Number
of Shares
	        10/29/2013	  	10/29/2013	  	 	Not Applicable	 	  	10,000,000
	        02/18/2014	  	02/18/2014	  	 	-5,500,000	 	  	4,500,000
	        12/16/2014	  	01/30/2015	  	 	900,000	 	  	5,400,000
	        04/20/2015	  	04/20/2015	  	 	4,390,122	 	  	9,790,122
	        06/03/2015	  	06/03/2015	  	 	500,000	 	  	10,290,122

 SUMMARY OF MODIFICATIONS AND AMENDMENTS TO THE PLAN 

The following is a summary of material modifications made to the Plan (including any material deviations from the Gunderson Dettmer precedent form used to
create the Plan): 
 DIMENSION THERAPEUTICS, INC. 2013 STOCK PLAN 

NOTICE OF STOCK OPTION GRANT (EARLY EXERCISE) 

The Optionee has been granted the following option to purchase shares of the Common Stock of Dimension Therapeutics, Inc.: 

 

			
	Name of Optionee:	  	«Name»
		
	Total Number of Shares:	  	«TotalShares»
		
	Type of Option:	  	«ISO» Incentive Stock Option (ISO)
		
		  	«NSO» Nonstatutory Stock Option (NSO)
		
	Exercise Price per Share:	  	$«PricePerShare»
		
	Date of Grant:	  	«DateGrant»
		
	Date Exercisable:	  	This option may be exercised at any time after the Date of Grant for all or any part of the Shares subject to this option.
		
	Vesting Commencement Date:	  	«VestComDate»
		
	Vesting Schedule:	  	The Right of Repurchase shall lapse with respect to the first «Percent»% of the Shares subject to this option when the Optionee completes «CliffPeriod» months of continuous Service beginning with the
Vesting Commencement Date set forth above. The









			
		  	Right of Repurchase shall lapse with respect to an additional «Fraction»% of the Shares subject to this option when the Optionee completes each month of continuous Service thereafter.
		
	Expiration Date:	  	«ExpDate». This option expires earlier if the Optionee’s Service terminates earlier, as provided in Section 6 of the Stock Option Agreement, or if the Company engages in certain corporate transactions, as
provided in Section 8(b) of the Plan.

 By signing below, the Optionee and the Company agree that this option is granted under, and governed by the terms and
conditions of, the 2013 Stock Plan and the Stock Option Agreement. Both of these documents are attached to, and made a part of, this Notice of Stock Option Grant. Section 14 of the Stock Option Agreement includes important acknowledgements
of the Optionee. 
  

					
	OPTIONEE:	 	           DIMENSION THERAPEUTICS, INC.


							
				
	  
	 		 	By:	 	  

		 		 	Title:  	 	  


 THE OPTION GRANTED PURSUANT TO THIS AGREEMENT AND THE SHARES ISSUABLE UPON THE EXERCISE THEREOF HAVE NOT BEEN REGISTERED
UNDER THE SECURITIES ACT OF 1933, AS AMENDED, AND MAY NOT BE SOLD, PLEDGED, OR OTHERWISE TRANSFERRED WITHOUT AN EFFECTIVE REGISTRATION THEREOF UNDER SUCH ACT OR AN OPINION OF COUNSEL, SATISFACTORY TO THE COMPANY AND ITS COUNSEL, THAT SUCH
REGISTRATION IS NOT REQUIRED. 
 DIMENSION THERAPEUTICS, INC. 2013 STOCK PLAN: 

STOCK OPTION AGREEMENT (EARLY EXERCISE) 
  

	SECTION 1.	  GRANT OF OPTION. 

 (a) Option. On the terms and conditions set forth
in the Notice of Stock Option Grant and this Agreement, the Company grants to the Optionee on the Date of Grant the option to purchase at the Exercise Price the number of Shares set forth in the Notice of Stock Option Grant. The Exercise Price is
agreed to be at least 100% of the Fair Market Value per Share on the Date of Grant (110% of Fair Market Value if this option is designated as an ISO in the Notice of Stock Option Grant and Section 3(b) of the Plan applies). This option is
intended to be an ISO or an NSO, as provided in the Notice of Stock Option Grant. 







 (b) $100,000 Limitation. Even if this option is designated as an ISO in the Notice of
Stock Option Grant, it shall be deemed to be an NSO to the extent (and only to the extent) required by the $100,000 annual limitation under Section 422(d) of the Code. 

(c) Stock Plan and Defined Terms. This option is granted pursuant to the Plan, a copy of which the Optionee acknowledges having
received. The provisions of the Plan are incorporated into this Agreement by this reference. Except as otherwise defined in this Agreement (including without limitation Section 15 hereof), capitalized terms shall have the meaning ascribed to
such terms in the Plan. 
  

	SECTION 2.	  RIGHT TO EXERCISE. 

 (a) Exercisability. Subject to Subsection
(b) below and the other conditions set forth in this Agreement, all or part of this option may be exercised prior to its expiration at the time or times set forth in the Notice of Stock Option Grant. Shares purchased by exercising this option
may be subject to the Right of Repurchase under Section 7. 
 (b) Stockholder Approval. Any other provision of this Agreement
notwithstanding, no portion of this option shall be exercisable at any time prior to the approval of the Plan by the Company’s stockholders. 
  

	SECTION 3.	  NO TRANSFER OR ASSIGNMENT OF OPTION. 

 Except as otherwise provided in this
Agreement, this option and the rights and privileges conferred hereby shall not be sold, pledged or otherwise transferred (whether by operation of law or otherwise) and shall not be subject to sale under execution, attachment, levy or similar
process. 
  

	SECTION 4.	  SECTION 4. EXERCISE PROCEDURES. 

 (a) Notice of Exercise. The Optionee
or the Optionee’s representative may exercise this option by: (i) signing and delivering written notice to the Company pursuant to Section 13(c) specifying the election to exercise this option, the number of Shares for which it is
being exercised and the form of payment and (ii) delivering payment, in a form permissible under Section 5, for the full amount of the Purchase Price (together with any applicable withholding taxes under Subsection (b)). In the event that
this option is being exercised by the representative of the Optionee, the notice shall be accompanied by proof (satisfactory to the Company) of the representative’s right to exercise this option. In the event of a partial exercise of this
option, Shares shall be deemed to have been purchased in the order in which they vest in accordance with the Notice of Stock Option Grant. 

(b) Withholding Taxes. In the event that the Company determines that it is required to withhold any tax (including without limitation
any income tax, social insurance contributions, payroll tax, payment on account or other tax-related items arising in connection with the Optionee’s participation in the Plan and legally applicable to the Optionee (the “Tax-Related
Items”)) as a result of the grant, vesting or exercise of this option, or as a result of the vesting or transfer of shares acquired upon exercise of this option, the Optionee, as a condition of this option, shall make arrangements
satisfactory to the Company to enable it to satisfy all Tax-







 
Related Items. The Optionee acknowledges that the responsibility for all Tax-Related Items is the Optionee’s and may exceed the amount actually withheld by the Company (or its affiliate or
agent). 
 (c) Issuance of Shares. After satisfying all requirements for exercise of this option, the Company shall cause to be
issued one or more certificates evidencing the Shares for which this option has been exercised. Such Shares shall be registered (i) in the name of the person exercising this option, (ii) in the names of such person and his or her spouse as
community property or as joint tenants with the right of survivorship or (iii) with the Company’s consent, in the name of a revocable trust. Until the issuance of the Shares has been entered into the books and records of the Company or a
duly authorized transfer agent of the Company, no right to vote, receive dividends or any other right as a stockholder will exist with respect to such Shares. In the case of Restricted Shares, the Company shall cause such certificates to be
deposited in escrow under Section 7(c). In the case of other Shares, the Company shall cause such certificates to be delivered to or upon the order of the person exercising this option. 

 

	SECTION 5.	  PAYMENT FOR STOCK. 

 (a) Cash. All or part of the Purchase Price may
be paid in cash or cash equivalents. 
 (b) Surrender of Stock. At the discretion of the Board of Directors, all or any part of the
Purchase Price may be paid by surrendering, or attesting to the ownership of, Shares that are already owned by the Optionee. Such Shares shall be surrendered to the Company in good form for transfer and shall be valued at their Fair Market Value as
of the date when this option is exercised. 
 (c) Exercise/Sale. All or part of the Purchase Price and any withholding taxes may be
paid by the delivery (on a form prescribed by the Company) of an irrevocable direction to a securities broker approved by the Company to sell Shares and to deliver all or part of the sales proceeds to the Company. However, payment pursuant to this
Subsection (c) shall be permitted only if (i) Stock then is publicly traded and (ii) such payment does not violate applicable law. 
  

	SECTION 6.	  TERM AND EXPIRATION. 

 (a) Basic Term. This option shall in any event
expire on the expiration date set forth in the Notice of Stock Option Grant, which date is 10 years after the Date of Grant (five years after the Date of Grant if this option is designated as an ISO in the Notice of Stock Option Grant and
Section 3(b) of the Plan applies). 
 (b) Termination of Service (Except by Death). If the Optionee’s Service terminates
for any reason other than death, then this option shall expire on the earliest of the following occasions: 
 (i) The
expiration date determined pursuant to Subsection (a) above; 







 (ii) The date three months after the termination of the Optionee’s Service
for any reason other than Disability; or 
 (iii) The date six months after the termination of the Optionee’s Service by
reason of Disability. 
 The Optionee may exercise all or part of this option at any time before its expiration under the preceding sentence, but only to
the extent that this option is exercisable for vested Shares on or before the date when the Optionee’s Service terminates. When the Optionee’s Service terminates, this option shall expire immediately with respect to the number of Shares
for which this option is not yet exercisable and with respect to any Restricted Shares. In the event that the Optionee dies after termination of Service but before the expiration of this option, all or part of this option may be exercised (prior to
expiration) by the executors or administrators of the Optionee’s estate or by any person who has acquired this option directly from the Optionee by beneficiary designation, bequest or inheritance, but only to the extent that this option was
exercisable for vested Shares on or before the date when the Optionee’s Service terminated. Once this option (or portion thereof) has terminated, the Optionee shall have no further rights with respect to the option (or portion thereof) or to
the underlying Shares. 
 (c) Death of the Optionee. If the Optionee dies while in Service, then this option shall expire on the
earlier of the following dates: 
 (i) The expiration date determined pursuant to Subsection (a) above; or 

(ii) The date 12 months after the Optionee’s death. 

All or part of this option may be exercised at any time before its expiration under the preceding sentence by the executors or administrators of the
Optionee’s estate or by any person who has acquired this option directly from the Optionee by beneficiary designation, bequest or inheritance, but only to the extent that this option is exercisable for vested Shares on or before the date of the
Optionee’s death. When the Optionee dies, this option shall expire immediately with respect to the number of Shares for which this option is not yet exercisable and with respect to any Restricted Shares. Once this option (or portion thereof)
has terminated, the Optionee shall have no further rights with respect to the option (or portion thereof) or to the underlying Shares. 

(d) Extension of Post-Termination Exercise Periods. Following the date on which the Company’s Stock is first listed for trading on
an established securities market, if during any part of the exercise period described in Subsections (b)(ii) or (iii) or Subsection (c)(ii) above the exercise of this option would be prohibited solely because the issuance of Shares upon such
exercise would violate the registration requirements under the Securities Act or a similar provision of other applicable law, then instead of terminating at the end of such prescribed period, the then-vested portion of this option will instead
remain outstanding and not expire until the earlier of (i) the expiration date determined pursuant to Section 6(a) above or (ii) the date on which the then-vested portion of this option has been exercisable without violation of
applicable law for the aggregate period (which need not be consecutive) after termination of the Optionee’s Service specified in the applicable Subsection above. 







 (e) Part-Time Employment and Leaves of Absence. If the Optionee commences working on a
part-time basis, then the Company may adjust the vesting schedule set forth in the Notice of Stock Option Grant. If the Optionee goes on a leave of absence, then the Company may adjust the vesting schedule set forth in the Notice of Stock Option
Grant in accordance with the Company’s leave of absence policy or the terms of such leave. Except as provided in the preceding sentence, Service shall be deemed to continue for any purpose under this Agreement while the Optionee is on a bona
fide leave of absence, if (i) such leave was approved by the Company in writing and (ii) continued crediting of Service for such purpose is expressly required by the terms of such leave or by applicable law (as determined by the
Company). Service shall be deemed to terminate when such leave ends, unless the Optionee immediately returns to active work. 
 (f)
Notice Concerning ISO Treatment. Even if this option is designated as an ISO in the Notice of Stock Option Grant, it ceases to qualify for favorable tax treatment as an ISO to the extent that it is exercised: 

(i) More than three months after the date when the Optionee ceases to be an Employee for any reason other than death or
permanent and total disability (as defined in Section 22(e)(3) of the Code); 
 (ii) More than 12 months after the date
when the Optionee ceases to be an Employee by reason of permanent and total disability (as defined in Section 22(e)(3) of the Code); or 

(iii) More than three months after the date when the Optionee has been on a leave of absence for three months, unless the
Optionee’s reemployment rights following such leave were guaranteed by statute or by contract. 
  

	SECTION 7.	  RIGHT OF REPURCHASE. 

 (a) Scope of Repurchase Right. Until they vest
in accordance with the Notice of Stock Option Grant and Subsection (b) below, the Shares acquired under this Agreement shall be Restricted Shares and shall be subject to the Company’s Right of Repurchase. The Company, however, may decline
to exercise its Right of Repurchase or may exercise its Right of Repurchase only with respect to a portion of the Restricted Shares. The Company may exercise its Right of Repurchase only during the Repurchase Period following the termination of the
Optionee’s Service, but the Right of Repurchase may be exercised automatically under Subsection (d) below. If the Right of Repurchase is exercised, the Company shall pay the Optionee an amount equal to the lower of (i) the Exercise
Price of each Restricted Share being repurchased or (ii) the Fair Market Value of such Restricted Share at the time the Right of Repurchase is exercised. 

(b) Lapse of Repurchase Right. The Right of Repurchase shall lapse with respect to the Restricted Shares in accordance with the vesting
schedule set forth in the Notice of Stock Option Grant. 
 (c) Escrow. Upon issuance, the certificate(s) for Restricted Shares shall
be deposited in escrow with the Company to be held in accordance with the provisions of this 







 
Agreement. Any additional or exchanged securities or other property described in Subsection (f) below shall immediately be delivered to the Company to be held in escrow. All ordinary cash
dividends on Restricted Shares (or on other securities held in escrow) shall be paid directly to the Optionee and shall not be held in escrow. Restricted Shares, together with any other assets held in escrow under this Agreement, shall be
(i) surrendered to the Company for repurchase upon exercise of the Right of Repurchase or the Right of First Refusal or (ii) released to the Optionee upon his or her request to the extent that the Shares have ceased to be Restricted Shares
(but not more frequently than once every six months). In any event, all Shares that have ceased to be Restricted Shares, together with any other vested assets held in escrow under this Agreement, shall be released within 90 days after the earlier of
(i) the termination of the Optionee’s Service or (ii) the lapse of the Right of First Refusal. 
 (d) Exercise of
Repurchase Right. The Company shall be deemed to have exercised its Right of Repurchase automatically for all Restricted Shares as of the commencement of the Repurchase Period, unless the Company during the Repurchase Period notifies the holder
of the Restricted Shares pursuant to Section 13(c) that it will not exercise its Right of Repurchase for some or all of the Restricted Shares. The Company shall pay to the holder of the Restricted Shares the purchase price determined under
Subsection (a) above for the Restricted Shares being repurchased. Payment shall be made in cash or cash equivalents and/or by canceling indebtedness to the Company incurred by the Optionee in the purchase of the Restricted Shares. The
certificate(s) representing the Restricted Shares being repurchased shall be delivered to the Company. 
 (e) Termination of Rights as
Stockholder. If the Right of Repurchase is exercised in accordance with this Section 7 and the Company makes available the consideration for the Restricted Shares being repurchased, then the person from whom the Restricted Shares are
repurchased shall no longer have any rights as a holder of the Restricted Shares (other than the right to receive payment of such consideration). Such Restricted Shares shall be deemed to have been repurchased pursuant to this Section 7,
whether or not the certificate(s) for such Restricted Shares have been delivered to the Company or the consideration for such Restricted Shares has been accepted. 

(f) Additional or Exchanged Securities and Property. In the event of a merger or consolidation of the Company, a sale of all or
substantially all of the Company’s stock or assets, any other corporate reorganization, a stock split, the declaration of a stock dividend, the declaration of an extraordinary dividend payable in a form other than stock, a spin-off, an
adjustment in conversion ratio, a recapitalization or a similar transaction affecting the Company’s outstanding securities, any securities or other property (including cash or cash equivalents) that are by reason of such transaction exchanged
for, or distributed with respect to, any Restricted Shares shall immediately be subject to the Right of Repurchase. Appropriate adjustments to reflect the exchange or distribution of such securities or property shall be made to the number and/or
class of the Restricted Shares. Appropriate adjustments shall also be made to the price per share to be paid upon the exercise of the Right of Repurchase, provided that the aggregate purchase price payable for the Restricted Shares shall remain the
same. In the event of any transaction described in Section 8(b) of the Plan or any other corporate reorganization, the Right of Repurchase may be exercised by the Company’s successor. 







 (g) Transfer of Restricted Shares. The Optionee shall not transfer, assign, encumber or
otherwise dispose of any Restricted Shares without the Company’s written consent, except as provided in the following sentence. The Optionee may transfer Restricted Shares to one or more members of the Optionee’s Immediate Family or to a
trust established by the Optionee for the benefit of the Optionee and/or one or more members of the Optionee’s Immediate Family, provided in either case that the Transferee agrees in writing on a form prescribed by the Company to be bound by
all provisions of this Agreement. If the Optionee transfers any Restricted Shares, then this Agreement shall apply to the Transferee to the same extent as to the Optionee. 

(h) Assignment of Repurchase Right. The Board of Directors may freely assign the Company’s Right of Repurchase, in whole or in
part. Any person who accepts an assignment of the Right of Repurchase from the Company shall assume all of the Company’s rights and obligations under this Section 7. 

 

	SECTION 8.	  RIGHT OF FIRST REFUSAL. 

 (a) Right of First Refusal. In the event
that the Optionee proposes to sell, pledge or otherwise transfer to a third party any Shares acquired under this Agreement, or any interest in such Shares, the Company shall have the Right of First Refusal with respect to all (and not less than all)
of such Shares. If the Optionee desires to transfer Shares acquired under this Agreement, the Optionee shall give a written Transfer Notice to the Company describing fully the proposed transfer, including the number of Shares proposed to be
transferred, the proposed transfer price, the name and address of the proposed Transferee and proof satisfactory to the Company that the proposed sale or transfer will not violate any applicable federal, State or foreign securities laws. The
Transfer Notice shall be signed both by the Optionee and by the proposed Transferee and must constitute a binding commitment of both parties to the transfer of the Shares. The Company shall have the right to purchase all, and not less than all, of
the Shares on the terms of the proposal described in the Transfer Notice (subject, however, to any change in such terms permitted under Subsection (b) below) by delivery of a notice of exercise of the Right of First Refusal within 30 days after
the date when the Transfer Notice was received by the Company. 
 (b) Transfer of Shares. If the Company fails to exercise its Right
of First Refusal within 30 days after the date when it received the Transfer Notice, the Optionee may, not later than 90 days following receipt of the Transfer Notice by the Company, conclude a transfer of the Shares subject to the Transfer Notice
on the terms and conditions described in the Transfer Notice, provided that any such sale is made in compliance with applicable federal, State and foreign securities laws and not in violation of any other contractual restrictions to which the
Optionee is bound. Any proposed transfer on terms and conditions different from those described in the Transfer Notice, as well as any subsequent proposed transfer by the Optionee, shall again be subject to the Right of First Refusal and shall
require compliance with the procedure described in Subsection (a) above. If the Company exercises its Right of First Refusal, the parties shall consummate the sale of the Shares on the terms set forth in the Transfer Notice within 60 days after
the date when the Company received the Transfer Notice (or within such longer period as may have been specified in the Transfer Notice); provided, however, that in the event the Transfer Notice provided that payment for the Shares was to be made in
a form 







 
other than cash or cash equivalents paid at the time of transfer, the Company shall have the option of paying for the Shares with cash or cash equivalents equal to the present value of the
consideration described in the Transfer Notice. 
 (c) Additional or Exchanged Securities and Property. In the event of a merger or
consolidation of the Company, a sale of all or substantially all of the Company’s stock or assets, any other corporate reorganization, a stock split, the declaration of a stock dividend, the declaration of an extraordinary dividend payable in a
form other than stock, a spin-off, an adjustment in conversion ratio, a recapitalization or a similar transaction affecting the Company’s outstanding securities, any securities or other property (including cash or cash equivalents) that are by
reason of such transaction exchanged for, or distributed with respect to, any Shares subject to this Section 8 shall immediately be subject to the Right of First Refusal. Appropriate adjustments to reflect the exchange or distribution of such
securities or property shall be made to the number and/or class of the Shares subject to this Section 8. 
 (d) Termination of Right
of First Refusal. Any other provision of this Section 8 notwithstanding, in the event that the Stock is readily tradable on an established securities market when the Optionee desires to transfer Shares, the Company shall have no Right of
First Refusal, and the Optionee shall have no obligation to comply with the procedures prescribed by Subsections (a) and (b) above. 

(e) Permitted Transfers. This Section 8 shall not apply to (i) a transfer by beneficiary designation, will or intestate
succession or (ii) a transfer to one or more members of the Optionee’s Immediate Family or to a trust established by the Optionee for the benefit of the Optionee and/or one or more members of the Optionee’s Immediate Family, provided
in either case that the Transferee agrees in writing on a form prescribed by the Company to be bound by all provisions of this Agreement. If the Optionee transfers any Shares acquired under this Agreement, either under this Subsection (e) or
after the Company has failed to exercise the Right of First Refusal, then this Agreement shall apply to the Transferee to the same extent as to the Optionee. 

(f) Termination of Rights as Stockholder. If the Company makes available, at the time and place and in the amount and form provided in
this Agreement, the consideration for the Shares to be purchased in accordance with this Section 8, then after such time the person from whom such Shares are to be purchased shall no longer have any rights as a holder of such Shares (other than
the right to receive payment of such consideration in accordance with this Agreement). Such Shares shall be deemed to have been purchased in accordance with the applicable provisions hereof, whether or not the certificate(s) therefor have been
delivered as required by this Agreement. 
 (g) Assignment of Right of First Refusal. The Board of Directors may freely assign the
Company’s Right of First Refusal, in whole or in part. Any person who accepts an assignment of the Right of First Refusal from the Company shall assume all of the Company’s rights and obligations under this Section 8. 








	SECTION 9.	  LEGALITY OF INITIAL ISSUANCE. 

 No Shares shall be issued upon the exercise
of this option unless and until the Company has determined that: 
 (a) It and the Optionee have taken any actions required
to register the Shares under the Securities Act or to perfect an exemption from the registration requirements thereof; 
 (b)
Any applicable listing requirement of any stock exchange or other securities market on which Stock is listed has been satisfied; and 

(c) Any other applicable provision of federal, State or foreign law has been satisfied. 

 

	SECTION 10.	  NO REGISTRATION RIGHTS. 

 The Company may, but shall not be obligated to,
register or qualify the sale of Shares under the Securities Act or any other applicable law. The Company shall not be obligated to take any affirmative action in order to cause the sale of Shares under this Agreement to comply with any law. 

 

	SECTION 11.	  RESTRICTIONS ON TRANSFER OF SHARES. 

 (a) Securities Law Restrictions.
Regardless of whether the offer and sale of Shares under the Plan have been registered under the Securities Act or have been registered or qualified under the securities laws of any State or other relevant jurisdiction, the Company at its discretion
may impose restrictions upon the sale, pledge or other transfer of such Shares (including the placement of appropriate legends on the stock certificates (or electronic equivalent) or the imposition of stop-transfer instructions) and may refuse to
transfer Shares acquired hereunder (or Shares proposed to be transferred in a subsequent transfer) if, in the judgment of the Company, such restrictions, legends or refusal are necessary or appropriate to achieve compliance with the Securities Act
or other relevant securities or other laws, including without limitation under Regulation S of the Securities Act or pursuant to another available exemption from registration. 

(b) Market Stand-Off. In connection with any underwritten public offering by the Company of its equity securities pursuant to an
effective registration statement filed under the Securities Act, including the Company’s initial public offering, the Optionee or a Transferee shall not directly or indirectly sell, make any short sale of, loan, hypothecate, pledge, offer,
grant or sell any option or other contract for the purchase of, purchase any option or other contract for the sale of, or otherwise dispose of or transfer, or agree to engage in any of the foregoing transactions with respect to, any Shares acquired
under this Agreement without the prior written consent of the Company or its managing underwriter. Such restriction (the “Market Stand-Off”) shall be in effect for such period of time following the date of the final prospectus for the
offering as may be requested by the Company or such underwriter. In no event, however, shall such period exceed 180 days plus such additional period as may reasonably be requested by the Company or such underwriter to accommodate regulatory
restrictions on (i) the publication 







 
or other distribution of research reports or (ii) analyst recommendations and opinions, including (without limitation) the restrictions set forth in Rule 2711(f)(4) of the National
Association of Securities Dealers and Rule 472(f)(4) of the New York Stock Exchange, as amended, or any similar successor rules. The Market Stand-Off shall in any event terminate two years after the date of the Company’s initial public
offering. In the event of the declaration of a stock dividend, a spin-off, a stock split, an adjustment in conversion ratio, a recapitalization or a similar transaction affecting the Company’s outstanding securities without receipt of
consideration, any new, substituted or additional securities which are by reason of such transaction distributed with respect to any Shares subject to the Market Stand-Off, or into which such Shares thereby become convertible, shall immediately be
subject to the Market Stand-Off. In order to enforce the Market Stand-Off, the Company may impose stop-transfer instructions with respect to the Shares acquired under this Agreement until the end of the applicable stand-off period. The
Company’s underwriters shall be beneficiaries of the agreement set forth in this Subsection (b). This Subsection (b) shall not apply to Shares registered in the public offering under the Securities Act. 

(c) Investment Intent at Grant. The Optionee represents and agrees that the Shares to be acquired upon exercising this option will be
acquired for investment, and not with a view to the sale or distribution thereof. 
 (d) Investment Intent at Exercise. In the event
that the sale of Shares under the Plan is not registered under the Securities Act but an exemption is available that requires an investment representation or other representation, the Optionee shall represent and agree at the time of exercise that
the Shares being acquired upon exercising this option are being acquired for investment, and not with a view to the sale or distribution thereof, and shall make such other representations as are deemed necessary or appropriate by the Company and its
counsel, including (if applicable) that as of the date of exercise the Optionee is (i) not a U.S. Person; (ii) not acquiring the Shares on behalf, or for the account or benefit, of a U.S. Person; and (iii) is not exercising the option
in the United States. 
 (e) Legends. All certificates evidencing Shares purchased under this Agreement shall bear the following
legend: 
 “THE SHARES REPRESENTED HEREBY MAY NOT BE SOLD, ASSIGNED, TRANSFERRED, ENCUMBERED OR IN ANY MANNER DISPOSED OF, EXCEPT IN
COMPLIANCE WITH THE TERMS OF A WRITTEN AGREEMENT BETWEEN THE COMPANY AND THE REGISTERED HOLDER OF THE SHARES (OR THE PREDECESSOR IN INTEREST TO THE SHARES). SUCH AGREEMENT GRANTS TO THE COMPANY CERTAIN RIGHTS OF FIRST REFUSAL UPON AN ATTEMPTED
TRANSFER OF THE SHARES AND CERTAIN REPURCHASE RIGHTS UPON TERMINATION OF SERVICE WITH THE COMPANY. IN ADDITION, THE SHARES ARE SUBJECT TO RESTRICTIONS ON TRANSFER FOR A LIMITED PERIOD FOLLOWING THE EFFECTIVE DATE OF THE UNDERWRITTEN PUBLIC OFFERING
OF THE COMPANY’S SECURITIES AND MAY NOT BE SOLD OR OTHERWISE DISPOSED OF BY THE HOLDER WITHOUT THE CONSENT OF THE COMPANY OR THE MANAGING UNDERWRITER. THE SECRETARY OF THE COMPANY WILL UPON WRITTEN REQUEST FURNISH A COPY OF SUCH AGREEMENT TO
THE HOLDER HEREOF WITHOUT CHARGE.” 







 All certificates evidencing Shares purchased under this Agreement in an unregistered transaction shall bear the
following legend (and such other restrictive legends as are required or deemed advisable under the provisions of any applicable law): 

“THE SHARES REPRESENTED HEREBY HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED (THE “ACT”) OR ANY
SECURITIES LAWS OF ANY U.S. STATE, AND MAY NOT BE SOLD, REOFFERED, PLEDGED, ASSIGNED, ENCUMBERED OR OTHERWISE TRANSFERRED OR DISPOSED WITHOUT AN EFFECTIVE REGISTRATION THEREOF UNDER SUCH ACT OR AN OPINION OF COUNSEL, SATISFACTORY TO THE COMPANY AND
ITS COUNSEL, THAT SUCH REGISTRATION IS NOT REQUIRED. IN THE ABSENCE OF REGISTRATION OR THE AVAILABILITY (CONFIRMED BY OPINION OF COUNSEL) OF AN ALTERNATIVE EXEMPTION FROM REGISTRATION UNDER THE ACT, THESE SHARES MAY NOT BE SOLD, REOFFERED, PLEDGED,
ASSIGNED, ENCUMBERED OR OTHERWISE TRANSFERRED OR DISPOSED OF EXCEPT IN ACCORDANCE WITH REGULATION S (RULES 901 THROUGH 905 AND PRELIMINARY NOTES) OF THE ACT. HEDGING TRANSACTIONS INVOLVING THESE SHARES MAY NOT BE CONDUCTED UNLESS IN COMPLIANCE WITH
THE ACT.” 
 (f) Removal of Legends. If, in the opinion of the Company and its counsel, any legend placed on a stock certificate
representing Shares sold under this Agreement is no longer required, the holder of such certificate shall be entitled to exchange such certificate for a certificate representing the same number of Shares but without such legend. 

(g) Administration. Any determination by the Company and its counsel in connection with any of the matters set forth in this
Section 11 shall be conclusive and binding on the Optionee and all other persons. 
  

	SECTION 12.	  ADJUSTMENT OF SHARES. 

 In the event of any transaction described in
Section 8(a) of the Plan, the terms of this option (including, without limitation, the number and kind of Shares subject to this option and the Exercise Price) shall be adjusted as set forth in Section 8(a) of the Plan. In the event that
the Company is a party to a merger or consolidation or in the event of a sale of all or substantially all of the Company’s stock or assets, this option shall be subject to the treatment provided by the Board of Directors in its sole discretion,
as provided in Section 8(b) of the Plan. 
  

	SECTION 13.	  MISCELLANEOUS PROVISIONS. 

 (a) Rights as a Stockholder. Neither the
Optionee nor the Optionee’s representative shall have any rights as a stockholder with respect to any Shares subject to this option until the Optionee or the Optionee’s representative becomes entitled to receive such Shares by filing a
notice of exercise and paying the Purchase Price pursuant to Sections 4 and 5. 







 (b) No Retention Rights. Nothing in this option or in the Plan shall confer upon the
Optionee any right to continue in Service for any period of specific duration or interfere with or otherwise restrict in any way the rights of the Company (or any Parent or Subsidiary employing or retaining the Optionee) or of the Optionee, which
rights are hereby expressly reserved by each, to terminate his or her Service at any time and for any reason, with or without cause. 
 (c)
Notice. Any notice required by the terms of this Agreement shall be given in writing. It shall be deemed effective upon (i) personal delivery, (ii) deposit with the United States Postal Service, by registered or certified mail, with
postage and fees prepaid, (iii) deposit with Federal Express Corporation, with shipping charges prepaid or (iv) deposit with any internationally recognized express mail courier service. Notice shall be addressed to the Company at its
principal executive office and to the Optionee at the address that he or she most recently provided to the Company in accordance with this Subsection (c). 

(d) Modifications and Waivers. No provision of this Agreement shall be modified, waived or discharged unless the modification, waiver
or discharge is agreed to in writing and signed by the Optionee and by an authorized officer of the Company (other than the Optionee). No waiver by either party of any breach of, or of compliance with, any condition or provision of this Agreement by
the other party shall be considered a waiver of any other condition or provision or of the same condition or provision at another time. 

(e) Entire Agreement. The Notice of Stock Option Grant, this Agreement and the Plan constitute the entire contract between the parties
hereto with regard to the subject matter hereof. They supersede any other agreements, representations or understandings (whether oral or written and whether express or implied) that relate to the subject matter hereof. 

(f) Choice of Law. This Agreement shall be governed by, and construed in accordance with, the laws of the State of Delaware, as such
laws are applied to contracts entered into and performed in such State. 
  

	SECTION 14.  	ACKNOWLEDGEMENTS OF THE OPTIONEE. 

 In addition to the other terms, conditions and
restrictions imposed on this option and the Shares issuable under this option pursuant to this Agreement and the Plan, the Optionee expressly acknowledges being subject to Sections 7 (Right of Repurchase), 8 (Right of First Refusal), 9 (Legality of
Initial Issuance) and 11 (Restrictions on Transfer of Shares, including without limitation the Market Stand-Off), as well as the following provisions: 

(a) Tax Consequences. The Optionee agrees that the Company does not have a duty to design or administer the Plan or its other
compensation programs in a manner that minimizes the Optionee’s tax liabilities. The Optionee shall not make any claim against the Company or its Board of Directors, officers or employees related to tax liabilities arising from this option or
the Optionee’s other compensation. In particular, any Optionee subject to U.S. taxation acknowledges that this option is exempt from Section 409A of the Code only if the Exercise Price is at least equal to the Fair Market Value per Share
on the Date of Grant. Since 







 
Shares are not traded on an established securities market, the determination of their Fair Market Value is made by the Board of Directors or by an independent valuation firm retained by the
Company. The Optionee acknowledges that there is no guarantee in either case that the Internal Revenue Service will agree with the valuation, and the Optionee shall not make any claim against the Company or its Board of Directors, officers or
employees in the event that the Internal Revenue Service asserts that the valuation was too low. 
 (b) Electronic Delivery of
Documents. The Optionee agrees to accept by email all documents relating to the Company, the Plan or this option and all other documents that the Company is required to deliver to its security holders (including, without limitation, disclosures
that may be required by the Securities and Exchange Commission). The Optionee also agrees that the Company may deliver these documents by posting them on a website maintained by the Company or by a third party under contract with the Company. If the
Company posts these documents on a website, it shall notify the Optionee by email of their availability. The Optionee acknowledges that he or she may incur costs in connection with electronic delivery, including the cost of accessing the internet
and printing fees, and that an interruption of internet access may interfere with his or her ability to access the documents. This consent shall remain in effect until this option expires or until the Optionee gives the Company written notice that
it should deliver paper documents. 
 (c) No Notice of Expiration Date. The Optionee agrees that the Company and its officers,
employees, attorneys and agents do not have any obligation to notify him or her prior to the expiration of this option pursuant to Section 6, regardless of whether this option will expire at the end of its full term or on an earlier date
related to the termination of the Optionee’s Service. The Optionee further agrees that he or she has the sole responsibility for monitoring the expiration of this option and for exercising this option, if at all, before it expires. This
Subsection (c) shall supersede any contrary representation that may have been made, orally or in writing, by the Company or by an officer, employee, attorney or agent of the Company. 

(d) Waiver of Statutory Information Rights. The Optionee acknowledges and agrees that, upon exercise of this option and until the first
sale of the Company’s Stock to the general public pursuant to a registration statement filed under the Securities Act, he or she will be deemed to have waived any rights the Optionee might otherwise have had under Section 220 of the
Delaware General Corporation Law to inspect for any proper purpose and to make copies and extracts from the Company’s stock ledger, a list of its stockholders and its other books and records or the books and records of any subsidiary. This
waiver applies only in the Optionee’s capacity as a stockholder and does not affect any other inspection rights the Optionee may have under other law or pursuant to a written agreement with the Company. 

(e) Plan Discretionary. The Optionee understands and acknowledges that (i) the Plan is entirely discretionary, (ii) the
Company and the Optionee’s employer have reserved the right to amend, suspend or terminate the Plan at any time, (iii) the grant of an option does not in any way create any contractual or other right to receive additional grants of options
(or benefits in lieu of options) at any time or in any amount and (iv) all determinations with respect to any additional grants, including (without limitation) the times when options will be granted, the number of Shares offered, the Exercise
Price and the vesting schedule, will be at the sole discretion of the Company. 







 (f) Termination of Service. The Optionee understands and acknowledges that participation
in the Plan ceases upon termination of his or her Service for any reason, except as may explicitly be provided otherwise in the Plan or this Agreement. 

(g) Extraordinary Compensation. The value of this option shall be an extraordinary item of compensation outside the scope of the
Optionee’s employment contract, if any, and shall not be considered a part of his or her normal or expected compensation for purposes of calculating severance, resignation, redundancy or end-of-service payments, bonuses, long-service awards,
pension or retirement benefits or similar payments. 
 (h) Authorization to Disclose. The Optionee hereby authorizes and directs the
Optionee’s employer to disclose to the Company or any Subsidiary any information regarding the Optionee’s employment, the nature and amount of the Optionee’s compensation and the fact and conditions of the Optionee’s
participation in the Plan, as the Optionee’s employer deems necessary or appropriate to facilitate the administration of the Plan. 

(i) Personal Data Authorization. The Optionee consents to the collection, use and transfer of personal data as described in this
Subsection (i). The Optionee understands and acknowledges that the Company, the Optionee’s employer and the Company’s other Subsidiaries hold certain personal information regarding the Optionee for the purpose of managing and administering
the Plan, including (without limitation) the Optionee’s name, home address, telephone number, date of birth, social insurance number, salary, nationality, job title, any Shares or directorships held in the Company and details of all options or
any other entitlements to Shares awarded, canceled, exercised, vested, unvested or outstanding in the Optionee’s favor (the “Data”). The Optionee further understands and acknowledges that the Company and/or its Subsidiaries will
transfer Data among themselves as necessary for the purpose of implementation, administration and management of the Optionee’s participation in the Plan and that the Company and/or any Subsidiary may each further transfer Data to any third
party assisting the Company in the implementation, administration and management of the Plan. The Optionee understands and acknowledges that the recipients of Data may be located in the United States or elsewhere. The Optionee authorizes such
recipients to receive, possess, use, retain and transfer Data, in electronic or other form, for the purpose of administering the Optionee’s participation in the Plan, including a transfer to any broker or other third party with whom the
Optionee elects to deposit Shares acquired under the Plan of such Data as may be required for the administration of the Plan and/or the subsequent holding of Shares on the Optionee’s behalf. The Optionee may, at any time, view the Data, require
any necessary modifications of Data or withdraw the consents set forth in this Subsection (i) by contacting the Company in writing. 
  

	SECTION 15.	  DEFINITIONS. 

 (a) “Agreement” shall mean this Stock Option
Agreement. 







 (b) “Board of Directors” shall mean the Board of Directors of the
Company, as constituted from time to time or, if a Committee has been appointed, such Committee. 
 (c) “Company”
shall mean Dimension Therapeutics, Inc., a Delaware corporation. 
 (d) “Immediate Family” shall mean any child,
stepchild, grandchild, parent, stepparent, grandparent, spouse, sibling, mother-in-law, father-in-law, son-in-law, daughter-in-law, brother-in-law or sister-in-law and shall include adoptive relationships. 

(e) “Optionee” shall mean the person named in the Notice of Stock Option Grant. 

(f) “Plan” shall mean the Dimension Therapeutics, Inc. 2013 Stock Plan, as in effect on the Date of Grant. 

(g) “Purchase Price” shall mean the Exercise Price multiplied by the number of Shares with respect to which this
option is being exercised. 
 (h) “Repurchase Period” shall mean a period of 90 consecutive days commencing
on the date when the Optionee’s Service terminates for any reason, including (without limitation) death or disability. 
 (i)
“Restricted Share” shall mean a Share that is subject to the Right of Repurchase. 
 (j) “Right of First
Refusal” shall mean the Company’s right of first refusal described in Section 8. 
 (k) “Right of Repurchase”
shall mean the Company’s right of repurchase described in Section 7. 
 (l) “Service” means service as an Employee,
Outside Director or Consultant. 
 (m) “Transferee” shall mean any person to whom the Optionee has directly or
indirectly transferred any Share acquired under this Agreement. 
 (n) “Transfer Notice” shall mean the notice of a
proposed transfer of Shares described in Section 8. 
 (o) “U.S. Person” shall mean a person described in Rule
902(k) of Regulation S of the Securities Act (or any successor rule or provision), which generally defines a U.S. person as any natural person resident in the United States, any estate of which any executor or administrator is a U.S. Person, or any
trust of which of any trustee is a U.S. Person. 







 DIMENSION THERAPEUTICS, INC. 2013 STOCK PLAN 

NOTICE OF STOCK OPTION GRANT (INSTALLMENT EXERCISE) 

The Optionee has been granted the following option to purchase shares of the Common Stock of Dimension Therapeutics, Inc.: 

 

			
		
	 Name of Optionee:
	  	«Name»
		
	 Total Number of Shares:
	  	«TotalShares»
		
	 Type of Option:
	  	«ISO» Incentive Stock Option (ISO)
		
		  	«NSO» Nonstatutory Stock Option (NSO)
		
	 Exercise Price per Share:
	  	$«PricePerShare»
		
	 Date of Grant:
	  	«DateGrant»
		
	 Date Exercisable:
	  	This option may be exercised with respect to the first «Percent»% of the Shares subject to this option when the Optionee completes «CliffPeriod» months of continuous Service beginning with the Vesting
Commencement Date set forth below. This option may be exercised with respect to an additional «Fraction»% of the Shares subject to this option when the Optionee completes each month of continuous Service thereafter. This option may
become exercisable on an accelerated basis under Section 2(a) of the Stock Option Agreement.
		
	 Vesting Commencement Date:
	  	«VestComDate»
		
	 Expiration Date:
	  	«ExpDate». This option expires earlier if the Optionee’s Service terminates earlier, as provided in Section 6 of the Stock Option Agreement, or if the Company engages in certain corporate transactions, as
provided in Section 8(b) of the Plan.

 By signing below, the Optionee and the Company agree that this option is granted under, and governed by the terms and
conditions of, the 2013 Stock Plan and the Stock Option Agreement. Both of these documents are attached to, and made a part of, this Notice of Stock Option Grant. 







 Section 13 of the Stock Option Agreement includes important acknowledgements of the Optionee. 

 

					
	OPTIONEE:	 	           DIMENSION THERAPEUTICS, INC.


							
				
	  
	 		 	By:	 	  

				
		 		 	Title:  	 	  


 THE OPTION GRANTED PURSUANT TO THIS AGREEMENT AND THE SHARES ISSUABLE UPON THE EXERCISE THEREOF HAVE NOT BEEN REGISTERED
UNDER THE SECURITIES ACT OF 1933, AS AMENDED, AND MAY NOT BE SOLD, PLEDGED, OR OTHERWISE TRANSFERRED WITHOUT AN EFFECTIVE REGISTRATION THEREOF UNDER SUCH ACT OR AN OPINION OF COUNSEL, SATISFACTORY TO THE COMPANY AND ITS COUNSEL, THAT SUCH
REGISTRATION IS NOT REQUIRED. 
 DIMENSION THERAPEUTICS, INC. 2013 STOCK PLAN: 

STOCK OPTION AGREEMENT (INSTALLMENT EXERCISE) 
  

	SECTION 1.	  GRANT OF OPTION. 

 (a) Option. On the terms and conditions set forth
in the Notice of Stock Option Grant and this Agreement, the Company grants to the Optionee on the Date of Grant the option to purchase at the Exercise Price the number of Shares set forth in the Notice of Stock Option Grant. The Exercise Price is
agreed to be at least 100% of the Fair Market Value per Share on the Date of Grant (110% of Fair Market Value if this option is designated as an ISO in the Notice of Stock Option Grant and Section 3(b) of the Plan applies). This option is
intended to be an ISO or an NSO, as provided in the Notice of Stock Option Grant. 
 (b) $100,000 Limitation. Even if this option is
designated as an ISO in the Notice of Stock Option Grant, it shall be deemed to be an NSO to the extent (and only to the extent) required by the $100,000 annual limitation under Section 422(d) of the Code. 

(c) Stock Plan and Defined Terms. This option is granted pursuant to the Plan, a copy of which the Optionee acknowledges having
received. The provisions of the Plan are incorporated into this Agreement by this reference. Except as otherwise defined in this Agreement (including without limitation Section 14 hereof), capitalized terms shall have the meaning ascribed to
such terms in the Plan. 
  

	SECTION 2.	  RIGHT TO EXERCISE. 

 (a) Exercisability. Subject to Subsection
(b) below and the other conditions set forth in this Agreement, all or part of this option may be exercised prior to its expiration at the time or times set forth in the Notice of Stock Option Grant. In addition, if the Optionee is subject to
an Involuntary Termination within 12 months following a Change in Control, [100]%of the shares subject to this option shall immediately vest and become exercisable. 







 (b) Stockholder Approval. Any other provision of this Agreement notwithstanding, no
portion of this option shall be exercisable at any time prior to the approval of the Plan by the Company’s stockholders. 
  

	SECTION 3.	  NO TRANSFER OR ASSIGNMENT OF OPTION. 

 Except as otherwise provided in this
Agreement, this option and the rights and privileges conferred hereby shall not be sold, pledged or otherwise transferred (whether by operation of law or otherwise) and shall not be subject to sale under execution, attachment, levy or similar
process. 
  

	SECTION 4.	  EXERCISE PROCEDURES. 

 (a) Notice of Exercise. The Optionee or the
Optionee’s representative may exercise this option by: (i) signing and delivering written notice to the Company pursuant to Section 12(c) specifying the election to exercise this option, the number of Shares for which it is being
exercised and the form of payment and (ii) delivering payment, in a form permissible under Section 5, for the full amount of the Purchase Price (together with any applicable withholding taxes under Subsection (b)). In the event that this
option is being exercised by the representative of the Optionee, the notice shall be accompanied by proof (satisfactory to the Company) of the representative’s right to exercise this option. 

(b) Withholding Taxes. In the event that the Company determines that it is required to withhold any tax (including without limitation
any income tax, social insurance contributions, payroll tax, payment on account or other tax-related items arising in connection with the Optionee’s participation in the Plan and legally applicable to the Optionee (the “Tax-Related
Items”)) as a result of the grant, vesting or exercise of this option, or as a result of the transfer of shares acquired upon exercise of this option, the Optionee, as a condition of this option, shall make arrangements satisfactory to the
Company to enable it to satisfy all Tax-Related Items. The Optionee acknowledges that the responsibility for all Tax-Related Items is the Optionee’s and may exceed the amount actually withheld by the Company (or its affiliate or agent). 

(c) Issuance of Shares. After satisfying all requirements for exercise of this option, the Company shall cause to be issued one or more
certificates evidencing the Shares for which this option has been exercised. Such Shares shall be registered (i) in the name of the person exercising this option, (ii) in the names of such person and his or her spouse as community property
or as joint tenants with the right of survivorship or (iii) with the Company’s consent, in the name of a revocable trust. Until the issuance of the Shares has been entered into the books and records of the Company or a duly authorized
transfer agent of the Company, no right to vote, receive dividends or any other right as a stockholder will exist with respect to such Shares. The Company shall cause such certificates to be delivered to or upon the order of the person exercising
this option. 








	SECTION 5.	  PAYMENT FOR STOCK. 

 (a) Cash. All or part of the Purchase Price may
be paid in cash or cash equivalents. 
 (b) Surrender of Stock. At the discretion of the Board of Directors, all or any part of the
Purchase Price may be paid by surrendering, or attesting to the ownership of, Shares that are already owned by the Optionee. Such Shares shall be surrendered to the Company in good form for transfer and shall be valued at their Fair Market Value as
of the date when this option is exercised. 
 (c) Exercise/Sale. All or part of the Purchase Price and any withholding taxes may be
paid by the delivery (on a form prescribed by the Company) of an irrevocable direction to a securities broker approved by the Company to sell Shares and to deliver all or part of the sales proceeds to the Company. However, payment pursuant to this
Subsection (c) shall be permitted only if (i) Stock then is publicly traded and (ii) such payment does not violate applicable law. 
  

	SECTION 6.	  TERM AND EXPIRATION. 

 (a) Basic Term. This option shall in any event
expire on the expiration date set forth in the Notice of Stock Option Grant, which date is 10 years after the Date of Grant (five years after the Date of Grant if this option is designated as an ISO in the Notice of Stock Option Grant and
Section 3(b) of the Plan applies). 
 (b) Termination of Service (Except by Death). If the Optionee’s Service terminates
for any reason other than death, then this option shall expire on the earliest of the following occasions: 
 (i) The
expiration date determined pursuant to Subsection (a) above; 
 (ii) The date three months after the termination of the
Optionee’s Service for any reason other than Disability; or 
 (iii) The date six months after the termination of the
Optionee’s Service by reason of Disability. 
 The Optionee may exercise all or part of this option at any time before its expiration under the
preceding sentence, but only to the extent that this option had become exercisable before the Optionee’s Service terminated. When the Optionee’s Service terminates, this option shall expire immediately with respect to the number of Shares
for which this option is not yet exercisable. In the event that the Optionee dies after termination of Service but before the expiration of this option, all or part of this option may be exercised (prior to expiration) by the executors or
administrators of the Optionee’s estate or by any person who has acquired this option directly from the Optionee by beneficiary designation, bequest or inheritance, but only to the extent that this option had become exercisable before the
Optionee’s Service terminated. Once this option (or portion thereof) has terminated, the Optionee shall have no further rights with respect to the option (or portion thereof) or to the underlying Shares. 







 (c) Death of the Optionee. If the Optionee dies while in Service, then this option shall
expire on the earlier of the following dates: 
 (i) The expiration date determined pursuant to Subsection (a) above; or

 (ii) The date 12 months after the Optionee’s death. 

All or part of this option may be exercised at any time before its expiration under the preceding sentence by the executors or administrators of the
Optionee’s estate or by any person who has acquired this option directly from the Optionee by beneficiary designation, bequest or inheritance, but only to the extent that this option had become exercisable before the Optionee’s death. When
the Optionee dies, this option shall expire immediately with respect to the number of Shares for which this option is not yet exercisable. Once this option (or portion thereof) has terminated, the Optionee shall have no further rights with respect
to the option (or portion thereof) or to the underlying Shares. 
 (d) Extension of Post-Termination Exercise Periods. Following the
date on which the Company’s Stock is first listed for trading on an established securities market, if during any part of the exercise period described in Subsections (b)(ii) or (iii) or Subsection (c)(ii) above the exercise of this option
would be prohibited solely because the issuance of Shares upon such exercise would violate the registration requirements under the Securities Act or a similar provision of other applicable law, then instead of terminating at the end of such
prescribed period, the then-vested portion of this option will instead remain outstanding and not expire until the earlier of (i) the expiration date determined pursuant to Section 6(a) above or (ii) the date on which the then-vested
portion of this option has been exercisable without violation of applicable law for the aggregate period (which need not be consecutive) after termination of the Optionee’s Service specified in the applicable Subsection above. 

(e) Part-Time Employment and Leaves of Absence. If the Optionee commences working on a part-time basis, then the Company may adjust the
vesting schedule set forth in the Notice of Stock Option Grant. If the Optionee goes on a leave of absence, then the Company may adjust the vesting schedule set forth in the Notice of Stock Option Grant in accordance with the Company’s leave of
absence policy or the terms of such leave. Except as provided in the preceding sentence, Service shall be deemed to continue for any purpose under this Agreement while the Optionee is on a bona fide leave of absence, if (i) such leave was
approved by the Company in writing and (ii) continued crediting of Service for such purpose is expressly required by the terms of such leave or by applicable law (as determined by the Company). Service shall be deemed to terminate when such
leave ends, unless the Optionee immediately returns to active work. 
 (f) Notice Concerning ISO Treatment. Even if this option is
designated as an ISO in the Notice of Stock Option Grant, it ceases to qualify for favorable tax treatment as an ISO to the extent that it is exercised: 







 (i) More than three months after the date when the Optionee ceases to be an
Employee for any reason other than death or permanent and total disability (as defined in Section 22(e)(3) of the Code); 

(ii) More than 12 months after the date when the Optionee ceases to be an Employee by reason of permanent and total disability
(as defined in Section 22(e)(3) of the Code); or 
 (iii) More than three months after the date when the Optionee has
been on a leave of absence for three months, unless the Optionee’s reemployment rights following such leave were guaranteed by statute or by contract. 
  

	SECTION 7.	  RIGHT OF FIRST REFUSAL. 

 (a) Right of First Refusal. In the event
that the Optionee proposes to sell, pledge or otherwise transfer to a third party any Shares acquired under this Agreement, or any interest in such Shares, the Company shall have the Right of First Refusal with respect to all (and not less than all)
of such Shares. If the Optionee desires to transfer Shares acquired under this Agreement, the Optionee shall give a written Transfer Notice to the Company describing fully the proposed transfer, including the number of Shares proposed to be
transferred, the proposed transfer price, the name and address of the proposed Transferee and proof satisfactory to the Company that the proposed sale or transfer will not violate any applicable federal, State or foreign securities laws. The
Transfer Notice shall be signed both by the Optionee and by the proposed Transferee and must constitute a binding commitment of both parties to the transfer of the Shares. The Company shall have the right to purchase all, and not less than all, of
the Shares on the terms of the proposal described in the Transfer Notice (subject, however, to any change in such terms permitted under Subsection (b) below) by delivery of a notice of exercise of the Right of First Refusal within 30 days after
the date when the Transfer Notice was received by the Company. 
 (b) Transfer of Shares. If the Company fails to exercise its Right
of First Refusal within 30 days after the date when it received the Transfer Notice, the Optionee may, not later than 90 days following receipt of the Transfer Notice by the Company, conclude a transfer of the Shares subject to the Transfer Notice
on the terms and conditions described in the Transfer Notice, provided that any such sale is made in compliance with applicable federal, State and foreign securities laws and not in violation of any other contractual restrictions to which the
Optionee is bound. Any proposed transfer on terms and conditions different from those described in the Transfer Notice, as well as any subsequent proposed transfer by the Optionee, shall again be subject to the Right of First Refusal and shall
require compliance with the procedure described in Subsection (a) above. If the Company exercises its Right of First Refusal, the parties shall consummate the sale of the Shares on the terms set forth in the Transfer Notice within 60 days after
the date when the Company received the Transfer Notice (or within such longer period as may have been specified in the Transfer Notice); provided, however, that in the event the Transfer Notice provided that payment for the Shares was to be made in
a form other than cash or cash equivalents paid at the time of transfer, the Company shall have the option of paying for the Shares with cash or cash equivalents equal to the present value of the consideration described in the Transfer Notice. 







 (c) Additional or Exchanged Securities and Property. In the event of a merger or
consolidation of the Company, a sale of all or substantially all of the Company’s stock or assets, any other corporate reorganization, a stock split, the declaration of a stock dividend, the declaration of an extraordinary dividend payable in a
form other than stock, a spin-off, an adjustment in conversion ratio, a recapitalization or a similar transaction affecting the Company’s outstanding securities, any securities or other property (including cash or cash equivalents) that are by
reason of such transaction exchanged for, or distributed with respect to, any Shares subject to this Section 7 shall immediately be subject to the Right of First Refusal. Appropriate adjustments to reflect the exchange or distribution of such
securities or property shall be made to the number and/or class of the Shares subject to this Section 7. 
 (d) Termination of Right
of First Refusal. Any other provision of this Section 7 notwithstanding, in the event that the Stock is readily tradable on an established securities market when the Optionee desires to transfer Shares, the Company shall have no Right of
First Refusal, and the Optionee shall have no obligation to comply with the procedures prescribed by Subsections (a) and (b) above. 

(e) Permitted Transfers. This Section 7 shall not apply to (i) a transfer by beneficiary designation, will or intestate
succession or (ii) a transfer to one or more members of the Optionee’s Immediate Family or to a trust established by the Optionee for the benefit of the Optionee and/or one or more members of the Optionee’s Immediate Family, provided
in either case that the Transferee agrees in writing on a form prescribed by the Company to be bound by all provisions of this Agreement. If the Optionee transfers any Shares acquired under this Agreement, either under this Subsection (e) or
after the Company has failed to exercise the Right of First Refusal, then this Agreement shall apply to the Transferee to the same extent as to the Optionee. 

(f) Termination of Rights as Stockholder. If the Company makes available, at the time and place and in the amount and form provided in
this Agreement, the consideration for the Shares to be purchased in accordance with this Section 7, then after such time the person from whom such Shares are to be purchased shall no longer have any rights as a holder of such Shares (other than
the right to receive payment of such consideration in accordance with this Agreement). Such Shares shall be deemed to have been purchased in accordance with the applicable provisions hereof, whether or not the certificate(s) therefor have been
delivered as required by this Agreement. 
 (g) Assignment of Right of First Refusal. The Board of Directors may freely assign the
Company’s Right of First Refusal, in whole or in part. Any person who accepts an assignment of the Right of First Refusal from the Company shall assume all of the Company’s rights and obligations under this Section 7. 

 

	SECTION 8.	  LEGALITY OF INITIAL ISSUANCE. 

 No Shares shall be issued upon the exercise
of this option unless and until the Company has determined that: 







 (a) It and the Optionee have taken any actions required to register the Shares
under the Securities Act or to perfect an exemption from the registration requirements thereof; 
 (b) Any applicable listing
requirement of any stock exchange or other securities market on which Stock is listed has been satisfied; and 
 (c) Any
other applicable provision of federal, State or foreign law has been satisfied. 
  

	SECTION 9.	  NO REGISTRATION RIGHTS. 

 The Company may, but shall not be obligated to,
register or qualify the sale of Shares under the Securities Act or any other applicable law. The Company shall not be obligated to take any affirmative action in order to cause the sale of Shares under this Agreement to comply with any law. 

 

	SECTION 10.	  RESTRICTIONS ON TRANSFER OF SHARES. 

 (a) Securities Law Restrictions.
Regardless of whether the offer and sale of Shares under the Plan have been registered under the Securities Act or have been registered or qualified under the securities laws of any State or other relevant jurisdiction, the Company at its discretion
may impose restrictions upon the sale, pledge or other transfer of such Shares (including the placement of appropriate legends on the stock certificates (or electronic equivalent) or the imposition of stop-transfer instructions) and may refuse to
transfer Shares acquired hereunder (or Shares proposed to be transferred in a subsequent transfer) if, in the judgment of the Company, such restrictions, legends or refusal are necessary or appropriate to achieve compliance with the Securities Act
or other relevant securities or other laws, including without limitation under Regulation S of the Securities Act or pursuant to another available exemption from registration. 

(b) Market Stand-Off. In connection with any underwritten public offering by the Company of its equity securities pursuant to an
effective registration statement filed under the Securities Act, including the Company’s initial public offering, the Optionee or a Transferee shall not directly or indirectly sell, make any short sale of, loan, hypothecate, pledge, offer,
grant or sell any option or other contract for the purchase of, purchase any option or other contract for the sale of, or otherwise dispose of or transfer, or agree to engage in any of the foregoing transactions with respect to, any Shares acquired
under this Agreement without the prior written consent of the Company or its managing underwriter. Such restriction (the “Market Stand-Off”) shall be in effect for such period of time following the date of the final prospectus for
the offering as may be requested by the Company or such underwriter. In no event, however, shall such period exceed 180 days plus such additional period as may reasonably be requested by the Company or such underwriter to accommodate regulatory
restrictions on (i) the publication or other distribution of research reports or (ii) analyst recommendations and opinions, including (without limitation) the restrictions set forth in Rule 2711(f)(4) of the National Association of
Securities Dealers and Rule 472(f)(4) of the New York Stock Exchange, as amended, or any similar successor rules. The Market Stand-Off shall in any event terminate 







 
two years after the date of the Company’s initial public offering. In the event of the declaration of a stock dividend, a spin-off, a stock split, an adjustment in conversion ratio, a
recapitalization or a similar transaction affecting the Company’s outstanding securities without receipt of consideration, any new, substituted or additional securities which are by reason of such transaction distributed with respect to any
Shares subject to the Market Stand-Off, or into which such Shares thereby become convertible, shall immediately be subject to the Market Stand-Off. In order to enforce the Market Stand-Off, the Company may impose stop-transfer instructions with
respect to the Shares acquired under this Agreement until the end of the applicable stand-off period. The Company’s underwriters shall be beneficiaries of the agreement set forth in this Subsection (b). This Subsection (b) shall not apply
to Shares registered in the public offering under the Securities Act. 
 (c) Investment Intent at Grant. The Optionee represents and
agrees that the Shares to be acquired upon exercising this option will be acquired for investment, and not with a view to the sale or distribution thereof. 

(d) Investment Intent at Exercise. In the event that the sale of Shares under the Plan is not registered under the Securities Act but
an exemption is available that requires an investment representation or other representation, the Optionee shall represent and agree at the time of exercise that the Shares being acquired upon exercising this option are being acquired for
investment, and not with a view to the sale or distribution thereof, and shall make such other representations as are deemed necessary or appropriate by the Company and its counsel, including (if applicable) that as of the date of exercise the
Optionee is (i) not a U.S. Person; (ii) not acquiring the Shares on behalf, or for the account or benefit, of a U.S. Person; and (iii) is not exercising the option in the United States. 

(e) Legends. All certificates evidencing Shares purchased under this Agreement shall bear the following legend: 

“THE SHARES REPRESENTED HEREBY MAY NOT BE SOLD, ASSIGNED, TRANSFERRED, ENCUMBERED OR IN ANY MANNER DISPOSED OF, EXCEPT IN COMPLIANCE WITH
THE TERMS OF A WRITTEN AGREEMENT BETWEEN THE COMPANY AND THE REGISTERED HOLDER OF THE SHARES (OR THE PREDECESSOR IN INTEREST TO THE SHARES). SUCH AGREEMENT GRANTS TO THE COMPANY CERTAIN RIGHTS OF FIRST REFUSAL UPON AN ATTEMPTED TRANSFER OF THE
SHARES. IN ADDITION, THE SHARES ARE SUBJECT TO RESTRICTIONS ON TRANSFER FOR A LIMITED PERIOD FOLLOWING THE EFFECTIVE DATE OF THE UNDERWRITTEN PUBLIC OFFERING OF THE COMPANY’S SECURITIES AND MAY NOT BE SOLD OR OTHERWISE DISPOSED OF BY THE HOLDER
WITHOUT THE CONSENT OF THE COMPANY OR THE MANAGING UNDERWRITER. THE SECRETARY OF THE COMPANY WILL UPON WRITTEN REQUEST FURNISH A COPY OF SUCH AGREEMENT TO THE HOLDER HEREOF WITHOUT CHARGE.” 







 All certificates evidencing Shares purchased under this Agreement in an unregistered transaction shall bear the
following legend (and such other restrictive legends as are required or deemed advisable under the provisions of any applicable law): 

“THE SHARES REPRESENTED HEREBY HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED (THE “ACT”) OR ANY
SECURITIES LAWS OF ANY U.S. STATE, AND MAY NOT BE SOLD, REOFFERED, PLEDGED, ASSIGNED, ENCUMBERED OR OTHERWISE TRANSFERRED OR DISPOSED WITHOUT AN EFFECTIVE REGISTRATION THEREOF UNDER SUCH ACT OR AN OPINION OF COUNSEL, SATISFACTORY TO THE COMPANY AND
ITS COUNSEL, THAT SUCH REGISTRATION IS NOT REQUIRED. IN THE ABSENCE OF REGISTRATION OR THE AVAILABILITY (CONFIRMED BY OPINION OF COUNSEL) OF AN ALTERNATIVE EXEMPTION FROM REGISTRATION UNDER THE ACT, THESE SHARES MAY NOT BE SOLD, REOFFERED, PLEDGED,
ASSIGNED, ENCUMBERED OR OTHERWISE TRANSFERRED OR DISPOSED OF EXCEPT IN ACCORDANCE WITH REGULATION S (RULES 901 THROUGH 905 AND PRELIMINARY NOTES) OF THE ACT. HEDGING TRANSACTIONS INVOLVING THESE SHARES MAY NOT BE CONDUCTED UNLESS IN COMPLIANCE WITH
THE ACT.” 
 (f) Removal of Legends. If, in the opinion of the Company and its counsel, any legend placed on a stock certificate
representing Shares sold under this Agreement is no longer required, the holder of such certificate shall be entitled to exchange such certificate for a certificate representing the same number of Shares but without such legend. 

(g) Administration. Any determination by the Company and its counsel in connection with any of the matters set forth in this
Section 10 shall be conclusive and binding on the Optionee and all other persons. 
  

	SECTION 11.	  ADJUSTMENT OF SHARES. 

 In the event of any transaction described in
Section 8(a) of the Plan, the terms of this option (including, without limitation, the number and kind of Shares subject to this option and the Exercise Price) shall be adjusted as set forth in Section 8(a) of the Plan. In the event that
the Company is a party to a merger or consolidation or in the event of a sale of all or substantially all of the Company’s stock or assets, this option shall be subject to the treatment provided by the Board of Directors in its sole discretion,
as provided in Section 8(b) of the Plan. 
  

	SECTION 12.	  MISCELLANEOUS PROVISIONS. 

 (a) Rights as a Stockholder. Neither the
Optionee nor the Optionee’s representative shall have any rights as a stockholder with respect to any Shares subject to this option until the Optionee or the Optionee’s representative becomes entitled to receive such Shares by filing a
notice of exercise and paying the Purchase Price pursuant to Sections 4 and 5. 
 (b) No Retention Rights. Nothing in this option or
in the Plan shall confer upon the Optionee any right to continue in Service for any period of specific duration or 







 
interfere with or otherwise restrict in any way the rights of the Company (or any Parent or Subsidiary employing or retaining the Optionee) or of the Optionee, which rights are hereby expressly
reserved by each, to terminate his or her Service at any time and for any reason, with or without cause. 
 (c) Notice. Any notice
required by the terms of this Agreement shall be given in writing. It shall be deemed effective upon (i) personal delivery, (ii) deposit with the United States Postal Service, by registered or certified mail, with postage and fees prepaid,
(iii) deposit with Federal Express Corporation, with shipping charges prepaid or (iv) deposit with any internationally recognized express mail courier service. Notice shall be addressed to the Company at its principal executive office and
to the Optionee at the address that he or she most recently provided to the Company in accordance with this Subsection (c). 
 (d)
Modifications and Waivers. No provision of this Agreement shall be modified, waived or discharged unless the modification, waiver or discharge is agreed to in writing and signed by the Optionee and by an authorized officer of the Company
(other than the Optionee). No waiver by either party of any breach of, or of compliance with, any condition or provision of this Agreement by the other party shall be considered a waiver of any other condition or provision or of the same condition
or provision at another time. 
 (e) Entire Agreement. The Notice of Stock Option Grant, this Agreement and the Plan constitute the
entire contract between the parties hereto with regard to the subject matter hereof. They supersede any other agreements, representations or understandings (whether oral or written and whether express or implied) that relate to the subject matter
hereof. 
 (f) Choice of Law. This Agreement shall be governed by, and construed in accordance with, the laws of the State of
Delaware, as such laws are applied to contracts entered into and performed in such State. 
 SECTION 13.   ACKNOWLEDGEMENTS OF THE
OPTIONEE. 
 In addition to the other terms, conditions and restrictions imposed on this option and the Shares issuable under this option
pursuant to this Agreement and the Plan, the Optionee expressly acknowledges being subject to Sections 7 (Right of First Refusal), 8 (Legality of Initial Issuance) and 10 (Restrictions on Transfer of Shares, including without limitation the Market
Stand-Off), as well as the following provisions: 
 (a) Tax Consequences (No Liability for Discounted Options). The Optionee agrees
that the Company does not have a duty to design or administer the Plan or its other compensation programs in a manner that minimizes the Optionee’s tax liabilities. The Optionee shall not make any claim against the Company or its Board of
Directors, officers or employees related to tax liabilities arising from this option or the Optionee’s other compensation. In particular, any Optionee subject to U.S. taxation acknowledges that this option is exempt from Section 409A of
the Code only if the Exercise Price is at least equal to the Fair Market Value per Share on the Date of Grant. Since Shares are not traded on an established securities market, the determination of their Fair Market Value is made by the Board of
Directors or by an independent 







 
valuation firm retained by the Company. The Optionee acknowledges that there is no guarantee in either case that the Internal Revenue Service will agree with the valuation, and the Optionee shall
not make any claim against the Company or its Board of Directors, officers or employees in the event that the Internal Revenue Service asserts that the valuation was too low. 

(b) Electronic Delivery of Documents. The Optionee agrees to accept by email all documents relating to the Company, the Plan or this
option and all other documents that the Company is required to deliver to its security holders (including, without limitation, disclosures that may be required by the Securities and Exchange Commission). The Optionee also agrees that the Company may
deliver these documents by posting them on a website maintained by the Company or by a third party under contract with the Company. If the Company posts these documents on a website, it shall notify the Optionee by email of their availability. The
Optionee acknowledges that he or she may incur costs in connection with electronic delivery, including the cost of accessing the internet and printing fees, and that an interruption of internet access may interfere with his or her ability to access
the documents. This consent shall remain in effect until this option expires or until the Optionee gives the Company written notice that it should deliver paper documents. 

(c) No Notice of Expiration Date. The Optionee agrees that the Company and its officers, employees, attorneys and agents do not have
any obligation to notify him or her prior to the expiration of this option pursuant to Section 6, regardless of whether this option will expire at the end of its full term or on an earlier date related to the termination of the Optionee’s
Service. The Optionee further agrees that he or she has the sole responsibility for monitoring the expiration of this option and for exercising this option, if at all, before it expires. This Subsection (c) shall supersede any contrary
representation that may have been made, orally or in writing, by the Company or by an officer, employee, attorney or agent of the Company. 

(d) Waiver of Statutory Information Rights. The Optionee acknowledges and agrees that, upon exercise of this option and until the first
sale of the Company’s Stock to the general public pursuant to a registration statement filed under the Securities Act, he or she will be deemed to have waived any rights the Optionee might otherwise have had under Section 220 of the
Delaware General Corporation Law to inspect for any proper purpose and to make copies and extracts from the Company’s stock ledger, a list of its stockholders and its other books and records or the books and records of any subsidiary. This
waiver applies only in the Optionee’s capacity as a stockholder and does not affect any other inspection rights the Optionee may have under other law or pursuant to a written agreement with the Company. 

(e) Plan Discretionary. The Optionee understands and acknowledges that (i) the Plan is entirely discretionary, (ii) the
Company and the Optionee’s employer have reserved the right to amend, suspend or terminate the Plan at any time, (iii) the grant of an option does not in any way create any contractual or other right to receive additional grants of options
(or benefits in lieu of options) at any time or in any amount and (iv) all determinations with respect to any additional grants, including (without limitation) the times when options will be granted, the number of Shares offered, the Exercise
Price and the vesting schedule, will be at the sole discretion of the Company. 







 (f) Termination of Service. The Optionee understands and acknowledges that participation
in the Plan ceases upon termination of his or her Service for any reason, except as may explicitly be provided otherwise in the Plan or this Agreement. 

(g) Extraordinary Compensation. The value of this option shall be an extraordinary item of compensation outside the scope of the
Optionee’s employment contract, if any, and shall not be considered a part of his or her normal or expected compensation for purposes of calculating severance, resignation, redundancy or end-of-service payments, bonuses, long-service awards,
pension or retirement benefits or similar payments. 
 (h) Authorization to Disclose. The Optionee hereby authorizes and directs the
Optionee’s employer to disclose to the Company or any Subsidiary any information regarding the Optionee’s employment, the nature and amount of the Optionee’s compensation and the fact and conditions of the Optionee’s
participation in the Plan, as the Optionee’s employer deems necessary or appropriate to facilitate the administration of the Plan. 

(i) Personal Data Authorization. The Optionee consents to the collection, use and transfer of personal data as described in this
Subsection (i). The Optionee understands and acknowledges that the Company, the Optionee’s employer and the Company’s other Subsidiaries hold certain personal information regarding the Optionee for the purpose of managing and administering
the Plan, including (without limitation) the Optionee’s name, home address, telephone number, date of birth, social insurance number, salary, nationality, job title, any Shares or directorships held in the Company and details of all options or
any other entitlements to Shares awarded, canceled, exercised, vested, unvested or outstanding in the Optionee’s favor (the “Data”). The Optionee further understands and acknowledges that the Company and/or its Subsidiaries will
transfer Data among themselves as necessary for the purpose of implementation, administration and management of the Optionee’s participation in the Plan and that the Company and/or any Subsidiary may each further transfer Data to any third
party assisting the Company in the implementation, administration and management of the Plan. The Optionee understands and acknowledges that the recipients of Data may be located in the United States or elsewhere. The Optionee authorizes such
recipients to receive, possess, use, retain and transfer Data, in electronic or other form, for the purpose of administering the Optionee’s participation in the Plan, including a transfer to any broker or other third party with whom the
Optionee elects to deposit Shares acquired under the Plan of such Data as may be required for the administration of the Plan and/or the subsequent holding of Shares on the Optionee’s behalf. The Optionee may, at any time, view the Data, require
any necessary modifications of Data or withdraw the consents set forth in this Subsection (i) by contacting the Company in writing. 
  

	SECTION 14.	  DEFINITIONS. 

 (a) “Agreement” shall mean this Stock Option
Agreement. 
 (b) “Board of Directors” shall mean the Board of Directors of the Company, as constituted from time to
time or, if a Committee has been appointed, such Committee. 







 (c) “Cause” shall mean (i) Optionee’s unauthorized use or
disclosure of the Company’s confidential information or trade secrets, which use or disclosure causes material harm to the Company; (ii) Optionee’s material breach of any agreement between Optionee and the Company;
(iii) Optionee’s material failure to comply with the Company’s written policies or rules; (iv) Optionee’s conviction of, or Optionee’s plea of “guilty” or “no contest” to, a felony under the laws of
the United States or any State; (v) Optionee’s gross negligence or willful misconduct; (vi) Optionee’s continuing failure to perform assigned duties after receiving written notification of such failure from the Company’s
Board of Directors or (vii) Optionee’s failure to cooperate in good faith with a governmental or internal investigation of the Company or its directors, officers or employees, if the Company has requested Optionee’s cooperation.

 (d) “Change in Control” shall mean (i) the consummation of a merger or consolidation of the Company with
or into another entity or (ii) the dissolution, liquidation or winding up of the Company. The foregoing notwithstanding, a merger or consolidation of the Company does not constitute a “Change in Control” if immediately after the
merger or consolidation a majority of the voting power of the capital stock of the continuing or surviving entity, or any direct or indirect parent corporation of the continuing or surviving entity, will be owned by the persons who were the
Company’s stockholders immediately prior to the merger or consolidation in substantially the same proportions as their ownership of the voting power of the Company’s capital stock immediately prior to the merger or consolidation.

 (e) “Code” shall mean the Internal Revenue Code of 1986, as amended. 

(f) “Committee” shall mean a committee of the Board of Directors, as described in Section 2 of the Plan. 

(g) “Company” shall mean Dimension Therapeutics, Inc., a Delaware corporation. 

(h) “Consultant” shall mean a person who performs bona fide services for the Company, a Parent or a Subsidiary as a
consultant or advisor, excluding Employees and Outside Directors. 
 (i) “Date of Grant” shall mean the date
of grant specified in the Notice of Stock Option Grant, which date shall be the later of (i) the date on which the Board of Directors resolved to grant this option or (ii) the first day of the Optionee’s Service. 

(j) “Disability” shall mean that the Optionee is unable to engage in any substantial gainful activity by reason of any
medically determinable physical or mental impairment which can be expected to result in death or which has lasted, or can be expected to last, for a continuous period of not less than 12 months. 

(k) “Employee” shall mean any individual who is a common-law employee of the Company, a Parent or a Subsidiary. 

(l) “Exercise Price” shall mean the amount for which one Share may be purchased upon exercise of this option, as
specified in the Notice of Stock Option Grant. 







 (m) “Fair Market Value” shall mean the fair market value of a Share, as
determined by the Board of Directors in good faith. Such determination shall be conclusive and binding on all persons. 
 (n)
“Good Reason” means that Optionee resigns within 12 months after one of the following conditions has come into existence without Optionee’s consent: 

(i) A reduction in Optionee’s base salary by more than 10%; 

(ii) A material diminution of Optionee’s authority, duties or responsibilities; or 

(iii) A relocation of Optionee’s principal workplace by more than 30 miles. 

A condition will not be considered “Good Reason” unless Optionee gives the Company written notice of the condition within 90 days after the
condition comes into existence and the Company fails to remedy the condition within 30 days after receiving Optionee’s written notice. 

(o) “Immediate Family” shall mean any child, stepchild, grandchild, parent, stepparent, grandparent, spouse, sibling,
mother-in-law, father-in-law, son-in-law, daughter-in-law, brother-in-law or sister-in-law and shall include adoptive relationships. 

(p) “Involuntary Termination” shall mean a Separation resulting from either (i) Optionee’s involuntary
discharge by the Company for reasons other than Cause or (ii) Optionee’s voluntary resignation for Good Reason. 
 (q)
“ISO” shall mean an employee incentive stock option described in Section 422(b) of the Code. 
 (r) “Notice of
Stock Option Grant” shall mean the document so entitled to which this Agreement is attached. 
 (s) “NSO” shall
mean a stock option not described in Sections 422(b) or 423(b) of the Code. 
 (t) “Optionee” shall mean the person named
in the Notice of Stock Option Grant. 
 (u) “Outside Director” shall mean a member of the Board of Directors who is not an
Employee. 
 (v) “Parent” shall mean any corporation (other than the Company) in an unbroken chain of corporations
ending with the Company, if each of the corporations other than the Company owns stock possessing 50% or more of the total combined voting power of all classes of stock in one of the other corporations in such chain. 

(w) “Plan” shall mean the EiQ Networks, Inc. 2008 Stock Plan, as in effect on the Date of Grant. 







 (x) “Purchase Price” shall mean the Exercise Price multiplied by the
number of Shares with respect to which this option is being exercised. 
 (y) “Right of First Refusal” shall mean
the Company’s right of first refusal described in Section 7. 
 (z) “Securities Act” shall mean the Securities Act of
1933, as amended. 
 (aa) “Separation” shall mean a “separation from service,” as defined in the regulations
under Section 409A of the Code. 
 (bb) “Service” shall mean service as an Employee, Outside Director or Consultant.

 (cc) “Share” shall mean one share of Stock, as adjusted in accordance with Section 8 of the Plan (if applicable).

 (dd) “Stock” shall mean the Common Stock of the Company. 

(ee) “Subsidiary” shall mean any corporation (other than the Company) in an unbroken chain of corporations beginning with the
Company, if each of the corporations other than the last corporation in the unbroken chain owns stock possessing 50% or more of the total combined voting power of all classes of stock in one of the other corporations in such chain. 

(ff) “Transferee” shall mean any person to whom the Optionee has directly or indirectly transferred any Share. 

(gg) “Transfer Notice” shall mean the notice of a proposed transfer of Shares described in Section 7. 







 DIMENSION THERAPEUTICS, INC. 2013 STOCK PLAN 

NOTICE OF STOCK OPTION GRANT (INSTALLMENT EXERCISE) 

The Optionee has been granted the following option to purchase shares of the Common Stock of Dimension Therapeutics, Inc.: 

 

			
	Name of Optionee:	  	«Name»
		
	Total Number of Shares:	  	«TotalShares»
		
	Type of Option:	  	«ISO» Incentive Stock Option (ISO)
		
		  	«NSO» Nonstatutory Stock Option (NSO)
		
	Exercise Price per Share:	  	$«PricePerShare»
		
	Date of Grant:	  	«DateGrant»
		
	Date Exercisable:	  	This option may be exercised with respect to the first «Percent»% of the Shares subject to this option when the Optionee completes «CliffPeriod» months of continuous Service beginning with the Vesting
Commencement Date set forth below. This option may be exercised with respect to an additional «Fraction»% of the Shares subject to this option when the Optionee completes each month of continuous Service thereafter.
		
	Vesting Commencement Date:	  	«VestComDate»
		
	Expiration Date:	  	«ExpDate». This option expires earlier if the Optionee’s Service terminates earlier, as provided in Section 6 of the Stock Option Agreement, or if the Company engages in certain corporate transactions, as
provided in Section 8(b) of the Plan.

 By signing below, the Optionee and the Company agree that this option is granted under, and governed by the terms and
conditions of, the 2013 Stock Plan and the Stock Option Agreement. Both of these documents are attached to, and made a part of, this Notice of Stock Option Grant. Section 13 of the Stock Option Agreement includes important acknowledgements
of the Optionee. 
  

					
	OPTIONEE:	 	           DIMENSION THERAPEUTICS, INC.


							
				
	  
	 		 	By:	 	  

		 		 	Title:  	 	  









 THE OPTION GRANTED PURSUANT TO THIS AGREEMENT AND THE SHARES ISSUABLE UPON THE EXERCISE THEREOF HAVE NOT BEEN
REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED, AND MAY NOT BE SOLD, PLEDGED, OR OTHERWISE TRANSFERRED WITHOUT AN EFFECTIVE REGISTRATION THEREOF UNDER SUCH ACT OR AN OPINION OF COUNSEL, SATISFACTORY TO THE COMPANY AND ITS COUNSEL, THAT SUCH
REGISTRATION IS NOT REQUIRED. 







 DIMENSION THERAPEUTICS, INC. 2013 STOCK PLAN: 

STOCK OPTION AGREEMENT (INSTALLMENT EXERCISE) 
  

	SECTION 1.	  GRANT OF OPTION. 

 (a) Option. On the terms and conditions set forth
in the Notice of Stock Option Grant and this Agreement, the Company grants to the Optionee on the Date of Grant the option to purchase at the Exercise Price the number of Shares set forth in the Notice of Stock Option Grant. The Exercise Price is
agreed to be at least 100% of the Fair Market Value per Share on the Date of Grant (110% of Fair Market Value if this option is designated as an ISO in the Notice of Stock Option Grant and Section 3(b) of the Plan applies). This option is
intended to be an ISO or an NSO, as provided in the Notice of Stock Option Grant. 
 (b) $100,000 Limitation. Even if this option is
designated as an ISO in the Notice of Stock Option Grant, it shall be deemed to be an NSO to the extent (and only to the extent) required by the $100,000 annual limitation under Section 422(d) of the Code. 

(c) Stock Plan and Defined Terms. This option is granted pursuant to the Plan, a copy of which the Optionee acknowledges having
received. The provisions of the Plan are incorporated into this Agreement by this reference. Except as otherwise defined in this Agreement (including without limitation Section 14 hereof), capitalized terms shall have the meaning ascribed to
such terms in the Plan. 
  

	SECTION 2.	  RIGHT TO EXERCISE. 

 (a) Exercisability. Subject to Subsection
(b) below and the other conditions set forth in this Agreement, all or part of this option may be exercised prior to its expiration at the time or times set forth in the Notice of Stock Option Grant. 

(b) Stockholder Approval. Any other provision of this Agreement notwithstanding, no portion of this option shall be exercisable at any
time prior to the approval of the Plan by the Company’s stockholders. 
  

	SECTION 3.	  NO TRANSFER OR ASSIGNMENT OF OPTION. 

 Except as otherwise provided in this
Agreement, this option and the rights and privileges conferred hereby shall not be sold, pledged or otherwise transferred (whether by operation of law or otherwise) and shall not be subject to sale under execution, attachment, levy or similar
process. 
  

	SECTION 4.	  EXERCISE PROCEDURES. 

 (a) Notice of Exercise. The Optionee or the
Optionee’s representative may exercise this option by: (i) signing and delivering written notice to the Company pursuant to Section 12(c) specifying the election to exercise this option, the number of Shares for which it is being
exercised and the form of payment and (ii) delivering payment, in a form permissible under Section 5, for the full amount of the Purchase Price (together with any applicable 







 
withholding taxes under Subsection (b)). In the event that this option is being exercised by the representative of the Optionee, the notice shall be accompanied by proof (satisfactory to the
Company) of the representative’s right to exercise this option. 
 (b) Withholding Taxes. In the event that the Company
determines that it is required to withhold any tax (including without limitation any income tax, social insurance contributions, payroll tax, payment on account or other tax-related items arising in connection with the Optionee’s participation
in the Plan and legally applicable to the Optionee (the “Tax-Related Items”)) as a result of the grant, vesting or exercise of this option, or as a result of the transfer of shares acquired upon exercise of this option, the
Optionee, as a condition of this option, shall make arrangements satisfactory to the Company to enable it to satisfy all Tax-Related Items. The Optionee acknowledges that the responsibility for all Tax-Related Items is the Optionee’s and may
exceed the amount actually withheld by the Company (or its affiliate or agent). 
 (c) Issuance of Shares. After satisfying all
requirements for exercise of this option, the Company shall cause to be issued one or more certificates evidencing the Shares for which this option has been exercised. Such Shares shall be registered (i) in the name of the person exercising
this option, (ii) in the names of such person and his or her spouse as community property or as joint tenants with the right of survivorship or (iii) with the Company’s consent, in the name of a revocable trust. Until the issuance of
the Shares has been entered into the books and records of the Company or a duly authorized transfer agent of the Company, no right to vote, receive dividends or any other right as a stockholder will exist with respect to such Shares. The Company
shall cause such certificates to be delivered to or upon the order of the person exercising this option. 
  

	SECTION 5.	  PAYMENT FOR STOCK. 

 (a) Cash. All or part of the Purchase Price may
be paid in cash or cash equivalents. 
 (b) Surrender of Stock. At the discretion of the Board of Directors, all or any part of the
Purchase Price may be paid by surrendering, or attesting to the ownership of, Shares that are already owned by the Optionee. Such Shares shall be surrendered to the Company in good form for transfer and shall be valued at their Fair Market Value as
of the date when this option is exercised. 
 (c) Exercise/Sale. All or part of the Purchase Price and any withholding taxes may be
paid by the delivery (on a form prescribed by the Company) of an irrevocable direction to a securities broker approved by the Company to sell Shares and to deliver all or part of the sales proceeds to the Company. However, payment pursuant to this
Subsection (c) shall be permitted only if (i) Stock then is publicly traded and (ii) such payment does not violate applicable law. 
  

	SECTION 6.	  TERM AND EXPIRATION. 

 (a) Basic Term. This option shall in any event
expire on the expiration date set forth in the Notice of Stock Option Grant, which date is 10 years after the Date of Grant (five years after the Date of Grant if this option is designated as an ISO in the Notice of Stock Option Grant and
Section 3(b) of the Plan applies). 







 (b) Termination of Service (Except by Death). If the Optionee’s Service terminates
for any reason other than death, then this option shall expire on the earliest of the following occasions: 
 (i) The
expiration date determined pursuant to Subsection (a) above; 
 (ii) The date three months after the termination of the
Optionee’s Service for any reason other than Disability; or 
 (iii) The date six months after the termination of the
Optionee’s Service by reason of Disability. 
 The Optionee may exercise all or part of this option at any time before its expiration under the
preceding sentence, but only to the extent that this option had become exercisable before the Optionee’s Service terminated. When the Optionee’s Service terminates, this option shall expire immediately with respect to the number of Shares
for which this option is not yet exercisable. In the event that the Optionee dies after termination of Service but before the expiration of this option, all or part of this option may be exercised (prior to expiration) by the executors or
administrators of the Optionee’s estate or by any person who has acquired this option directly from the Optionee by beneficiary designation, bequest or inheritance, but only to the extent that this option had become exercisable before the
Optionee’s Service terminated. Once this option (or portion thereof) has terminated, the Optionee shall have no further rights with respect to the option (or portion thereof) or to the underlying Shares. 

(c) Death of the Optionee. If the Optionee dies while in Service, then this option shall expire on the earlier of the following dates:

 (i) The expiration date determined pursuant to Subsection (a) above; or 

(ii) The date 12 months after the Optionee’s death. 

All or part of this option may be exercised at any time before its expiration under the preceding sentence by the executors or administrators of the
Optionee’s estate or by any person who has acquired this option directly from the Optionee by beneficiary designation, bequest or inheritance, but only to the extent that this option had become exercisable before the Optionee’s death. When
the Optionee dies, this option shall expire immediately with respect to the number of Shares for which this option is not yet exercisable. Once this option (or portion thereof) has terminated, the Optionee shall have no further rights with respect
to the option (or portion thereof) or to the underlying Shares. 
 (d) Extension of Post-Termination Exercise Periods. Following the
date on which the Company’s Stock is first listed for trading on an established securities market, if during any part of the exercise period described in Subsections (b)(ii) or (iii) or Subsection








 
(c)(ii) above the exercise of this option would be prohibited solely because the issuance of Shares upon such exercise would violate the registration requirements under the Securities Act or a
similar provision of other applicable law, then instead of terminating at the end of such prescribed period, the then-vested portion of this option will instead remain outstanding and not expire until the earlier of (i) the expiration date
determined pursuant to Section 6(a) above or (ii) the date on which the then-vested portion of this option has been exercisable without violation of applicable law for the aggregate period (which need not be consecutive) after termination
of the Optionee’s Service specified in the applicable Subsection above. 
 (e) Part-Time Employment and Leaves of Absence. If
the Optionee commences working on a part-time basis, then the Company may adjust the vesting schedule set forth in the Notice of Stock Option Grant. If the Optionee goes on a leave of absence, then the Company may adjust the vesting schedule set
forth in the Notice of Stock Option Grant in accordance with the Company’s leave of absence policy or the terms of such leave. Except as provided in the preceding sentence, Service shall be deemed to continue for any purpose under this
Agreement while the Optionee is on a bona fide leave of absence, if (i) such leave was approved by the Company in writing and (ii) continued crediting of Service for such purpose is expressly required by the terms of such leave or by
applicable law (as determined by the Company). Service shall be deemed to terminate when such leave ends, unless the Optionee immediately returns to active work. 

(f) Notice Concerning ISO Treatment. Even if this option is designated as an ISO in the Notice of Stock Option Grant, it ceases to
qualify for favorable tax treatment as an ISO to the extent that it is exercised: 
 (i) More than three months after the
date when the Optionee ceases to be an Employee for any reason other than death or permanent and total disability (as defined in Section 22(e)(3) of the Code); 

(ii) More than 12 months after the date when the Optionee ceases to be an Employee by reason of permanent and total disability
(as defined in Section 22(e)(3) of the Code); or 
 (iii) More than three months after the date when the Optionee has
been on a leave of absence for three months, unless the Optionee’s reemployment rights following such leave were guaranteed by statute or by contract. 
  

	SECTION 7.	  RIGHT OF FIRST REFUSAL. 

 (a) Right of First Refusal. In the event
that the Optionee proposes to sell, pledge or otherwise transfer to a third party any Shares acquired under this Agreement, or any interest in such Shares, the Company shall have the Right of First Refusal with respect to all (and not less than all)
of such Shares. If the Optionee desires to transfer Shares acquired under this Agreement, the Optionee shall give a written Transfer Notice to the Company describing fully the proposed transfer, including the number of Shares proposed to be
transferred, the proposed transfer price, the name and address of the proposed Transferee and proof satisfactory to the Company that the proposed sale or transfer will not violate any applicable federal, State or








 
foreign securities laws. The Transfer Notice shall be signed both by the Optionee and by the proposed Transferee and must constitute a binding commitment of both parties to the transfer of the
Shares. The Company shall have the right to purchase all, and not less than all, of the Shares on the terms of the proposal described in the Transfer Notice (subject, however, to any change in such terms permitted under Subsection (b) below) by
delivery of a notice of exercise of the Right of First Refusal within 30 days after the date when the Transfer Notice was received by the Company. 

(b) Transfer of Shares. If the Company fails to exercise its Right of First Refusal within 30 days after the date when it received the
Transfer Notice, the Optionee may, not later than 90 days following receipt of the Transfer Notice by the Company, conclude a transfer of the Shares subject to the Transfer Notice on the terms and conditions described in the Transfer Notice,
provided that any such sale is made in compliance with applicable federal, State and foreign securities laws and not in violation of any other contractual restrictions to which the Optionee is bound. Any proposed transfer on terms and conditions
different from those described in the Transfer Notice, as well as any subsequent proposed transfer by the Optionee, shall again be subject to the Right of First Refusal and shall require compliance with the procedure described in Subsection
(a) above. If the Company exercises its Right of First Refusal, the parties shall consummate the sale of the Shares on the terms set forth in the Transfer Notice within 60 days after the date when the Company received the Transfer Notice (or
within such longer period as may have been specified in the Transfer Notice); provided, however, that in the event the Transfer Notice provided that payment for the Shares was to be made in a form other than cash or cash equivalents paid at the time
of transfer, the Company shall have the option of paying for the Shares with cash or cash equivalents equal to the present value of the consideration described in the Transfer Notice. 

(c) Additional or Exchanged Securities and Property. In the event of a merger or consolidation of the Company, a sale of all or
substantially all of the Company’s stock or assets, any other corporate reorganization, a stock split, the declaration of a stock dividend, the declaration of an extraordinary dividend payable in a form other than stock, a spin-off, an
adjustment in conversion ratio, a recapitalization or a similar transaction affecting the Company’s outstanding securities, any securities or other property (including cash or cash equivalents) that are by reason of such transaction exchanged
for, or distributed with respect to, any Shares subject to this Section 7 shall immediately be subject to the Right of First Refusal. Appropriate adjustments to reflect the exchange or distribution of such securities or property shall be made
to the number and/or class of the Shares subject to this Section 7. 
 (d) Termination of Right of First Refusal. Any other
provision of this Section 7 notwithstanding, in the event that the Stock is readily tradable on an established securities market when the Optionee desires to transfer Shares, the Company shall have no Right of First Refusal, and the Optionee
shall have no obligation to comply with the procedures prescribed by Subsections (a) and (b) above. 
 (e) Permitted
Transfers. This Section 7 shall not apply to (i) a transfer by beneficiary designation, will or intestate succession or (ii) a transfer to one or more members of the Optionee’s Immediate Family or to a trust established by
the Optionee for the benefit of the Optionee and/or one or more members of the Optionee’s Immediate Family, provided in 







 
either case that the Transferee agrees in writing on a form prescribed by the Company to be bound by all provisions of this Agreement. If the Optionee transfers any Shares acquired under this
Agreement, either under this Subsection (e) or after the Company has failed to exercise the Right of First Refusal, then this Agreement shall apply to the Transferee to the same extent as to the Optionee. 

(f) Termination of Rights as Stockholder. If the Company makes available, at the time and place and in the amount and form provided in
this Agreement, the consideration for the Shares to be purchased in accordance with this Section 7, then after such time the person from whom such Shares are to be purchased shall no longer have any rights as a holder of such Shares (other than
the right to receive payment of such consideration in accordance with this Agreement). Such Shares shall be deemed to have been purchased in accordance with the applicable provisions hereof, whether or not the certificate(s) therefor have been
delivered as required by this Agreement. 
 (g) Assignment of Right of First Refusal. The Board of Directors may freely assign the
Company’s Right of First Refusal, in whole or in part. Any person who accepts an assignment of the Right of First Refusal from the Company shall assume all of the Company’s rights and obligations under this Section 7. 

 

	SECTION 8.	  LEGALITY OF INITIAL ISSUANCE. 

 No Shares shall be issued upon the exercise
of this option unless and until the Company has determined that: 
 (a) It and the Optionee have taken any actions required to register the
Shares under the Securities Act or to perfect an exemption from the registration requirements thereof; 
 (b) Any applicable listing
requirement of any stock exchange or other securities market on which Stock is listed has been satisfied; and 
 (c) Any other applicable
provision of federal, State or foreign law has been satisfied. 
  

	SECTION 9.	  NO REGISTRATION RIGHTS. 

 The Company may, but shall not be obligated to,
register or qualify the sale of Shares under the Securities Act or any other applicable law. The Company shall not be obligated to take any affirmative action in order to cause the sale of Shares under this Agreement to comply with any law. 

 

	SECTION 10.	  RESTRICTIONS ON TRANSFER OF SHARES. 

 (a) Securities Law Restrictions.
Regardless of whether the offer and sale of Shares under the Plan have been registered under the Securities Act or have been registered or qualified under the securities laws of any State or other relevant jurisdiction, the Company at its discretion
may impose restrictions upon the sale, pledge or other transfer of such 







 
Shares (including the placement of appropriate legends on the stock certificates (or electronic equivalent) or the imposition of stop-transfer instructions) and may refuse to transfer Shares
acquired hereunder (or Shares proposed to be transferred in a subsequent transfer) if, in the judgment of the Company, such restrictions, legends or refusal are necessary or appropriate to achieve compliance with the Securities Act or other relevant
securities or other laws, including without limitation under Regulation S of the Securities Act or pursuant to another available exemption from registration. 

(b) Market Stand-Off. In connection with any underwritten public offering by the Company of its equity securities pursuant to an
effective registration statement filed under the Securities Act, including the Company’s initial public offering, the Optionee or a Transferee shall not directly or indirectly sell, make any short sale of, loan, hypothecate, pledge, offer,
grant or sell any option or other contract for the purchase of, purchase any option or other contract for the sale of, or otherwise dispose of or transfer, or agree to engage in any of the foregoing transactions with respect to, any Shares acquired
under this Agreement without the prior written consent of the Company or its managing underwriter. Such restriction (the “Market Stand-Off”) shall be in effect for such period of time following the date of the final prospectus for
the offering as may be requested by the Company or such underwriter. In no event, however, shall such period exceed 180 days plus such additional period as may reasonably be requested by the Company or such underwriter to accommodate regulatory
restrictions on (i) the publication or other distribution of research reports or (ii) analyst recommendations and opinions, including (without limitation) the restrictions set forth in Rule 2711(f)(4) of the National Association of
Securities Dealers and Rule 472(f)(4) of the New York Stock Exchange, as amended, or any similar successor rules. The Market Stand-Off shall in any event terminate two years after the date of the Company’s initial public offering. In the event
of the declaration of a stock dividend, a spin-off, a stock split, an adjustment in conversion ratio, a recapitalization or a similar transaction affecting the Company’s outstanding securities without receipt of consideration, any new,
substituted or additional securities which are by reason of such transaction distributed with respect to any Shares subject to the Market Stand-Off, or into which such Shares thereby become convertible, shall immediately be subject to the Market
Stand-Off. In order to enforce the Market Stand-Off, the Company may impose stop-transfer instructions with respect to the Shares acquired under this Agreement until the end of the applicable stand-off period. The Company’s underwriters shall
be beneficiaries of the agreement set forth in this Subsection (b). This Subsection (b) shall not apply to Shares registered in the public offering under the Securities Act. 

(c) Investment Intent at Grant. The Optionee represents and agrees that the Shares to be acquired upon exercising this option will be
acquired for investment, and not with a view to the sale or distribution thereof. 
 (d) Investment Intent at Exercise. In the event
that the sale of Shares under the Plan is not registered under the Securities Act but an exemption is available that requires an investment representation or other representation, the Optionee shall represent and agree at the time of exercise that
the Shares being acquired upon exercising this option are being acquired for investment, and not with a view to the sale or distribution thereof, and shall make such other representations as are deemed necessary or appropriate by the Company and its
counsel, including (if applicable) that as of the date of exercise the Optionee is (i) not a U.S. Person; (ii) not acquiring the Shares on behalf, or for the account or benefit, of a U.S. Person; and (iii) is not exercising the option
in the United States. 







 (e) Legends. All certificates evidencing Shares purchased under this Agreement shall bear
the following legend: 
 “THE SHARES REPRESENTED HEREBY MAY NOT BE SOLD, ASSIGNED, TRANSFERRED, ENCUMBERED OR IN ANY MANNER DISPOSED OF,
EXCEPT IN COMPLIANCE WITH THE TERMS OF A WRITTEN AGREEMENT BETWEEN THE COMPANY AND THE REGISTERED HOLDER OF THE SHARES (OR THE PREDECESSOR IN INTEREST TO THE SHARES). SUCH AGREEMENT GRANTS TO THE COMPANY CERTAIN RIGHTS OF FIRST REFUSAL UPON AN
ATTEMPTED TRANSFER OF THE SHARES. IN ADDITION, THE SHARES ARE SUBJECT TO RESTRICTIONS ON TRANSFER FOR A LIMITED PERIOD FOLLOWING THE EFFECTIVE DATE OF THE UNDERWRITTEN PUBLIC OFFERING OF THE COMPANY’S SECURITIES AND MAY NOT BE SOLD OR OTHERWISE
DISPOSED OF BY THE HOLDER WITHOUT THE CONSENT OF THE COMPANY OR THE MANAGING UNDERWRITER. THE SECRETARY OF THE COMPANY WILL UPON WRITTEN REQUEST FURNISH A COPY OF SUCH AGREEMENT TO THE HOLDER HEREOF WITHOUT CHARGE.” 

All certificates evidencing Shares purchased under this Agreement in an unregistered transaction shall bear the following legend (and such other restrictive
legends as are required or deemed advisable under the provisions of any applicable law): 
 “THE SHARES REPRESENTED HEREBY HAVE NOT BEEN
REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED (THE “ACT”) OR ANY SECURITIES LAWS OF ANY U.S. STATE, AND MAY NOT BE SOLD, REOFFERED, PLEDGED, ASSIGNED, ENCUMBERED OR OTHERWISE TRANSFERRED OR DISPOSED WITHOUT AN EFFECTIVE
REGISTRATION THEREOF UNDER SUCH ACT OR AN OPINION OF COUNSEL, SATISFACTORY TO THE COMPANY AND ITS COUNSEL, THAT SUCH REGISTRATION IS NOT REQUIRED. IN THE ABSENCE OF REGISTRATION OR THE AVAILABILITY (CONFIRMED BY OPINION OF COUNSEL) OF AN ALTERNATIVE
EXEMPTION FROM REGISTRATION UNDER THE ACT, THESE SHARES MAY NOT BE SOLD, REOFFERED, PLEDGED, ASSIGNED, ENCUMBERED OR OTHERWISE TRANSFERRED OR DISPOSED OF EXCEPT IN ACCORDANCE WITH REGULATION S (RULES 901 THROUGH 905 AND PRELIMINARY NOTES) OF THE
ACT. HEDGING TRANSACTIONS INVOLVING THESE SHARES MAY NOT BE CONDUCTED UNLESS IN COMPLIANCE WITH THE ACT.” 
 (f) Removal of
Legends. If, in the opinion of the Company and its counsel, any legend placed on a stock certificate representing Shares sold under this Agreement is no longer required, the holder of such certificate shall be entitled to exchange such
certificate for a certificate representing the same number of Shares but without such legend. 







 (g) Administration. Any determination by the Company and its counsel in connection with
any of the matters set forth in this Section 10 shall be conclusive and binding on the Optionee and all other persons. 
  

	SECTION 11.	  ADJUSTMENT OF SHARES. 

 In the event of any transaction described in
Section 8(a) of the Plan, the terms of this option (including, without limitation, the number and kind of Shares subject to this option and the Exercise Price) shall be adjusted as set forth in Section 8(a) of the Plan. In the event that
the Company is a party to a merger or consolidation or in the event of a sale of all or substantially all of the Company’s stock or assets, this option shall be subject to the treatment provided by the Board of Directors in its sole discretion,
as provided in Section 8(b) of the Plan. 
  

	SECTION 12.	  MISCELLANEOUS PROVISIONS. 

 (a) Rights as a Stockholder. Neither the
Optionee nor the Optionee’s representative shall have any rights as a stockholder with respect to any Shares subject to this option until the Optionee or the Optionee’s representative becomes entitled to receive such Shares by filing a
notice of exercise and paying the Purchase Price pursuant to Sections 4 and 5. 
 (b) No Retention Rights. Nothing in this option or
in the Plan shall confer upon the Optionee any right to continue in Service for any period of specific duration or interfere with or otherwise restrict in any way the rights of the Company (or any Parent or Subsidiary employing or retaining the
Optionee) or of the Optionee, which rights are hereby expressly reserved by each, to terminate his or her Service at any time and for any reason, with or without cause. 

(c) Notice. Any notice required by the terms of this Agreement shall be given in writing. It shall be deemed effective upon
(i) personal delivery, (ii) deposit with the United States Postal Service, by registered or certified mail, with postage and fees prepaid, (iii) deposit with Federal Express Corporation, with shipping charges prepaid or
(iv) deposit with any internationally recognized express mail courier service. Notice shall be addressed to the Company at its principal executive office and to the Optionee at the address that he or she most recently provided to the Company in
accordance with this Subsection (c). 
 (d) Modifications and Waivers. No provision of this Agreement shall be modified, waived or
discharged unless the modification, waiver or discharge is agreed to in writing and signed by the Optionee and by an authorized officer of the Company (other than the Optionee). No waiver by either party of any breach of, or of compliance with, any
condition or provision of this Agreement by the other party shall be considered a waiver of any other condition or provision or of the same condition or provision at another time. 

(e) Entire Agreement. The Notice of Stock Option Grant, this Agreement and the Plan constitute the entire contract between the parties
hereto with regard to the subject matter hereof. They supersede any other agreements, representations or understandings (whether oral or written and whether express or implied) that relate to the subject matter hereof. 







 (f) Choice of Law. This Agreement shall be governed by, and construed in accordance with,
the laws of the State of Delaware, as such laws are applied to contracts entered into and performed in such State. 
  

	SECTION 13.	  ACKNOWLEDGEMENTS OF THE OPTIONEE. 

 In addition to the other terms,
conditions and restrictions imposed on this option and the Shares issuable under this option pursuant to this Agreement and the Plan, the Optionee expressly acknowledges being subject to Sections 7 (Right of First Refusal), 8 (Legality of Initial
Issuance) and 10 (Restrictions on Transfer of Shares, including without limitation the Market Stand-Off), as well as the following provisions: 

(a) Tax Consequences (No Liability for Discounted Options). The Optionee agrees that the Company does not have a duty to design or
administer the Plan or its other compensation programs in a manner that minimizes the Optionee’s tax liabilities. The Optionee shall not make any claim against the Company or its Board of Directors, officers or employees related to tax
liabilities arising from this option or the Optionee’s other compensation. In particular, any Optionee subject to U.S. taxation acknowledges that this option is exempt from Section 409A of the Code only if the Exercise Price is at least
equal to the Fair Market Value per Share on the Date of Grant. Since Shares are not traded on an established securities market, the determination of their Fair Market Value is made by the Board of Directors or by an independent valuation firm
retained by the Company. The Optionee acknowledges that there is no guarantee in either case that the Internal Revenue Service will agree with the valuation, and the Optionee shall not make any claim against the Company or its Board of Directors,
officers or employees in the event that the Internal Revenue Service asserts that the valuation was too low. 
 (b) Electronic Delivery
of Documents. The Optionee agrees to accept by email all documents relating to the Company, the Plan or this option and all other documents that the Company is required to deliver to its security holders (including, without limitation,
disclosures that may be required by the Securities and Exchange Commission). The Optionee also agrees that the Company may deliver these documents by posting them on a website maintained by the Company or by a third party under contract with the
Company. If the Company posts these documents on a website, it shall notify the Optionee by email of their availability. The Optionee acknowledges that he or she may incur costs in connection with electronic delivery, including the cost of accessing
the internet and printing fees, and that an interruption of internet access may interfere with his or her ability to access the documents. This consent shall remain in effect until this option expires or until the Optionee gives the Company written
notice that it should deliver paper documents. 
 (c) No Notice of Expiration Date. The Optionee agrees that the Company and its
officers, employees, attorneys and agents do not have any obligation to notify him or her prior to the expiration of this option pursuant to Section 6, regardless of whether this option will expire at the end of its full term or on an earlier
date related to the termination of the Optionee’s Service. The Optionee further agrees that he or she has the sole responsibility for 







 
monitoring the expiration of this option and for exercising this option, if at all, before it expires. This Subsection (c) shall supersede any contrary representation that may have been
made, orally or in writing, by the Company or by an officer, employee, attorney or agent of the Company. 
 (d) Waiver of Statutory
Information Rights. The Optionee acknowledges and agrees that, upon exercise of this option and until the first sale of the Company’s Stock to the general public pursuant to a registration statement filed under the Securities Act, he or she
will be deemed to have waived any rights the Optionee might otherwise have had under Section 220 of the Delaware General Corporation Law to inspect for any proper purpose and to make copies and extracts from the Company’s stock ledger, a
list of its stockholders and its other books and records or the books and records of any subsidiary. This waiver applies only in the Optionee’s capacity as a stockholder and does not affect any other inspection rights the Optionee may have
under other law or pursuant to a written agreement with the Company. 
 (e) Plan Discretionary. The Optionee understands and
acknowledges that (i) the Plan is entirely discretionary, (ii) the Company and the Optionee’s employer have reserved the right to amend, suspend or terminate the Plan at any time, (iii) the grant of an option does not in any way
create any contractual or other right to receive additional grants of options (or benefits in lieu of options) at any time or in any amount and (iv) all determinations with respect to any additional grants, including (without limitation) the
times when options will be granted, the number of Shares offered, the Exercise Price and the vesting schedule, will be at the sole discretion of the Company. 

(f) Termination of Service. The Optionee understands and acknowledges that participation in the Plan ceases upon termination of his or
her Service for any reason, except as may explicitly be provided otherwise in the Plan or this Agreement. 
 (g) Extraordinary
Compensation. The value of this option shall be an extraordinary item of compensation outside the scope of the Optionee’s employment contract, if any, and shall not be considered a part of his or her normal or expected compensation for
purposes of calculating severance, resignation, redundancy or end-of-service payments, bonuses, long-service awards, pension or retirement benefits or similar payments. 

(h) Authorization to Disclose. The Optionee hereby authorizes and directs the Optionee’s employer to disclose to the Company or
any Subsidiary any information regarding the Optionee’s employment, the nature and amount of the Optionee’s compensation and the fact and conditions of the Optionee’s participation in the Plan, as the Optionee’s employer deems
necessary or appropriate to facilitate the administration of the Plan. 
 (i) Personal Data Authorization. The Optionee consents to
the collection, use and transfer of personal data as described in this Subsection (i). The Optionee understands and acknowledges that the Company, the Optionee’s employer and the Company’s other Subsidiaries hold certain personal
information regarding the Optionee for the purpose of managing and administering the Plan, including (without limitation) the Optionee’s name, home address, telephone number, date of birth, social insurance number, salary, nationality, job
title, any Shares or directorships held in the Company and details of all options or any other 







 
entitlements to Shares awarded, canceled, exercised, vested, unvested or outstanding in the Optionee’s favor (the “Data”). The Optionee further understands and acknowledges
that the Company and/or its Subsidiaries will transfer Data among themselves as necessary for the purpose of implementation, administration and management of the Optionee’s participation in the Plan and that the Company and/or any Subsidiary
may each further transfer Data to any third party assisting the Company in the implementation, administration and management of the Plan. The Optionee understands and acknowledges that the recipients of Data may be located in the United States or
elsewhere. The Optionee authorizes such recipients to receive, possess, use, retain and transfer Data, in electronic or other form, for the purpose of administering the Optionee’s participation in the Plan, including a transfer to any broker or
other third party with whom the Optionee elects to deposit Shares acquired under the Plan of such Data as may be required for the administration of the Plan and/or the subsequent holding of Shares on the Optionee’s behalf. The Optionee may, at
any time, view the Data, require any necessary modifications of Data or withdraw the consents set forth in this Subsection (i) by contacting the Company in writing. 
  

	SECTION 14.	  DEFINITIONS. 

 (a) “Agreement” shall mean this Stock Option
Agreement. 
 (b) “Board of Directors” shall mean the Board of Directors of the Company, as constituted from time to
time or, if a Committee has been appointed, such Committee. 
 (c) “Company” shall mean Dimension Therapeutics,
Inc., a Delaware corporation. 
 (d) “Immediate Family” shall mean any child, stepchild, grandchild, parent,
stepparent, grandparent, spouse, sibling, mother-in-law, father-in-law, son-in-law, daughter-in-law, brother-in-law or sister-in-law and shall include adoptive relationships. 

(e) “Optionee” shall mean the person named in the Notice of Stock Option Grant. 

(f) “Plan” shall mean the Dimension Therapeutics, Inc. 2013 Stock Plan, as in effect on the Date of Grant. 

(g) “Purchase Price” shall mean the Exercise Price multiplied by the number of Shares with respect to which this
option is being exercised. 
 (h) “Right of First Refusal” shall mean the Company’s right of first refusal
described in Section 7. 
 (i) “Transferee” shall mean any person to whom the Optionee has directly or indirectly
transferred any Share acquired under this Agreement. 
 (j) “Transfer Notice” shall mean the notice of a proposed
transfer of Shares described in Section 7. 
 (k) “U.S. Person” shall mean a person described in Rule 902(k) of
Regulation S of the Securities Act (or any successor rule or provision), which generally defines a U.S. person as any natural person resident in the United States, any estate of which any executor or administrator is a U.S. Person, or any trust of
which of any trustee is a U.S. Person. 







 DIMENSION THERAPEUTICS, INC. 2013 STOCK PLAN: 

SUMMARY OF STOCK GRANT (FOR SERVICES) 

The Transferee is acquiring shares of the Common Stock of Dimension Therapeutics, Inc. on the following terms: 

 

			
	Name of Transferee:	  	«Name»
		
	Total Number of Transferred Shares:	  	«TotalShares»
		
	Date of Transfer:	  	«DateTransfer»
		
	Vesting Commencement Date:	  	«VestComDate»
		
	Vesting Schedule:	  	The Forfeiture Condition shall lapse with respect to the first «Percent»% of the Transferred Shares when the Transferee completes «CliffPeriod» months of continuous Service beginning with the Vesting
Commencement Date set forth above. The Forfeiture Condition shall lapse with respect to an additional «Fraction»% of the Transferred Shares when the Transferee completes each month of continuous Service thereafter.

 By signing below, the Transferee and the Company agree that the acquisition of the Transferred Shares is governed by the terms
and conditions of the 2013 Stock Plan and the Stock Grant Agreement. Both of these documents are attached to, and made a part of, this Summary of Stock Grant. The Transferee agrees to accept by email all documents relating to the Company, the Plan
or this grant and all other documents that the Company is required to deliver to its security holders (including, without limitation, disclosures that may be required by the Securities and Exchange Commission). The Transferee also agrees that the
Company may deliver these documents by posting them on a website maintained by the Company or by a third party under contract with the Company. If the Company posts these documents on a website, it shall notify the Transferee by email of their
availability. The Transferee acknowledges that he or she may incur costs in connection with electronic delivery, including the cost of accessing the internet and printing fees, and that an interruption of internet access may interfere with his or
her ability to access the documents. This consent shall remain in effect until the Transferee gives the Company written notice that it should deliver paper documents. 








					
	TRANSFEREE:	 	           DIMENSION THERAPEUTICS, INC.


							
				
		 		 	By:	 	
	  
	 		 		 	  

	Address for Mailing Stock Certificate:	 		 	Title:  	 	  

				
	  
	 		 		 	
	  
	 		 		 	








 DIMENSION THERAPEUTICS, INC. 2013 STOCK PLAN: 

STOCK GRANT AGREEMENT (FOR SERVICES) 
  

	SECTION 1.	  ACQUISITION OF SHARES. 

 (a) Transfer. On the terms and conditions set
forth in the Summary of Stock Grant and this Agreement, the Company agrees to transfer to the Transferee the number of Shares set forth in the Summary of Stock Grant. The transfer shall occur at the offices of the Company on the date of transfer set
forth in the Summary of Stock Grant or at such other place and time as the parties may agree. 
 (b) Consideration. The Transferee
and the Company agree that the Transferred Shares are being issued to the Transferee as consideration for a portion of the services performed by the Transferee for the Company. The value of such portion is agreed to be not less than 100% of the Fair
Market Value of the Transferred Shares. 
 (c) Stock Plan and Defined Terms. The transfer of the Transferred Shares is subject to the
Plan, a copy of which the Transferee acknowledges having received. The provisions of the Plan are incorporated into this Agreement by this reference. Except as otherwise defined in this Agreement (including without limitation Section 11
hereof), capitalized terms shall have the meaning ascribed to such terms in the Plan. 
  

	SECTION 2.	  FORFEITURE CONDITION. 

 (a) Scope of Forfeiture Condition. All
Transferred Shares initially shall be Restricted Shares and shall be subject to forfeiture to the Company. The Transferee shall not transfer, assign, encumber or otherwise dispose of any Restricted Shares without the Company’s written consent,
except as provided in the following sentence. The Transferee may transfer Restricted Shares to one or more members of the Transferee’s Immediate Family or to a trust established by the Transferee for the benefit of the Transferee and/or one or
more members of the Transferee’s Immediate Family, provided in either case that the Transferee agrees in writing on a form prescribed by the Company to be bound by all provisions of this Agreement. If the Transferee transfers any Restricted
Shares, then this Agreement shall apply to the Subsequent Transferee to the same extent as to the Transferee. 
 (b) Vesting. The
Forfeiture Condition shall lapse and the Restricted Shares shall become vested in accordance with the vesting schedule set forth in the Summary of Stock Grant. 

(c) Execution of Forfeiture. The Forfeiture Condition shall be applicable only if the Transferee’s Service terminates for any
reason, with or without cause, including (without limitation) death or disability, before all Restricted Shares have become vested. In the event that the Transferee’s Service terminates for any reason, the certificate(s) representing any
remaining Restricted Shares shall be delivered to the Company. The Company shall make no payment for Restricted Shares that are forfeited. 







 (d) Additional or Exchanged Securities and Property. In the event of a merger or
consolidation of the Company, a sale of all or substantially all of the Company’s stock or assets, any other corporate reorganization, the declaration of a stock dividend, the declaration of an extraordinary dividend payable in a form other
than stock, a spin-off, a stock split, an adjustment in conversion ratio, a recapitalization or a similar transaction affecting the Company’s outstanding securities, any securities or other property (including cash or cash equivalents) that are
by reason of such transaction exchanged for, or distributed with respect to, any Restricted Shares or into which such Restricted Shares thereby become convertible shall immediately be subject to the Forfeiture Condition. Appropriate adjustments to
reflect the exchange or distribution of such securities or property shall be made to the number and/or class of the Restricted Shares. 

(e) Termination of Rights as Stockholder. If Restricted Shares are forfeited in accordance with this Section 2, then the person
who is to forfeit such Restricted Shares shall no longer have any rights as a holder of such Restricted Shares. Such Restricted Shares shall be deemed to have been forfeited in accordance with the applicable provisions hereof, whether or not the
certificate(s) therefor have been delivered as required by this Agreement. 
 (f) Escrow. Upon issuance, the certificates for
Restricted Shares shall be deposited in escrow with the Company to be held in accordance with the provisions of this Agreement. Any new, substituted or additional securities or other property described in Subsection (d) above shall immediately
be delivered to the Company to be held in escrow, but only to the extent the Transferred Shares are at the time Restricted Shares. All regular cash dividends on Restricted Shares (or other securities at the time held in escrow) shall be paid
directly to the Transferee and shall not be held in escrow. Restricted Shares, together with any other assets or securities held in escrow hereunder, shall be (i) surrendered to the Company for forfeiture and cancellation in the event that the
Forfeiture Condition or Right of First Refusal applies or (ii) released to the Transferee upon the Transferee’s request to the extent the Transferred Shares are no longer Restricted Shares (but not more frequently than once every six
months). In any event, all Transferred Shares that have vested (and any other vested assets and securities attributable thereto) shall be released within 60 days after the earlier of (i) the termination of the Transferee’s Service or
(ii) the lapse of the Right of First Refusal. 
 (g) Part-Time Employment and Leaves of Absence. If the Transferee commences
working on a part-time basis, then the Company may adjust the vesting schedule set forth in the Summary of Stock Grant. If the Transferee goes on a leave of absence, then the Company may adjust the vesting schedule set forth in the Summary of Stock
Grant in accordance with the Company’s leave of absence policy or the terms of such leave. Except as provided in the preceding sentence, Service shall be deemed to continue while the Transferee is on a bona fide leave of absence, if
(i) such leave was approved by the Company in writing and (ii) continued crediting of Service is expressly required by the terms of such leave or by applicable law (as determined by the Company). Service shall be deemed to terminate when
such leave ends, unless the Transferee immediately returns to active work. 








	SECTION 3.	  RIGHT OF FIRST REFUSAL. 

 (a) Right of First Refusal. In the event
that the Transferee proposes to sell, pledge or otherwise transfer to a third party any Transferred Shares, or any interest in Transferred Shares, the Company shall have the Right of First Refusal with respect to all (and not less than all) of such
Transferred Shares. If the Transferee desires to transfer Transferred Shares, the Transferee shall give a written Transfer Notice to the Company describing fully the proposed transfer, including the number of Transferred Shares proposed to be
transferred, the proposed transfer price, the name and address of the proposed Subsequent Transferee and proof satisfactory to the Company that the proposed sale or transfer will not violate any applicable federal, State or foreign securities laws.
The Transfer Notice shall be signed both by the Transferee and by the proposed Subsequent Transferee and must constitute a binding commitment of both parties to the transfer of the Transferred Shares. The Company shall have the right to purchase
all, and not less than all, of the Transferred Shares on the terms of the proposal described in the Transfer Notice (subject, however, to any change in such terms permitted under Subsection (b) below) by delivery of a notice of exercise of the
Right of First Refusal within 30 days after the date when the Transfer Notice was received by the Company. 
 (b) Transfer of Shares.
If the Company fails to exercise its Right of First Refusal within 30 days after receiving the Transfer Notice, the Transferee may, not later than 90 days after the Company received the Transfer Notice, conclude a transfer of the Transferred Shares
subject to the Transfer Notice on the terms and conditions described in the Transfer Notice, provided that any such sale is made in compliance with applicable federal, State and foreign securities laws and not in violation of any other contractual
restrictions to which the Transferee is bound. Any proposed transfer on terms and conditions different from those described in the Transfer Notice, as well as any subsequent proposed transfer by the Transferee, shall again be subject to the Right of
First Refusal and shall require compliance with the procedure described in Subsection (a) above. If the Company exercises its Right of First Refusal, the parties shall consummate the sale of the Transferred Shares on the terms set forth in the
Transfer Notice within 60 days after the Company received the Transfer Notice (or within such longer period as may have been specified in the Transfer Notice); provided, however, that in the event the Transfer Notice provided that payment for the
Transferred Shares was to be made in a form other than cash or cash equivalents paid at the time of transfer, the Company shall have the option of paying for the Transferred Shares with cash or cash equivalents equal to the present value of the
consideration described in the Transfer Notice. 
 (c) Additional or Exchanged Securities and Property. In the event of a merger or
consolidation of the Company, a sale of all or substantially all of the Company’s stock or assets, any other corporate reorganization, a stock split, the declaration of a stock dividend, the declaration of an extraordinary dividend payable in a
form other than stock, a spin-off, an adjustment in conversion ratio, a recapitalization or a similar transaction affecting the Company’s outstanding securities, any securities or other property (including cash or cash equivalents) that are by
reason of such transaction exchanged for, or distributed with respect to, any Transferred Shares subject to this Section 3 shall immediately be subject to the Right of First Refusal. Appropriate adjustments to reflect the exchange or
distribution of such securities or property shall be made to the number and/or class of the Transferred Shares subject to this Section 3. 







 (d) Termination of Right of First Refusal. Any other provision of this Section 3
notwithstanding, in the event that the Stock is readily tradable on an established securities market when the Transferee desires to transfer Transferred Shares, the Company shall have no Right of First Refusal, and the Transferee shall have no
obligation to comply with the procedures prescribed by Subsections (a) and (b) above. 
 (e) Permitted Transfers. This
Section 3 shall not apply to (i) a transfer by beneficiary designation, will or intestate succession or (ii) a transfer to one or more members of the Transferee’s Immediate Family or to a trust established by the Transferee for
the benefit of the Transferee and/or one or more members of the Transferee’s Immediate Family, provided in either case that the Transferee agrees in writing on a form prescribed by the Company to be bound by all provisions of this Agreement. If
the Transferee transfers any Transferred Shares, either under this Subsection (e) or after the Company has failed to exercise the Right of First Refusal, then this Agreement shall apply to the Subsequent Transferee to the same extent as to the
Transferee. 
 (f) Termination of Rights as Stockholder. If the Company makes available, at the time and place and in the amount and
form provided in this Agreement, the consideration for the Shares to be purchased in accordance with this Section 3, then after such time the person from whom such Shares are to be purchased shall no longer have any rights as a holder of such
Shares (other than the right to receive payment of such consideration in accordance with this Agreement). Such Shares shall be deemed to have been purchased in accordance with the applicable provisions hereof, whether or not the certificate(s)
therefor have been delivered as required by this Agreement. 
 (g) Assignment of Right of First Refusal. The Board of Directors may
freely assign the Company’s Right of First Refusal, in whole or in part. Any person who accepts an assignment of the Right of First Refusal from the Company shall assume all of the Company’s rights and obligations under this
Section 3. 
  

	SECTION 4.	 OTHER RESTRICTIONS ON TRANSFER. 

 (a) Transferee Representations. In
connection with the issuance and acquisition of Shares under this Agreement, the Transferee hereby represents and warrants to the Company as follows: 

(i) The Transferee is acquiring and will hold the Transferred Shares for investment for his or her account only and not with a
view to, or for resale in connection with, any “distribution” thereof within the meaning of the Securities Act. 

(ii) The Transferee understands that the Transferred Shares have not been registered under the Securities Act by reason of a
specific exemption therefrom and that the Transferred Shares must be held indefinitely, unless their sale or other transfer is subsequently registered under the Securities Act or the Transferee obtains an opinion of counsel, in form and substance
satisfactory to the Company and its counsel, that such registration is not required. The Transferee further acknowledges and understands that the Company is under no obligation to register the Transferred Shares. 







 (iii) The Transferee is aware of Rule 144 under the Securities Act, which permits
limited public resales of securities acquired in a non-public offering, subject to the satisfaction of certain conditions. These conditions may include (without limitation) that certain current public information about the issuer be available, that
the resale occur only after a holding period required by Rule 144 has been satisfied, that the sale occur through an unsolicited “broker’s transaction,” and that the amount of securities being sold during any three-month period not
exceed specified limitations. The Transferee acknowledges and understands that the conditions for resale set forth in Rule 144 have not been satisfied as of the Date of Transfer and that the Company is not required to take action to satisfy any such
conditions. 
 (iv) The Transferee will not sell, transfer or otherwise dispose of the Transferred Shares in violation of the
Securities Act, the Securities Exchange Act of 1934, or the rules promulgated thereunder, including Rule 144 under the Securities Act. The Transferee agrees that he or she will not dispose of the Transferred Shares unless and until he or she has
complied with all requirements of this Agreement applicable to the disposition of Transferred Shares and he or she has provided the Company with written assurances, in substance and form satisfactory to the Company, that (A) the proposed
disposition does not require registration of the Transferred Shares under the Securities Act or all appropriate action necessary for compliance with the registration requirements of the Securities Act or with any exemption from registration
available under the Securities Act (including Rule 144) has been taken and (B) the proposed disposition will not result in the contravention of any transfer restrictions applicable to the Transferred Shares under applicable state law. 

(v) The Transferee has received and has had access to such information as he or she considers necessary or appropriate for
deciding whether to invest in the Transferred Shares, and the Transferee has had an opportunity to ask questions and receive answers from the Company regarding the terms and conditions of the issuance of the Transferred Shares. 

(vi) The Transferee is aware that his or her investment in the Company is a speculative investment that has limited liquidity
and is subject to the risk of complete loss. The Transferee is able, without impairing his or her financial condition, to hold the Transferred Shares for an indefinite period and to suffer a complete loss of his or her investment in the Transferred
Shares. 
 (b) Securities Law Restrictions. Regardless of whether the offer and sale of Shares under the Plan have been registered
under the Securities Act or have been registered or qualified under the securities laws of any State or other relevant jurisdiction, the Company at its discretion may impose restrictions upon the sale, pledge or other transfer of the Transferred
Shares (including the placement of appropriate legends on the stock certificates (or electronic equivalent) or the imposition of stop-transfer instructions) and may refuse to transfer Shares acquired hereunder (or Shares proposed to be transferred
in a subsequent transfer) if, in the judgment of the Company, such restrictions, legends or refusal are necessary or appropriate to achieve compliance with the Securities Act or other relevant securities or other laws, including without limitation
under Regulation S of the Securities Act or pursuant to another available exemption from registration. 







 (c) Market Stand-Off. In connection with any underwritten public offering by the Company
of its equity securities pursuant to an effective registration statement filed under the Securities Act, including the Company’s initial public offering, the Transferee or a Subsequent Transferee shall not directly or indirectly sell, make any
short sale of, loan, hypothecate, pledge, offer, grant or sell any option or other contract for the purchase of, purchase any option or other contract for the sale of, or otherwise dispose of or transfer, or agree to engage in any of the foregoing
transactions with respect to, any Transferred Shares without the prior written consent of the Company or its managing underwriter. Such restriction (the “Market Stand-Off”) shall be in effect for such period of time following the date of
the final prospectus for the offering as may be requested by the Company or such underwriter. In no event, however, shall such period exceed 180 days plus such additional period as may reasonably be requested by the Company or such underwriter to
accommodate regulatory restrictions on (i) the publication or other distribution of research reports or (ii) analyst recommendations and opinions, including (without limitation) the restrictions set forth in Rule 2711(f)(4) of the National
Association of Securities Dealers and Rule 472(f)(4) of the New York Stock Exchange, as amended, or any similar successor rules. The Market Stand-Off shall in any event terminate two years after the date of the Company’s initial public
offering. In the event of the declaration of a stock dividend, a spin-off, a stock split, an adjustment in conversion ratio, a recapitalization or a similar transaction affecting the Company’s outstanding securities without receipt of
consideration, any new, substituted or additional securities which are by reason of such transaction distributed with respect to any Shares subject to the Market Stand-Off, or into which such Shares thereby become convertible, shall immediately be
subject to the Market Stand-Off. In order to enforce the Market Stand-Off, the Company may impose stop-transfer instructions with respect to the Transferred Shares until the end of the applicable stand-off period. The Company’s underwriters
shall be beneficiaries of the agreement set forth in this Subsection (c). This Subsection (c) shall not apply to Shares registered in the public offering under the Securities Act. 

(d) Rights of the Company. The Company shall not be required to (i) transfer on its books any Transferred Shares that have been
sold or transferred in contravention of this Agreement or (ii) treat as the owner of Transferred Shares, or otherwise to accord voting, dividend or liquidation rights to, any Subsequent Transferee to whom Transferred Shares have been
transferred in contravention of this Agreement. 
  

	SECTION 5.	  SUCCESSORS AND ASSIGNS. 

 Except as otherwise expressly provided to the
contrary, the provisions of this Agreement shall inure to the benefit of, and be binding upon, the Company and its successors and assigns and be binding upon the Transferee and the Transferee’s legal representatives, heirs, legatees,
distributees, assigns and transferees by operation of law, whether or not any such person has become a party to this Agreement or has agreed in writing to join herein and to be bound by the terms, conditions and restrictions hereof. 








	SECTION 6.	  NO RETENTION RIGHTS. 

 Nothing in this Agreement or in the Plan shall confer
upon the Transferee any right to continue providing services to the Company for any period of specific duration or interfere with or otherwise restrict in any way the rights of the Company (or any Parent or Subsidiary employing or retaining the
Transferee) or of the Transferee, which rights are hereby expressly reserved by each, to terminate his or her Service at any time and for any reason, with or without cause. 
  

	SECTION 7.	  TAX ELECTION. 

 The acquisition of the Transferred Shares may result in
adverse tax consequences that may be avoided or mitigated by filing an election under Code Section 83(b). Such election may be filed only within 30 days after the date of transfer set forth in the Summary of Stock Grant. The form for making the
Code Section 83(b) election is attached to this Agreement as an Exhibit. The Transferee should consult with his or her tax advisor to determine the tax consequences of acquiring the Transferred Shares and the advantages and disadvantages of
filing the Code Section 83(b) election. The Transferee acknowledges that it is his or her sole responsibility, and not the Company’s, to file a timely election under Code Section 83(b), even if the Transferee requests the Company or
its representatives to make this filing on his or her behalf. 
  

	SECTION 8.	  LEGENDS. 

 All certificates evidencing Transferred Shares shall bear the following legends:

 “THE SHARES REPRESENTED HEREBY MAY NOT BE SOLD, ASSIGNED, TRANSFERRED, ENCUMBERED OR IN ANY MANNER DISPOSED OF, EXCEPT IN COMPLIANCE
WITH THE TERMS OF A WRITTEN AGREEMENT BETWEEN THE COMPANY AND THE REGISTERED HOLDER OF THE SHARES (OR THE PREDECESSOR IN INTEREST TO THE SHARES). SUCH AGREEMENT GRANTS TO THE COMPANY CERTAIN RIGHTS OF FIRST REFUSAL UPON AN ATTEMPTED TRANSFER OF THE
SHARES AND IMPOSES CERTAIN FORFEITURE CONDITIONS UPON TERMINATION OF SERVICE WITH THE COMPANY. IN ADDITION, THE SHARES ARE SUBJECT TO RESTRICTIONS ON TRANSFER FOR A LIMITED PERIOD FOLLOWING THE EFFECTIVE DATE OF THE UNDERWRITTEN PUBLIC OFFERING OF
THE COMPANY’S SECURITIES AND MAY NOT BE SOLD OR OTHERWISE DISPOSED OF BY THE HOLDER WITHOUT THE CONSENT OF THE COMPANY OR THE MANAGING UNDERWRITER. THE SECRETARY OF THE COMPANY WILL UPON WRITTEN REQUEST FURNISH A COPY OF SUCH AGREEMENT TO THE
HOLDER HEREOF WITHOUT CHARGE.” 
 All certificates evidencing the Transferred Shares acquired under this Agreement in an unregistered transaction shall
bear the following legend (and such other restrictive legends as are required or deemed advisable under the provisions of any applicable law): 







 “THE SHARES REPRESENTED HEREBY HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS
AMENDED (THE “ACT”) OR ANY SECURITIES LAWS OF ANY U.S. STATE, AND MAY NOT BE SOLD, REOFFERED, PLEDGED, ASSIGNED, ENCUMBERED OR OTHERWISE TRANSFERRED OR DISPOSED WITHOUT AN EFFECTIVE REGISTRATION THEREOF UNDER SUCH ACT OR AN OPINION OF
COUNSEL, SATISFACTORY TO THE COMPANY AND ITS COUNSEL, THAT SUCH REGISTRATION IS NOT REQUIRED. IN THE ABSENCE OF REGISTRATION OR THE AVAILABILITY (CONFIRMED BY OPINION OF COUNSEL) OF AN ALTERNATIVE EXEMPTION FROM REGISTRATION UNDER THE ACT, THESE
SHARES MAY NOT BE SOLD, REOFFERED, PLEDGED, ASSIGNED, ENCUMBERED OR OTHERWISE TRANSFERRED OR DISPOSED OF EXCEPT IN ACCORDANCE WITH REGULATION S (RULES 901 THROUGH 905 AND PRELIMINARY NOTES) OF THE ACT. HEDGING TRANSACTIONS INVOLVING THESE SHARES MAY
NOT BE CONDUCTED UNLESS IN COMPLIANCE WITH THE ACT.” 
 If required by the authorities of any State in connection with the issuance of the Transferred
Shares, the legend or legends required by such State authorities shall also be endorsed on all such certificates. 
  

	SECTION 9.	  MISCELLANEOUS PROVISIONS. 

 (a) Choice of Law. This Agreement shall be
governed by, and construed in accordance with, the laws of the State of Delaware, as such laws are applied to contracts entered into and performed in such State. 

(b) Notice. Any notice required by the terms of this Agreement shall be given in writing. It shall be deemed effective upon
(i) personal delivery, (ii) deposit with the United States Postal Service, by registered or certified mail, with postage and fees prepaid, (iii) deposit with Federal Express Corporation, with shipping charges prepaid or
(iv) deposit with any internationally recognized express mail courier service. Notice shall be addressed to the Company at its principal executive office and to the Transferee at the address that he or she most recently provided to the Company
in accordance with this Subsection (c). 
 (c) Entire Agreement. The Summary of Stock Grant, this Agreement and the Plan constitute
the entire contract between the parties hereto with regard to the subject matter hereof. They supersede any other agreements, representations or understandings (whether oral or written and whether express or implied) that relate to the subject
matter hereof. 
  

	SECTION 10.	  ACKNOWLEDGEMENTS OF THE TRANSFEREE. 

 In addition to the other terms,
conditions and restrictions imposed on the Shares acquired pursuant to this Agreement, the Transferee expressly acknowledges being subject to Sections 2 (Forfeiture Condition), 3 (Right of First Refusal) and 4 (Other Restrictions on Transfer,
including without limitation the Market Stand-Off), as well as the following provisions: 







 (a) Waiver of Statutory Information Rights. The Transferee acknowledges and agrees that,
until the first sale of the Company’s Stock to the general public pursuant to a registration statement filed under the Securities Act, he or she will be deemed to have waived any rights the Transferee might otherwise have had under
Section 220 of the Delaware General Corporation Law to inspect for any proper purpose and to make copies and extracts from the Company’s stock ledger, a list of its stockholders and its other books and records or the books and records of
any subsidiary. This waiver applies only in the Transferee’s capacity as a stockholder and does not affect any other inspection rights the Transferee may have under other law or pursuant to a written agreement with the Company. 

(b) Plan Discretionary. The Transferee understands and acknowledges that (i) the Plan is entirely discretionary, (ii) the
Company and the Transferee’s employer have reserved the right to amend, suspend or terminate the Plan at any time, (iii) the transfer of the Transferred Shares does not in any way create any contractual or other right to receive additional
awards under the Plan at any time or in any amount and (iv) all determinations with respect to any additional awards, including (without limitation) the times when awards will be granted, the number of Shares offered and the vesting schedule,
will be at the sole discretion of the Company. 
 (c) Termination of Service. The Transferee understands and acknowledges that
participation in the Plan ceases upon termination of his or her Service for any reason, except as may explicitly be provided otherwise in the Plan or this Agreement. 

(d) Extraordinary Compensation. The value of the Transferred Shares shall be an extraordinary item of compensation outside the scope of
the Transferee’s employment contract, if any, and shall not be considered a part of his or her normal or expected compensation for purposes of calculating severance, resignation, redundancy or end-of-service payments, bonuses, long-service
awards, pension or retirement benefits or similar payments. 
 (e) Authorization to Disclose. The Transferee hereby authorizes and
directs the Transferee’s employer to disclose to the Company or any Subsidiary any information regarding the Transferee’s employment, the nature and amount of the Transferee’s compensation and the fact and conditions of the
Transferee’s participation in the Plan, as the Transferee’s employer deems necessary or appropriate to facilitate the administration of the Plan. 

(f) Personal Data Authorization. The Transferee consents to the collection, use and transfer of personal data as described in this
Subsection (f). The Transferee understands and acknowledges that the Company, the Transferee’s employer and the Company’s other Subsidiaries hold certain personal information regarding the Transferee for the purpose of managing and
administering the Plan, including (without limitation) the Transferee’s name, home address, telephone number, date of birth, social insurance number, salary, nationality, job title, any Shares or directorships held in the Company and details of
all options or any other entitlements to Shares awarded, canceled, exercised, vested, unvested or outstanding in the Transferee’s favor (the “Data”). The Transferee further understands and acknowledges that the Company and/or
its Subsidiaries will transfer Data among themselves as necessary for the purpose of implementation, administration and management of the Transferee’s participation in the Plan and that the Company and/or any Subsidiary may each further
transfer Data to any third party assisting the Company in the implementation, administration and management of the Plan. 







 
The Transferee understands and acknowledges that the recipients of Data may be located in the United States or elsewhere. The Transferee authorizes such recipients to receive, possess, use,
retain and transfer Data, in electronic or other form, for the purpose of administering the Transferee’s participation in the Plan, including a transfer to any broker or other third party with whom the Transferee elects to deposit Shares
acquired under the Plan of such Data as may be required for the administration of the Plan and/or the subsequent holding of Shares on the Transferee’s behalf. The Transferee may, at any time, view the Data, require any necessary modifications
of Data or withdraw the consents set forth in this Subsection (f) by contacting the Company in writing. 
  

	SECTION 11.	  DEFINITIONS. 

 (a) “Agreement” shall mean this Stock Grant
Agreement. 
 (b) “Board of Directors” shall mean the Board of Directors of the Company, as constituted from time to time
or, if a Committee has been appointed, such Committee. 
 (c) “Company” shall mean Dimension Therapeutics, Inc., a Delaware
corporation. 
 (d) “Forfeiture Condition” shall mean the forfeiture condition described in Section 2. 

(e) “Immediate Family” shall mean any child, stepchild, grandchild, parent, stepparent, grandparent, spouse, sibling,
mother-in-law, father-in-law, son-in-law, daughter-in-law, brother-in-law or sister-in-law and shall include adoptive relationships. 
 (f)
“Plan” shall mean the Dimension Therapeutics, Inc. 2013 Stock Plan, as amended. 
 (g) “Restricted Share”
shall mean a Transferred Share that is subject to the Forfeiture Condition. 
 (h) “Right of First Refusal” shall mean the
Company’s right of first refusal described in Section 3. 
 (i) “Subsequent Transferee” shall mean any person to
whom the Transferee has directly or indirectly transferred any Transferred Shares. 
 (j) “Transferee” shall mean the
individual named in the Summary of Stock Grant. 
 (k) “Transfer Notice” shall mean the notice of a proposed transfer of
Transferred Shares described in Section 3. 
 (l) “Transferred Shares” shall mean the Shares acquired by the
Transferee pursuant to this Agreement. 







 EXHIBIT I 

SECTION 83(b) ELECTION 
 The undersigned
taxpayer hereby elects, pursuant to Section 83(b) of the Internal Revenue Code of 1986, as amended, and pursuant to Treasury Regulations Section 1.83-2, to include in gross income as compensation for services the fair market value of the shares
described below. 
  

	 	(1)	The taxpayer who performed the services is: 

  

	 	    	Name:                            
                                         
                    

  

	 	    	Address:                         
                                         
                   

  

	 	    	Social Security No.:                        
                                         
  

  

	 	(2)	The property with respect to which the election is made is                  shares of the common stock of Dimension Therapeutics,
Inc. 

  

	 	(3)	The property was transferred to the taxpayer on                 ,
                    . 

  

	 	(4)	The taxable year for which the election is made is the calendar year             . 


 

	 	(5)	The property is subject to forfeiture if for any reason taxpayer’s service with the issuer terminates. The forfeiture condition lapses in a series of installments over a
        -year period ending on                 ,
                    . 

  

	 	(6)	The fair market value of such property at the time of transfer (determined without regard to any restriction other than a restriction that by its terms will never lapse) is
$        per share x                  shares = $        . 


 

	 	(7)	No amount was paid for such property. 

  

	 	(8)	The amount to include in gross income is $        . [The amount in Line 6.] 

  

	 	(9)	A copy of this statement was furnished to Dimension Therapeutics, Inc., for whom taxpayer rendered the services underlying the transfer of such property. 


 

	 	(10)	This statement is executed on                 ,
                    . 

  

			
	  
	  	  

	Spouse (if any)	  	Taxpayer

 Within 30 days after the date of transfer of the property, this election must be filed with the Internal Revenue Service
office where the taxpayer files his or her annual federal income tax return. The filing should be made by registered or certified mail, return receipt requested. The taxpayer must (a) include a copy of the completed form with his or her federal
income tax return for the taxable year in which the property is transferred and (b) deliver an additional copy to the Company.