Document:

EX-10.10

 Exhibit 10.10 
  

 
 HOMOLOGY MEDICINES, INC. 

March 31, 2016 
 Arthur O. Tzianabos 

[***] 
 Dear Arthur: 

It is my pleasure to offer you employment with Homology Medicines, Inc. (the “Company”). Your position Will be President and Chief
Executive Officer, reporting to the Board of Directors. Your effective date of hire as a regular, full-time employee Will be March 31, 2016 (your “Start Date”). 

Compensation: This is an exempt position. The Company will pay you an annual salary of $410,000.00, payable in accordance with the
Company’s regular payroll practices and subject to all applicable tax reporting and withholding requirements. 
 Signing Bonus: The
Company agrees to pay you a one-time cash signing bonus in an amount equal to $15,000 (the “Signing Bonus”), payable upon the Company’s first regular payroll date following the Start Date
and subject to all applicable tax reporting and withholding requirements. Notwithstanding the foregoing, you acknowledge and agree that if you voluntarily resign or the Company terminates your employment for Cause (each, a “Termination
Event”) prior to the first anniversary of your Start Date, you shall repay 50% of the Signing Bonus to the Company within 30 days of such Termination Event without reduction for any taxes withheld by the Company upon its payment to you of
the Signing Bonus. By signing below, you authorize the Company to immediately offset against and reduce any amounts otherwise due to you for any amounts due to the Company in respect of your obligation to repay the Signing Bonus under this
paragraph. 
 Performance Bonus: During employment, you will be eligible for yearly bonuses with an initial bonus target of 40% of your base
salary, based on the achievement, as determined by the Company’s Board of Directors (the “Board”), of goals and objectives established by the Board. Any bonus you receive will be paid on or before March 31 of the year
following the calendar year in which it is earned, provided that you remain employed by the Company at the time of such payment (unless you have died prior to the time of such payment, in which case the amount of any bonus earned by you prior to
your death will be paid to your estate). Your compensation package, including your base salary and target bonus, will be reviewed at least annually by the Company following discussion between yourself and the Board. 

Equity: As soon as practicable following your Start Date, and subject to Board approval, the Company will grant you an incentive stock option
(the “Option”) to purchase 3,432,221 shares of the Company’s common stock, $0.0001 par value per share (the “Common Stock”), at an 

 exercise price equal to the fair market value per share on the date of grant. The Options will vest (i.e., become
exercisable) as to 25% of the underlying shares on the first anniversary of your Start Date, and as to an additional 2.0833% of the underlying shares every month thereafter, so that the Options will be fully vested on the four-year anniversary of
your Start Date. In addition, if you are terminated without Cause (as defined below) within twelve (12) months of a Change in Control Transaction (as defined in the Company’s 2015 Stock Incentive Plan) or during the thirty (30) day
period preceding a Change in Control Transaction in anticipation of such Change in Control Transaction, 100% of the shares subject to the Options that are unvested at the time of such termination shall immediately vest. The Options will be subject
to the Company’s 2015 Stock Incentive Plan and to the terms of the Company’s standard form of incentive stock option agreement. 

Additional Benefits: As a regular, full-time employee you are eligible to participate in any employee benefit plans which the Company may offer
to its senior executives. These plans may, from time to time, be amended or terminated with or without prior notice. You will also be eligible for twenty (20) days of paid time off (PTO), five (5) of which may be carried over from year to
year if unused. You will also be entitled to paid legal holidays in accordance with the Company’s normal policies. Additionally, you shall be entitled to indemnification to the full extent authorized under the Company’s Certificate of
Incorporation and By-laws as in effect from time to time. 
 Expense Reimbursement. You will be
reimbursed for all travel, lodging, meals and other out-of-pocket expenses, including but not limited to, all costs associated with your smart phone, incurred by you in
the performance of your duties, in accordance with the expense reimbursement policies of the Company for senior executives as in effect from time to time. 

Term of Employment: Your employment at all times will be at will, meaning that you are not being offered employment for a definite period and
that either you or the Company may terminate the employment relationship at any time for any reason. We request, however, that you provide us with at least thirty days’ advance notice in the event that you intend to terminate your employment.

 Termination by the Company Without Cause or Resignation by you with Good Reason: If your employment is terminated by the Company without
Cause (as defined below) or if you voluntarily terminate your employment for Good Reason (as defined below), you will receive a severance of nine (9) months base salary continuation (paid on the Company’s normal payroll cycle) and nine
(9) months reimbursement of COBRA premiums for health and dental benefit coverage for you and your immediate family (less applicable contributions by you and paid on the Company’s normal payroll cycle for benefits), as in effect
immediately prior to such termination or resignation, so long as you are eligible to elect comparable COBRA benefits, provided, however that if the Company’s health and dental insurance plan is terminated during such nine (9) months period
such that you are no longer eligible for COBRA benefits, the Company will pay to you the amount of the COBRA premiums for health and benefit coverage that otherwise would have been paid to the health and dental insurance provider, which amount shall
be paid on the same schedule as the Company would have paid such premiums to the provider for the remaining portion of such nine (9) months period. 

 As used herein, “Cause” shall mean: (i) any gross or willful misconduct or material breach
of any Company policy, agreement with, or duty owed to the Company, (ii) theft, fraud, embezzlement, or other intentional misappropriation of Company funds, (iii) destruction of Company property with intent or through gross negligence,
(iv) any material breach of any obligation or duty to the Company or any of its affiliates (whether arising by statute, common law, or agreement) relating to confidentiality, noncompetition,
non-solicitation, or proprietary rights; or (iv) commission of, indictment or conviction of, or pleading guilty or nolo contendere to, a misdemeanor where imprisonment is imposed, other than for a
traffic-related offense, a felony, or any crime involving fraud or dishonesty. 
 As a condition to the Company’s obligation to make the severance
payments described above, you must (i) timely execute a customary release of work-related claims against the Company and its affiliates that is in the form substantially similar to the one attached hereto as Exhibit A, which will be presented
to you at time of termination and (ii) abide by your post-termination obligations contained in the NDA (as defined below). 
 As use herein,
“Good Reason” shall mean the occurrence of any of the following events without your consent: 
 (i)
assignment to you of any duties inconsistent in any material respect with to your position (including titles and reporting relationships), authority, duties and/or responsibilities as contemplated by this Agreement; and/or 

(ii) any material failure by the Company to comply with any of the provisions regarding your Base Salary payable to you under
this Agreement. 
 Notwithstanding the foregoing, no Good Reason will have occurred unless and until you have: (a) provided the
Company, within 60 days of your knowledge of the occurrence of the facts and circumstances underlying the Good Reason event, written notice stating with specificity the applicable facts and circumstances underlying such finding of Good Reason;
(b) provided the Company with an opportunity to cure the same within 30 days after the receipt of such notice; and (c) your resignation of employment must occur within 30 days after expiration of the 30 day period set forth in the
foregoing clause (b). 
 Employment Eligibility: The Immigration Reform and Control Act requires employers to verify employment eligibility
and identity of new employees. On your first day of employment, you must provide us with appropriate documents to establish your eligibility to work in the United States (e.g., social security card, driver’s license, US passport). We will not
be able to employ you if you fail to comply with this requirement. 
 Work Environment: The Company maintains a smoke-free, drug-free
workplace policy and supports equal employment opportunities for all of its employees. 
 No Conflicts: By accepting this offer below, you
represent (i) that you are subject to no agreements which might restrict your conduct at the Company, and (ii) that you understand that if you become aware at any time during your employment with the Company that you are subject to any
agreements which might restrict your conduct at the Company, you are required to immediately inform the Company of the existence of such agreements or your employment by the Company will be subject to immediate termination. 

 Employee NDA: Enclosed for your review is a copy of the Company’s Non-Disclosure, Non-Competition and Assignment of Intellectual Property Agreement (“NDA’’). This offer is conditioned on your signing the NDA and your
continued willingness thereafter to abide by the terms of the NDA. You are required to sign the NDA when you countersign this offer letter. 

Background Check: This offer is contingent upon satisfactory completion of a background check. The enclosed Background Check Disclosure and
Authorization details the scope of the background check that may be done by the Company. Please review this disclosure and promptly return your signed authorization. If you decide not to authorize a background check, your offer will be rescinded and
your acceptance of our offer declined. 
 Miscellaneous: 

This letter constitutes our entire offer regarding the terms and conditions of your employment with the Company. It supersedes any other agreements or promises
made to you by anyone regarding the subject matter hereof, whether oral or written. This letter is binding on its successors and assigns. 
 Please indicate
your acceptance of this offer by signing below and returning a signed copy of this letter to my attention at the Company by Monday April 4, 2016. 
 We
are looking forward to working with you. Please contact me if you have any questions or need more information. 
  

					
		 		  	Sincerely,
			
		 		  	 /s/ Kush Parmar

		 		  	Kush Parmar, President
			
	 Accepted and agreed:
	 		  	
			
	 /s/ Arthur Tzianabos
	 		  	3/31/2016
	 Arthur Tzianabos
	 		  	DateEX-10.11

 Exhibit 10.11 

Employment Agreement 

This Employment Agreement (this “Agreement”), dated as of March 18, 2018, is made by and between Homology Medicines, Inc., a
Delaware corporation (together with any successor thereto, the “Company”), and Siyamak Rasty (“Executive”) (collectively referred to herein as the “Parties” or individually referred to as a
“Party”). 
 RECITALS 
  

	A.	It is the desire of the Company to assure itself of the services of Executive as of the Effective Date (as defined below) and thereafter by entering into this Agreement. 

 

	B.	Executive and the Company mutually desire that Executive provide services to the Company on the terms herein provided. 

AGREEMENT 
 NOW,
THEREFORE, in consideration of the foregoing and of the respective covenants and agreements set forth below, the Parties hereto agree as follows: 
  

	1.	Employment. 

 (a) General. Effective on the consummation of the
Company’s initial public offering of stock pursuant to an effective registration statement filed under the Securities Act of 1933, as amended (the “Effective Date”), the Company shall employ Executive, and Executive shall
remain in the employ of the Company, for the period and in the positions set forth in this Section 1, and subject to the other terms and conditions herein provided. 

(b) At-Will Employment. The Company and Executive acknowledge that Executive’s employment
is and shall continue to be at-will, as defined under applicable law, and that Executive’s employment with the Company may be terminated by either Party at any time for any or no reason (subject to the
notice requirements of Section 3(b)). This “at-will” nature of Executive’s employment shall remain unchanged during Executive’s tenure as an employee and may not
be changed, except in an express writing signed by Executive and a duly authorized officer of the Company. If Executive’s employment terminates for any reason, Executive shall not be entitled to any payments, benefits, damages, award or
compensation other than as provided in this Agreement or otherwise agreed to in writing by the Company or as provided by applicable law. The term of this Agreement (the “Term”) shall commence on the Effective Date and end on the
date this Agreement is terminated under Section 3. 
 (c) Positions and Duties. During the Term, Executive
shall serve as Chief Operating Officer of the Company, with such responsibilities, duties and authority normally associated with such position and as may from time to time be assigned to Executive by the Chief Executive Officer of the Company.
Executive shall devote substantially all of Executive’s working time and efforts to the business and affairs of the Company (which shall include service to its affiliates, if applicable) and shall not engage in outside business activities
(including serving on outside boards or committees) without the consent of the Board of Directors of the Company or an authorized committee of the Board (in either case, the “Board”), provided that Executive shall be permitted to
(i) manage Executive’s personal, financial and legal affairs, (ii) participate in trade associations, and (iii) serve on the board of directors of
not-for-profit or tax-exempt charitable organizations, in each case, subject to compliance with this Agreement and provided that
such activities do not materially interfere with Executive’s performance of Executive’s duties and responsibilities hereunder. Executive agrees to observe and comply with the rules and policies of the Company as adopted by the Company from
time to time, in each case, as amended from time to time, and as delivered or made available to Executive (each, a “Policy”). 

	2.	Compensation and Related Matters. 

 (a) Annual Base Salary. During the
Term, Executive shall receive a base salary at a rate of $358,852 per annum, which shall be paid in accordance with the customary payroll practices of the Company and shall be pro-rated for partial years of
employment. Such annual base salary shall be reviewed (and may be adjusted) from time to time by the Board (such annual base salary, as it may be adjusted from time to time, the “Annual Base Salary”). 

(b) Annual Cash Bonus Opportunity. During the Term, Executive will be eligible to participate in an annual incentive program
established by the Board. Executive’s annual incentive compensation under such incentive program (the “Annual Bonus”) shall be targeted at 35% of Executive’s Annual Base Salary (such target, as may be increased by the
Board from time to time, the “Target Annual Bonus”). The Annual Bonus payable under the incentive program shall be based on the achievement of performance goals to be determined by the Board. The payment of any Annual Bonus pursuant
to the incentive program shall be subject to Executive’s continued employment with the Company through the date of payment, except as otherwise provided in Section 4(b). 

(c) Benefits. During the Term, Executive shall be eligible to participate in employee benefit plans, programs and arrangements of the
Company (including medical, dental and 401(k) plans), subject to the terms and eligibility requirements thereof and as such plans, programs and arrangements may be amended from time to time. In no event shall Executive be eligible to participate in
any severance plan or program of the Company, except as set forth in Section 4 of this Agreement. 
 (d)
Vacation. During the Term, Executive shall be entitled to paid personal leave in accordance with the Company’s Policies. Any vacation shall be taken at the reasonable and mutual convenience of the Company and Executive. 

(e) Business Expenses. During the Term, the Company shall reimburse Executive for all reasonable travel and other business expenses
incurred by Executive in the performance of Executive’s duties to the Company in accordance with the Company’s expense reimbursement Policy. 

(f) Key Person Insurance. At any time during the Term, the Company shall have the right (but not the obligation) to insure the life of
Executive for the Company’s sole benefit. The Company shall have the right to determine the amount of insurance and the type of policy. Executive shall reasonably cooperate with the Company in obtaining such insurance by submitting to physical
examinations, by supplying all information reasonably required by any insurance carrier, and by executing all necessary documents reasonably required by any insurance carrier, provided that any information provided to an insurance company or broker
shall not be provided to the Company without the prior written authorization of Executive. Executive shall incur no financial obligation by executing any required document, and shall have no interest in any such policy. 

 

	3.	Termination. 

 Executive’s employment hereunder and the Term may be
terminated by the Company or Executive, as applicable, without any breach of this Agreement under the following circumstances and the Term will end on the Date of Termination: 

  
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 (a) Circumstances. 

(i) Death. Executive’s employment hereunder shall terminate upon Executive’s death. 

(ii) Disability. If Executive has incurred a Disability, as defined below, the Company may terminate Executive’s
employment. 
 (iii) Termination for Cause. The Company may terminate Executive’s employment for Cause, as
defined below. 
 (iv) Termination without Cause. The Company may terminate Executive’s employment without Cause.

 (v) Resignation from the Company with Good Reason. Executive may resign Executive’s employment with the
Company with Good Reason, as defined below. 
 (vi) Resignation from the Company without Good Reason. Executive may
resign Executive’s employment with the Company for any reason other than Good Reason or for no reason. 
 (b) Notice of
Termination. Any termination of Executive’s employment by the Company or by Executive under this Section 3 (other than termination pursuant to Section 3(a)(i)) shall be communicated by a
written notice to the other Party hereto (i) indicating the specific termination provision in this Agreement relied upon, (ii) setting forth in reasonable detail the facts and circumstances claimed to provide a basis for termination of
Executive’s employment under the provision so indicated, if applicable, and (iii) specifying a Date of Termination which, if submitted by Executive, shall be at least thirty (30) days following the date of such notice (a
“Notice of Termination”); provided, however, that in the event that Executive delivers a Notice of Termination to the Company, the Company may, in its sole discretion, change the Date of Termination to any date that occurs
following the date of Company’s receipt of such Notice of Termination and is prior to the date specified in such Notice of Termination, but the termination will still be considered a resignation by Executive. A Notice of Termination submitted
by the Company may provide for a Date of Termination on the date Executive receives the Notice of Termination, or any date thereafter elected by the Company. The failure by either Party to set forth in the Notice of Termination any fact or
circumstance which contributes to a showing of Cause or Good Reason shall not waive any right of the Party hereunder or preclude the Party from asserting such fact or circumstance in enforcing the Party’s rights hereunder. 

(c) Company Obligations upon Termination. Upon termination of Executive’s employment pursuant to any of the circumstances listed in
this Section 3, Executive (or Executive’s estate) shall be entitled to receive the sum of: (i) the portion of Executive’s Annual Base Salary earned through the Date of Termination, but not yet paid to
Executive; (ii) any expense reimbursements owed to Executive pursuant to Section 2(e); and (iii) any amount accrued and arising from Executive’s participation in, or benefits accrued under any employee
benefit plans, programs or arrangements, which amounts shall be payable in accordance with the terms and conditions of such employee benefit plans, programs or arrangements (collectively, the “Company Arrangements”). Except as
otherwise expressly required by law (e.g., COBRA) or as specifically provided herein, all of Executive’s rights to salary, severance, benefits, bonuses and other compensatory amounts hereunder (if any) shall cease upon the termination of
Executive’s employment hereunder. In the event that Executive’s employment is terminated by the Company for any reason, Executive’s sole and exclusive remedy shall be to receive the payments and benefits described in this
Section 3(c) or Section 4, as applicable. 

  
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 (d) Deemed Resignation. Upon termination of Executive’s employment for any reason,
Executive shall be deemed to have resigned from all offices and directorships, if any, then held with the Company or any of its subsidiaries. 
  

	4.	Severance Payments. 

 (a) Termination for Cause, or Termination Upon Death,
Disability or Resignation from the Company Without Good Reason. If Executive’s employment shall terminate as a result of Executive’s death pursuant to Section 3(a)(i) or Disability pursuant to
Section 3(a)(ii), pursuant to Section 3(a)(iii) for Cause, or pursuant to Section 3(a)(iv) for Executive’s resignation from the Company without Good Reason, then
Executive shall not be entitled to any severance payments or benefits, except as provided in Section 3(c). 
 (b)
Termination without Cause, or Resignation from the Company with Good Reason. If Executive’s employment terminates without Cause pursuant to Section 3(a)(iv), or pursuant to
Section 3(a)(v) due to Executive’s resignation with Good Reason, then, subject to Executive signing on or before the 21st day following Executive’s Separation from Service (as defined below), and not revoking, a
release of claims substantially in the form attached as Exhibit A to this Agreement (the “Release”), and Executive’s continued compliance with Section 5, Executive shall receive, in addition to
payments and benefits set forth in Section 3(c), the following: 
 (i) an amount in cash equal to
0.75 times the Annual Base Salary, payable in the form of salary continuation in regular installments over the 9-month period following the date of Executive’s Separation from Service (the
“Severance Period”) in accordance with the Company’s normal payroll practices; 
 (ii) to the extent
unpaid as of the Date of Termination, an amount of cash equal to any Annual Bonus earned by Executive for the Company’s fiscal year prior to the fiscal year in which the Date of Termination occurs, as determined by the Board in its discretion
based upon actual performance achieved, which Annual Bonus, if any, shall be paid to Executive in the fiscal year in which the Date of Termination occurs when bonuses for such prior fiscal year are paid in the ordinary course to actively employed
senior executives of the Company; and 
 (iii) if Executive timely elects to receive continued medical, dental or vision
coverage under one or more of the Company’s group medical, dental or vision plans pursuant to the Consolidated Omnibus Budget Reconciliation Act of 1985, as amended (“COBRA”), then the Company shall directly pay, or reimburse
Executive for, the COBRA premiums for Executive and Executive’s covered dependents under such plans, less the amount Executive would have had to pay to receive such coverage as an active employee based on the cost sharing levels in effect on
the Date of Termination, during the period commencing on Executive’s Separation from Service and ending upon the earliest of (X) the last day of the Severance Period, (Y) the date that Executive and/or Executive’s covered
dependents become no longer eligible for COBRA or (Z) the date Executive becomes eligible to receive medical, dental or vision coverage, as applicable, from a subsequent employer (and Executive agrees to promptly notify the Company of such
eligibility). Notwithstanding the foregoing, if the Company determines in its sole discretion that it cannot provide the foregoing benefit without potentially violating applicable law (including, without limitation, Section 2716 of the Public
Health Service Act) or incurring an excise tax, the Company shall in lieu thereof provide to Executive a taxable monthly payment in an amount equal to the monthly COBRA premium that Executive would be required to pay to continue Executive’s and
Executive’s covered dependents’ group health coverage in effect on the Date of Termination (which amount shall be based on the premium for the first month of COBRA 

  
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coverage), less the amount Executive would have had to pay to receive group health coverage as an active employee for Executive and his or her covered dependents based on the cost sharing levels
in effect on the Date of Termination, which payments shall be made regardless of whether Executive elects COBRA continuation coverage and shall commence in the month following the month in which the Date of Termination occurs and shall end on the
earliest of (X) the last day of the Severance Period, (Y) the date that Executive and/or Executive’s covered dependents become no longer eligible for COBRA or (Z) the date Executive becomes eligible to receive healthcare coverage
from a subsequent employer (and Executive agrees to promptly notify the Company of such eligibility). 
 (c) Change in Control. In
lieu of the payments and benefits set forth in Section 4(b), in the event Executive’s employment terminates without Cause pursuant to Section 3(a)(iv), or pursuant to
Section 3(a)(v) due to Executive’s resignation with Good Reason, in either case, on or within twelve (12) months following the date of a Change in Control, subject to Executive signing on or before the 21st day
following Executive’s Separation from Service, and not revoking, the Release, Executive shall receive, in addition to payments and benefits set forth in Section 3(c), the following: 

(i) an amount in cash equal to 1.0 times the sum of (A) the Annual Base Salary plus (B) the Target Annual Bonus,
payable in equal installments over the 12-month period following the date of Executive’s Separation from Service (the “CIC Severance Period”) in accordance with the Company’s normal
payroll practices; 
 (ii) the payment set forth in Section 4(b)(ii); 

(iii) the benefits set forth in Section 4(b)(iii), provided that the “Severance Period” will mean the CIC
Severance Period; and 
 (iv) all unvested equity or equity-based awards held by Executive under any Company equity
compensation plans that vest solely based on the passage of time shall immediately become 100% vested (for the avoidance of doubt, with any such awards that vest in whole or in part based on the attainment of performance-vesting conditions being
governed by the terms of the applicable award agreement). 
 (d) Survival. Notwithstanding anything to the contrary in this Agreement,
the provisions of Sections 5 through 9 will survive the termination of Executive’s employment and the termination of the Term. 

5.     Restrictive Covenants. As a condition to the effectiveness of this Agreement, Executive will have
executed and delivered to the Company no later than contemporaneously herewith the Employee Proprietary Information and Inventions Assignment Agreement attached as Exhibit B (the “Restrictive Covenant Agreement”). Executive
agrees to abide by the terms of the Restrictive Covenant Agreement, which are hereby incorporated by reference into this Agreement. Executive acknowledges that the provisions of the Restrictive Covenant Agreement will survive the termination of
Executive’s employment and the termination of the Term for the periods set forth in the Restrictive Covenant Agreement. 
  

	6.	Assignment and Successors. 

 The Company may assign its rights and obligations
under this Agreement to any of its affiliates or to any successor to all or substantially all of the business or the assets of the Company (by merger or otherwise), and may assign or encumber this Agreement and its rights hereunder as security for

  
 5 

 
indebtedness of the Company and its affiliates. This Agreement shall be binding upon and inure to the benefit of the Company, Executive and their respective successors, assigns, personnel and
legal representatives, executors, administrators, heirs, distributees, devisees, and legatees, as applicable. None of Executive’s rights or obligations may be assigned or transferred by Executive, other than Executive’s rights to payments
hereunder, which may be transferred only by will or operation of law. Notwithstanding the foregoing, Executive shall be entitled, to the extent permitted under applicable law and applicable Company Arrangements, to select and change a beneficiary or
beneficiaries to receive compensation hereunder following Executive’s death by giving written notice thereof to the Company. 
  

	7.	Certain Definitions. 

 (a) Cause. The Company shall have “Cause”
to terminate Executive’s employment hereunder upon: 
 (i) The Board’s reasonable, good faith determination that
Executive has refused to (A) substantially perform the duties associated with Executive’s position with the Company or (B) carry out the reasonable and lawful instructions of the Board concerning duties or actions consistent with the
Executive’s position with the Company; 
 (ii) Executive’s breach of a material provision of this Agreement that,
to the extent capable of cure, has remained uncured for a period of thirty (30) days following written notice from the Company; 

(iii) Executive’s conviction, plea of no contest, plea of nolo contendere, or imposition of unadjudicated probation
for any felony or crime involving moral turpitude; 
 (iv) Executive’s unlawful use (including being under the
influence) or possession of illegal drugs on the Company’s (or any of its affiliate’s) premises or while performing Executive’s duties and responsibilities under this Agreement; or 

(v) Executive’s commission of any act of fraud, embezzlement, misappropriation, willful misconduct, or breach of fiduciary
duty against the Company or any of its affiliates. 
 (b) Change in Control. “Change in Control” shall have the meaning set
forth in the Homology Medicines, Inc. 2018 Incentive Award Plan. 
 (c) Code. “Code” shall mean the Internal Revenue Code of
1986, as amended, and the regulations and guidance promulgated thereunder. 
 (d) Date of Termination. “Date of Termination”
shall mean (i) if Executive’s employment is terminated by Executive’s death, the date of Executive’s death; or (ii) if Executive’s employment is terminated pursuant to Section 3(a)(ii) –
(vi) either the date indicated in the Notice of Termination or the date specified by the Company pursuant to Section 3(b), whichever is earlier. 

(e) Disability. “Disability” shall mean, at any time the Company or any of its affiliates sponsors a long-term disability plan
for the Company’s employees, “disability” as defined in such long-term disability plan for the purpose of determining a participant’s eligibility for benefits, provided, however, if the long-term disability plan contains
multiple definitions of disability, “Disability” shall refer to that definition of disability which, if Executive qualified for such disability benefits, would provide coverage for the longest period of time. The determination of whether
Executive has a Disability shall be made by the person or persons required to make disability determinations under the long-term disability 

  
 6 

 
plan. At any time the Company does not sponsor a long-term disability plan for its employees, “Disability” shall mean Executive’s inability to perform, with or without reasonable
accommodation, the essential functions of Executive’s positions hereunder for a total of three months during any six-month period as a result of incapacity due to mental or physical illness as determined
by a physician selected by the Company or its insurers and acceptable to Executive or Executive’s legal representative, with such agreement as to acceptability not to be unreasonably withheld or delayed. Any refusal by Executive to submit to a
medical examination for the purpose of determining Disability shall be deemed to constitute conclusive evidence of Executive’s Disability. 

(f) Good Reason. For the sole purpose of determining Executive’s right to severance payments and benefits as described above,
Executive’s resignation will be with “Good Reason” if Executive resigns within ninety (90) days after any of the following events, unless Executive consents in writing to the applicable event: (i) a reduction in
Executive’s Annual Base Salary or Target Annual Bonus, (ii) a material decrease in Executive’s authority or areas of responsibility as are commensurate with Executive’s title or position with the Company, (iii) the
relocation of Executive’s primary office to a location more than twenty-five (25) miles from the Executive’s primary office as of the date of this Agreement or (iv) the Company’s breach of a material provision of this
Agreement. Notwithstanding the foregoing, no Good Reason will have occurred unless and until Executive has: (a) provided the Company, within sixty (60) days of Executive’s knowledge of the occurrence of the facts and circumstances
underlying the Good Reason event, written notice stating with specificity the applicable facts and circumstances underlying such finding of Good Reason; (b) provided the Company with an opportunity to cure the same within thirty (30) days
after the receipt of such notice; and (c) the Company shall have failed to so cure within such period. 
  

	8.	Parachute Payments. 

 (a) Notwithstanding any other provisions of this Agreement
or any Company equity plan or agreement, in the event that any payment or benefit by the Company or otherwise to or for the benefit of Executive, whether paid or payable or distributed or distributable pursuant to the terms of this Agreement or
otherwise (all such payments and benefits, including the payments and benefits under Section 4 hereof, being hereinafter referred to as the “Total Payments”), would be subject (in whole or in part) to the
excise tax imposed by Section 4999 of the Code (the “Excise Tax”), then the Total Payments shall be reduced (in the order provided in Section 8(b)) to the minimum extent necessary to avoid the
imposition of the Excise Tax on the Total Payments, but only if (i) the net amount of such Total Payments, as so reduced (and after subtracting the net amount of federal, state and local income and employment taxes on such reduced Total
Payments and after taking into account the phase out of itemized deductions and personal exemptions attributable to such reduced Total Payments), is greater than or equal to (ii) the net amount of such Total Payments without such reduction (but
after subtracting the net amount of federal, state and local income and employment taxes on such Total Payments and the amount of the Excise Tax to which Executive would be subject in respect of such unreduced Total Payments and after taking into
account the phase out of itemized deductions and personal exemptions attributable to such unreduced Total Payments). 
 (b) The Total
Payments shall be reduced in the following order: (i) reduction on a pro-rata basis of any cash severance payments that are exempt from Section 409A of the Code
(“Section 409A”), (ii) reduction on a pro-rata basis of any non-cash severance payments or benefits that are exempt from
Section 409A, (iii) reduction on a pro-rata basis of any other payments or benefits that are exempt from Section 409A, and (iv) reduction of any payments or benefits otherwise payable to
Executive on a pro-rata basis or such other manner that complies with Section 409A; provided, in case of clauses (ii), (iii) and (iv), that reduction of any payments attributable to the acceleration of
vesting of Company equity awards shall be first applied to Company equity awards that would otherwise vest last in time. 

  
 7 

 (c) All determinations regarding the application of this Section 8
shall be made by an accounting firm or consulting group with experience in performing calculations regarding the applicability of Section 280G of the Code and the Excise Tax selected by the Company (the “Independent Advisors”).
For purposes of determinations, no portion of the Total Payments shall be taken into account which, in the opinion of the Independent Advisors, (i) does not constitute a “parachute payment” within the meaning of
Section 280G(b)(2) of the Code (including by reason of Section 280G(b)(4)(A) of the Code) or (ii) constitutes reasonable compensation for services actually rendered, within the meaning of Section 280G(b)(4)(B) of the Code, in
excess of the “base amount” (as defined in Section 280G(b)(3) of the Code) allocable to such reasonable compensation. The costs of obtaining such determination and all related fees and expenses (including related fees and expenses
incurred in any later audit) shall be borne by the Company. 
 (d) In the event it is later determined that a greater reduction in the Total
Payments should have been made to implement the objective and intent of this Section 8, the excess amount shall be returned promptly by Executive to the Company. 

 

	9.	Miscellaneous Provisions. 

 (a) Governing Law. This Agreement shall be
governed, construed, interpreted and enforced in accordance with its express terms, and otherwise in accordance with the substantive laws of the Commonwealth of Massachusetts without reference to the principles of conflicts of law of the
Commonwealth of Massachusetts or any other jurisdiction that would result in the application of the laws of a jurisdiction other than the Commonwealth of Massachusetts, and where applicable, the laws of the United States. 

(b) Validity. The invalidity or unenforceability of any provision or provisions of this Agreement shall not affect the validity or
enforceability of any other provision of this Agreement, which shall remain in full force and effect. 
 (c) Notices. Any notice,
request, claim, demand, document and other communication hereunder to any Party shall be effective upon receipt (or refusal of receipt) and shall be in writing and delivered personally or sent by facsimile or certified or registered mail, postage
prepaid, as follows: 
 (i) If to the Company, to the [Chief Executive Officer][Chief Financial Officer] of the Company at
the Company’s headquarters, 
 (ii) If to Executive, to the last address that the Company has in its personnel records
for Executive, or 
 (iii) At any other address as any Party shall have specified by notice in writing to the other Party.

 (d) Counterparts. This Agreement may be executed in several counterparts, each of which shall be deemed to be an original, but all
of which together will constitute one and the same Agreement. Signatures delivered by facsimile or PDF shall be deemed effective for all purposes. 

(e) Entire Agreement. The terms of this Agreement, and the Restrictive Covenant Agreement incorporated herein by reference as set forth
in Section 5, are intended by the Parties to be the final expression of their agreement with respect to the subject matter hereof and supersede all prior understandings and agreements, whether written or oral, including any
prior employment offer letter or employment agreement between Executive and the Company. The Parties further intend that this Agreement shall constitute the complete and exclusive statement of their terms and that no extrinsic evidence whatsoever
may be introduced in any judicial, administrative, or other legal proceeding to vary the terms of this Agreement. 

  
 8 

 (f) Amendments; Waivers. This Agreement may not be modified, amended, or terminated except
by an instrument in writing, signed by Executive and a duly authorized officer of Company. By an instrument in writing similarly executed, Executive or a duly authorized officer of the Company may waive compliance by the other Party with any
specifically identified provision of this Agreement that such other Party was or is obligated to comply with or perform; provided, however, that such waiver shall not operate as a waiver of, or estoppel with respect to, any other or
subsequent failure. No failure to exercise and no delay in exercising any right, remedy, or power hereunder will preclude any other or further exercise of any other right, remedy, or power provided herein or by law or in equity. 

(g) Construction. This Agreement shall be deemed drafted equally by both the Parties. Its language shall be construed as a whole and
according to its fair meaning. Any presumption or principle that the language is to be construed against any Party shall not apply. The headings in this Agreement are only for convenience and are not intended to affect construction or
interpretation. Any references to paragraphs, subparagraphs, sections or subsections are to those parts of this Agreement, unless the context clearly indicates to the contrary. Also, unless the context clearly indicates to the contrary, (i) the
plural includes the singular and the singular includes the plural; (ii) “and” and “or” are each used both conjunctively and disjunctively; (iii) “any,” “all,” “each,” or “every” means
“any and all,” and “each and every”; (iv) “includes” and “including” are each “without limitation”; (v) “herein,” “hereof,” “hereunder” and other similar compounds of
the word “here” refer to the entire Agreement and not to any particular paragraph, subparagraph, section or subsection; and (vi) all pronouns and any variations thereof shall be deemed to refer to the masculine, feminine, neuter,
singular or plural as the identity of the entities or persons referred to may require. 
 (h) Arbitration. Any controversy, claim or
dispute arising out of or relating to this Agreement, shall be settled solely and exclusively by a binding arbitration process administered by JAMS/Endispute in Boston, Massachusetts. Such arbitration shall be conducted in accordance with the
then-existing JAMS/Endispute Rules of Practice and Procedure, with the following exceptions if in conflict: (i) one arbitrator who is a retired judge shall be chosen by JAMS/Endispute; (ii) each Party to the arbitration will pay one-half of the expenses and fees of the arbitrator, together with other expenses of the arbitration incurred or approved by the arbitrator; and (iii) arbitration may proceed in the absence of any Party if
written notice (pursuant to the JAMS/Endispute rules and regulations) of the proceedings has been given to such Party. Each Party shall bear its own attorney’s fees and expenses; provided that the arbitrator may assess the prevailing
Party’s fees and costs against the non-prevailing Party as part of the arbitrator’s award. The Parties agree to abide by all decisions and awards rendered in such proceedings. Such decisions and
awards rendered by the arbitrator shall be final and conclusive. All such controversies, claims or disputes shall be settled in this manner in lieu of any action at law or equity; provided, however, that nothing in this subsection shall be construed
as precluding the bringing of an action for injunctive relief or specific performance as provided in this Agreement or the Restrictive Covenant Agreement. This dispute resolution process and any arbitration hereunder shall be confidential and
neither any Party nor the neutral arbitrator shall disclose the existence, contents or results of such process without the prior written consent of all Parties, except where necessary or compelled in a court to enforce this arbitration provision or
an award from such arbitration or otherwise in a legal proceeding. If JAMS/Endispute no longer exists or is otherwise unavailable, the Parties agree that the American Arbitration Association (“AAA”) shall administer the arbitration
in accordance with its then-existing rules as modified by this subsection. In such event, all references herein to JAMS/Endispute shall mean AAA. Notwithstanding the foregoing, Executive and the Company each have the right to resolve any issue or
dispute over intellectual property rights by court action instead of arbitration. 

  
 9 

 (i) Enforcement. If any provision of this Agreement is held to be illegal, invalid or
unenforceable under present or future laws effective during the Term, such provision shall be fully severable; this Agreement shall be construed and enforced as if such illegal, invalid or unenforceable provision had never comprised a portion of
this Agreement; and the remaining provisions of this Agreement shall remain in full force and effect and shall not be affected by the illegal, invalid or unenforceable provision or by its severance from this Agreement. Furthermore, in lieu of such
illegal, invalid or unenforceable provision there shall be added automatically as part of this Agreement a provision as similar in terms to such illegal, invalid or unenforceable provision as may be possible and be legal, valid and enforceable. 

(j) Withholding. The Company shall be entitled to withhold from any amounts payable under this Agreement any federal, state, local or
foreign withholding or other taxes or charges which the Company is required to withhold. The Company shall be entitled to rely on the advice of counsel if any questions as to the amount or requirement of withholding shall arise. 

(k) Section 409A. 

(i) General. The intent of the Parties is that the payments and benefits under this Agreement comply with or be exempt
from Section 409A and, accordingly, to the maximum extent permitted, this Agreement shall be interpreted to be in compliance therewith. 

(ii) Separation from Service. Notwithstanding anything in this Agreement to the contrary, any compensation or benefits
payable under this Agreement that is designated under this Agreement as payable upon Executive’s termination of employment shall be payable only upon Executive’s “separation from service” with the Company within the meaning of
Section 409A (a “Separation from Service”) and, except as provided below, any such compensation or benefits described in Section 2 shall not be paid, or, in the case of installments, shall not commence
payment, until the thirtieth (30th) day following Executive’s Separation from Service (the “First Payment Date”). Any installment payments that would have been made to Executive during the thirty (30) day period
immediately following Executive’s Separation from Service but for the preceding sentence shall be paid to Executive on the First Payment Date and the remaining payments shall be made as provided in this Agreement. 

(iii) Specified Employee. Notwithstanding anything in this Agreement to the contrary, if Executive is deemed by the
Company at the time of Executive’s Separation from Service to be a “specified employee” for purposes of Section 409A, to the extent delayed commencement of any portion of the benefits to which Executive is entitled under this
Agreement is required in order to avoid a prohibited distribution under Section 409A, such portion of Executive’s benefits shall not be provided to Executive prior to the earlier of (i) the expiration of the six-month period measured from the date of Executive’s Separation from Service with the Company or (ii) the date of Executive’s death. Upon the first business day following the expiration of the
applicable Section 409A period, all payments deferred pursuant to the preceding sentence shall be paid in a lump sum to Executive (or Executive’s estate or beneficiaries), and any remaining payments due to Executive under this Agreement
shall be paid as otherwise provided herein. 
 (iv) Expense Reimbursements. To the extent that any reimbursements
under this Agreement are subject to Section 409A, any such reimbursements payable to Executive shall be paid to Executive no later than December 31 of the year following the year in which the expense was incurred; provided, that Executive
submits Executive’s reimbursement request promptly following the date the expense is incurred, the amount of expenses reimbursed in one year shall 

  
 10 

 
not affect the amount eligible for reimbursement in any subsequent year, other than medical expenses referred to in Section 105(b) of the Code, and Executive’s right to reimbursement
under this Agreement will not be subject to liquidation or exchange for another benefit. 
 (v) Installments.
Executive’s right to receive any installment payments under this Agreement, including without limitation any continuation salary payments that are payable on Company payroll dates, shall be treated as a right to receive a series of separate
payments and, accordingly, each such installment payment shall at all times be considered a separate and distinct payment as permitted under Section 409A. Except as otherwise permitted under Section 409A, no payment hereunder shall be
accelerated or deferred unless such acceleration or deferral would not result in additional tax or interest pursuant to Section 409A. 
  

	10.	Executive Acknowledgement. 

 Executive acknowledges that Executive has read and
understands this Agreement, is fully aware of its legal effect, has not acted in reliance upon any representations or promises made by the Company other than those contained in writing herein, and has entered into this Agreement freely based on
Executive’s own judgment. 
 [Signature Page Follows] 

  
 11 

 IN WITNESS WHEREOF, the Parties have executed this Agreement on the date and year first above
written. 
  

			
	HOMOLOGY MEDICINES, INC.
		
	By:	 	 /s/ Arthur O. Tzianabos, Ph.D.

		 	Name: Arthur O. Tzianabos, Ph.D.
		 	Title: President and Chief Executive Officer

  

	
	EXECUTIVE
	
	     /s/ Siyamak Rasty

	Siyamak Rasty

 [Signature Page to Employment Agreement] 

 EXHIBIT A 

Separation Agreement and Release 

This Separation Agreement and Release (“Agreement”) is made by and between
                     (“Executive”) and Homology Medicines, Inc. (the “Company”) (collectively referred to as the
“Parties” or individually referred to as a “Party”). Capitalized terms used but not defined in this Agreement shall have the meanings set forth in the Employment Agreement (as defined below). 

WHEREAS, the Parties have previously entered into that certain Employment Agreement, dated as of
            , 2018 (the “Employment Agreement”); and 

WHEREAS, in connection with Executive’s termination of employment with the Company or a subsidiary or affiliate of the Company effective
            , 20    , the Parties wish to resolve any and all disputes, claims, complaints, grievances, charges, actions, petitions, and demands that Executive may have
against the Company and any of the Releasees as defined below, including, but not limited to, any and all claims arising out of or in any way related to Executive’s employment with or separation from the Company or its subsidiaries or
affiliates but, for the avoidance of doubt, nothing herein will be deemed to release any rights or remedies in connection with Executive’s ownership of vested equity securities of the Company or Executive’s right to indemnification by the
Company or any of its affiliates pursuant to contract or applicable law (collectively, the “Retained Claims”). 
 NOW,
THEREFORE, in consideration of the severance payments and benefits described in Section 4 of the Employment Agreement, which, pursuant to the Employment Agreement, are conditioned on Executive’s execution and
non-revocation of this Agreement, and in consideration of the mutual promises made herein, the Company and Executive hereby agree as follows: 

1. Severance Payments and Benefits; Salary and Benefits. The Company agrees to provide Executive with the severance payments and
benefits described in Section 4 of the Employment Agreement, payable at the times set forth in, and subject to the terms and conditions of, the Employment Agreement. In addition, to the extent not already paid, and subject to the terms and
conditions of the Employment Agreement, the Company shall pay or provide to Executive all other payments or benefits described in Section 3(c) of the Employment Agreement, subject to and in accordance with the terms thereof. 

2. Release of Claims. Executive agrees that, other than with respect to the Retained Claims, the foregoing consideration represents
settlement in full of all outstanding obligations owed to Executive by the Company, any of its direct or indirect subsidiaries and affiliates, and any of its or their current and former officers, directors, equityholders, managers, employees,
agents, investors, attorneys, shareholders, administrators, affiliates, benefit plans, plan administrators, insurers, trustees, divisions, and subsidiaries and predecessor and successor corporations and assigns (collectively, the
“Releasees”). Executive, on Executive’s own behalf and on behalf of any of Executive’s affiliated companies or entities and any of their respective heirs, family members, executors, agents, and assigns, other than with
respect to the Retained Claims, hereby and forever releases the Releasees from, and agrees not to sue concerning, or in any manner to institute, prosecute, or pursue, any claim, complaint, charge, duty, obligation, or cause of action relating to any
matters of any kind, whether presently known or unknown, suspected or unsuspected, that Executive may possess against any of the Releasees arising from any omissions, acts, facts, or damages that have occurred up until and including the Effective
Date of this Agreement (as defined in Section 7 below), including, without limitation: 

 (a) any and all claims relating to or arising from Executive’s employment or service
relationship with the Company or any of its direct or indirect subsidiaries or affiliates and the termination of that relationship; 
 (b)
any and all claims relating to, or arising from, Executive’s right to purchase, or actual purchase of any shares of stock or other equity interests of the Company or any of its affiliates, including, without limitation, any claims for fraud,
misrepresentation, breach of fiduciary duty, breach of duty under applicable state law, and securities fraud under any state or federal law; 

(c) any and all claims for wrongful discharge of employment; termination in violation of public policy; discrimination; harassment;
retaliation; breach of contract, both express and implied; breach of covenant of good faith and fair dealing, both express and implied; promissory estoppel; negligent or intentional infliction of emotional distress; fraud; negligent or intentional
misrepresentation; negligent or intentional interference with contract or prospective economic advantage; unfair business practices; defamation; libel; slander; negligence; personal injury; assault; battery; invasion of privacy; false imprisonment;
conversion; and disability benefits; 
 (d) any and all claims for violation of any federal, state, or municipal statute, including, but not
limited to, Title VII of the Civil Rights Act of 1964; the Civil Rights Act of 1991; the Rehabilitation Act of 1973; the Americans with Disabilities Act of 1990; the Equal Pay Act; the Fair Labor Standards Act; the Fair Credit Reporting Act; the Age
Discrimination in Employment Act of 1967; the Older Workers Benefit Protection Act; the Employee Retirement Income Security Act of 1974; the Worker Adjustment and Retraining Notification Act; the Family and Medical Leave Act; and the Sarbanes-Oxley
Act of 2002; 
 (e) any and all claims for violation of the federal or any state constitution; 

(f) any and all claims arising out of any other laws and regulations relating to employment or employment discrimination; 

(g) any claim for any loss, cost, damage, or expense arising out of any dispute over the
non-withholding or other tax treatment of any of the proceeds received by Executive as a result of this Agreement; and 

(h) any and all claims for attorneys’ fees and costs. 

Executive agrees that the release set forth in this section shall be and remain in effect in all respects as a complete general release as to the matters
released. This release does not release claims that cannot be released as a matter of law, including, but not limited to, Executive’s right to report possible violations of federal law or regulation to any governmental agency or entity in
accordance with the provisions of and rules promulgated under Section 21F of the Securities Exchange Act of 1934 or Section 806 of the Sarbanes-Oxley Act of 2002, or any other whistleblower protection provisions of state or federal law or
regulation and any right to receive an award for information provided thereunder, Executive’s right to file a charge with or participate in a charge by the Equal Employment Opportunity Commission, or any other local, state, or federal
administrative body or government agency that is authorized to enforce or administer laws related to employment, against the Company for discrimination (with the understanding that Executive’s release of claims herein bars Executive from
recovering such monetary relief from the Company or any Releasee for any alleged discriminatory treatment), claims for unemployment compensation or any state disability insurance benefits pursuant to the terms of applicable state law,

  
 A-2 

 
claims to continued participation in certain of the Company’s group benefit plans pursuant to the terms and conditions of COBRA, claims to any benefit entitlements vested as the date of
separation of Executive’s employment, pursuant to written terms of any employee benefit plan of the Company or its affiliates and Executive’s right under applicable law and any Retained Claims. This release further does not release claims
for breach of Section 3(c) or Section 4 of the Employment Agreement. 
 3. Acknowledgment of Waiver of Claims under ADEA.
Executive understands and acknowledges that Executive is waiving and releasing any rights Executive may have under the Age Discrimination in Employment Act of 1967 (“ADEA”), and that this waiver and release is knowing and voluntary.
Executive understands and agrees that this waiver and release does not apply to any rights or claims that may arise under the ADEA after the Effective Date of this Agreement. Executive understands and acknowledges that the consideration given for
this waiver and release is in addition to anything of value to which Executive was already entitled. Executive further understands and acknowledges that Executive has been advised by this writing that: (a) Executive should consult with an
attorney prior to executing this Agreement; (b) Executive has 21 days within which to consider this Agreement; (c) Executive has 7 days following Executive’s execution of this Agreement to revoke this Agreement pursuant to written
notice to the General Counsel of the Company; (d) this Agreement shall not be effective until after the revocation period has expired; and (e) nothing in this Agreement prevents or precludes Executive from challenging or seeking a
determination in good faith of the validity of this waiver under the ADEA, nor does it impose any condition precedent, penalties, or costs for doing so, unless specifically authorized by federal law. In the event Executive signs this Agreement and
returns it to the Company in less than the 21 day period identified above, Executive hereby acknowledges that Executive has freely and voluntarily chosen to waive the time period allotted for considering this Agreement. 

4. Severability. In the event that any provision or any portion of any provision hereof or any surviving agreement made a part hereof
becomes or is declared by a court of competent jurisdiction or arbitrator to be illegal, unenforceable, or void, this Agreement shall continue in full force and effect without said provision or portion of provision. 

5. No Oral Modification. This Agreement may only be amended in a writing signed by Executive and a duly authorized officer of the
Company. 
 6. Governing Law; Dispute Resolution. This Agreement shall be subject to the provisions of Sections 9(a), 9(c), and 9(h)
of the Employment Agreement. 
 7. Effective Date. If Executive has attained or is over the age of 40 as of the date of
Executive’s termination of employment, then each Party has seven days after that Party signs this Agreement to revoke it and this Agreement will become effective on the eighth day after Executive signed this Agreement, so long as it has been
signed by the Parties and has not been revoked by either Party before that date (the “Effective Date”). If Executive has not attained the age of 40 as of the date of Executive’s termination of employment, then the
“Effective Date” shall be the date on which Executive signs this Agreement. 
 8. Voluntary Execution of Agreement.
Executive understands and agrees that Executive executed this Agreement voluntarily, without any duress or undue influence on the part or behalf of the Company or any third party, with the full intent of releasing all of Executive’s claims
against the Company and any of the other Releasees. Executive acknowledges that: (a) Executive has read this Agreement; (b) Executive has not relied upon any representations or statements made by the Company

  
 A-3 

 
that are not specifically set forth in this Agreement; (c) Executive has been represented in the preparation, negotiation, and execution of this Agreement by legal counsel of
Executive’s own choice or has elected not to retain legal counsel; (d) Executive understands the terms and consequences of this Agreement and of the releases it contains; and (e) Executive is fully aware of the legal and binding
effect of this Agreement. 
 IN WITNESS WHEREOF, the Parties have executed this Agreement on the respective dates set forth below. 

 

							
		 		 	EXECUTIVE
			
	Dated:                         	 		 	  

			
		 		 	HOMOLOGY MEDICINES, INC.
				
	Dated:                         	 		 	By:	 	  

		 		 		 	 Name:
 Title:

  
 A-4 

 EXHIBIT B 

Restrictive Covenant Agreement 

[attached] 

 HOMOLOGY MEDICINES, INC. 

EMPLOYEE PROPRIETARY INFORMATION AND INVENTIONS ASSIGNMENT AGREEMENT 

In consideration and as a condition of my employment by Homology Medicines, Inc., a Delaware corporation (together with any of its successors
or assigns collectively, the “Company”), and my receipt of the compensation paid to me by the Company in the context of that employment, and for other good and valuable consideration, the receipt and sufficiency of which is hereby
acknowledged, effective as of the date set forth below, I, the undersigned, agree as follows: 
 1. Proprietary Information. During
the term of my employment, I may receive and otherwise be exposed, directly or indirectly, to confidential and proprietary information of the Company whether in graphic, written, electronic or oral form, including without
limitation, information relating to the Company’s business, strategies, designs, products, services and technologies and any derivatives, improvements and enhancements relating to any of the foregoing, or to the Company’s
suppliers, customers or business partners (collectively “Proprietary Information”). Proprietary Information may be identified at the time of disclosure as confidential or proprietary or information which by its context would reasonably be
deemed to be confidential or proprietary. “Proprietary Information” may also include without limitation (i)(a) unpublished patent disclosures and patent applications and other filings, know-how,
trade secrets, works of authorship and other intellectual property, as well as any information regarding ideas, Inventions (as defined in Section 5), technology, and processes, including without limitation assays, sketches, schematics,
techniques, drawings, designs, descriptions, specifications and technical documentation, (b) information relating to physical, chemical or biological materials and compounds (including without limitation reagents, gene sequences, nucleic acids,
amino acids, cell lines, media, antibodies, antibody fragments, compounds, c-DNAs, antisense nucleotides, proteins, peptides and vectors), their structures, compositions, and formulations, and methods for
their handling, use, and manufacture, and processes, apparatus, models relating thereto, (c) specifications, protocols, models, designs, equipment, engineering, algorithms, software programs, software source documents, formulae,
(d) information concerning or resulting from any research and development or other project, including without limitation nonclinical and clinical data, experimental work, clinical and product development plans, results from clinical studies,
regulatory compliance information, and research, development and regulatory strategies, and (e) business and financial information, including without limitation purchasing, procurement, manufacturing, customer lists, information relating to
investors, employees, business and contractual relationships, business forecasts, sales and merchandising, business and marketing plans, product plans, and business strategies, including without limitation information the Company provides regarding
third parties, such as, but not limited to, suppliers, customers, employees, investors, or vendors; and (ii) any other information, to the extent such information contains, reflects or is based upon any of the foregoing Proprietary Information.
The Proprietary Information may also include information of a third party that is disclosed to you by the Company or such third party at the Company’s direction. 

 2. Obligations of Non-Use and Nondisclosure. I
acknowledge the confidential and secret character of the Proprietary Information, and agree that the Proprietary Information is the sole, exclusive and valuable property of the Company. Accordingly, I agree not to use the Proprietary Information
except in the performance of my authorized duties as an employee of the Company, and not to disclose all or any part of the Proprietary Information in any form to any third party, either during or after the term of my employment, without the prior
written consent of the Company on a case-by-case basis. Upon termination of my employment, I agree to cease using and to return to the Company all whole and partial
copies and derivatives of the Proprietary Information, whether in my possession or under my direct or indirect control, provided that I am entitled to retain my personal copies of (i) my compensation records, (ii) materials
distributed to stockholders generally, and (iii) this Employee Proprietary Information and Inventions Assignment Agreement (this “Agreement”). I understand that my obligations of nondisclosure with respect to Proprietary
Information shall not apply to information that I can establish by competent proof (i) was actually in the public domain at the time of disclosure or enters the public domain following disclosure other than as a result of a breach of this
Agreement, (ii) is already in my possession without breach of any obligations of confidentiality at the time of disclosure by the Company as shown by my files and records immediately prior to the time of disclosure, or (iii) is obtained by
me from a third party not under confidentiality obligations and without a breach of any obligations of confidentiality. If I become compelled by law, regulation (including without limitation the rules of any applicable securities exchange), court
order, or other governmental authority to disclose the Proprietary Information, I shall, to the extent possible and permissible under applicable law, first give the Company prompt notice. I agree to cooperate reasonably with the Company in any
proceeding to obtain a protective order or other remedy. If such protective order or other remedy is not obtained, I shall only disclose that portion of such Proprietary Information required to be disclosed, in the opinion of my legal counsel. I
shall request that confidential treatment be accorded such Proprietary Information, where available. Compulsory disclosures made pursuant to this section shall not relieve me of my obligations of confidentiality and
non-use with respect to non-compulsory disclosures. I understand that nothing herein is intended to or shall prevent me from communicating directly with, cooperating
with, or providing information to, any federal, state or local government regulator, including, but not limited to, the U.S. Securities and Exchange Commission, the U.S. Commodity Futures Trading Commission, or the U.S. Department of Justice. I
shall promptly notify my supervisor or any officer of the Company if I learn of any possible unauthorized use or disclosure of Proprietary Information and shall cooperate fully with the Company to enforce its rights in such information. 

3. Defend Trade Secrets Act Notice of Immunity Rights. I acknowledge that the Company has provided me with the following notice of
immunity rights in compliance with the requirements of the Defend Trade Secrets Act: (i) I shall not be held criminally or civilly liable under any Federal or State trade secret law for the disclosure of Proprietary Information that is made in
confidence to a Federal, State, or local government official or to an attorney solely for the purpose of reporting or investigating a suspected violation of law, (ii) I shall not be held criminally or civilly liable under any Federal or State
trade secret law for the disclosure of Proprietary Information that is made in a complaint or other document filed in a lawsuit or other proceeding, if such filing is made under seal and (iii) if I file a lawsuit for retaliation by the Company
for reporting a suspected violation of law, I may disclose the Proprietary Information to my attorney and use the Proprietary Information in the court proceeding, if I file any document containing the Proprietary Information under seal, and do not
disclose the Proprietary Information, except pursuant to court order. 

  
 2 

 4. Property of the Company. I acknowledge and agree that all notes, memoranda, reports,
drawings, blueprints, manuals, materials, data, emails and other papers and records of every kind, or other tangible or intangible materials which shall come into my possession in the course of my employment with the Company, relating to any
Proprietary Information, shall be the sole and exclusive property of the Company and I hereby assign any rights or interests I may obtain in any of the foregoing. I agree to surrender this property to the Company immediately upon termination of my
employment with the Company, or at any time upon request by the Company. I further agree that any property situated on the Company’s data systems or on the Company’s premises and owned by the Company, including without limitation
electronic storage media, filing cabinets or other work areas, is subject to inspection by Company personnel at any time with or without notice. I further agree that in the event of termination of my employment with the Company I will execute a
Termination Certificate substantially in the form attached hereto as Exhibit A. 
 5. Inventions. 

5.1 Disclosure and Assignment of Inventions. For purposes of this Agreement, an “Invention” shall mean any idea, invention or
work of authorship, including, without limitation, any documentation, formula, design, device, code, method, software, technique, process, discovery, concept, improvement, enhancement, development, machine or contribution, in each case whether or
not patentable or copyrightable. I will disclose all Inventions promptly in writing to an officer of the Company or to attorneys of the Company in accordance with the Company’s policies and procedures, I will, and hereby do, assign to the
Company, without requirement of further writing, without royalty or any other further consideration, my entire right, title and interest throughout the world in and to all Inventions created, conceived, made, developed, and/or reduced to practice by
me in the course of my employment by the Company and all intellectual property rights therein. I hereby waive, and agree to waive, any moral rights I may have in any copyrightable work I create or have created on behalf of the Company. I also hereby
agree, that for a period of one year after my employment with the Company, I shall disclose to the Company any Inventions that I create, conceive, make, develop, reduce to practice or work on that relate to the work I performed for the Company. The
Company agrees that it will use commercially reasonable measures to keep Inventions disclosed to it pursuant to this Section 5.1 that do not constitute Inventions to be owned by the Company in confidence and shall not use any Inventions for its
own advantage, unless in either case those Inventions are assigned or assignable to the Company pursuant to this Section 5.1 or otherwise. 

5.2 Certain Exemptions. The obligations to assign Inventions set forth in Section 5.1 apply with respect to all Inventions
(a) whether or not such Inventions are conceived, made, developed or worked on by me during my regular hours of employment with the Company; (b) whether or not the Invention was made at the suggestion of the Company; (c) whether or
not the Invention was reduced to drawings, written description, documentation, models or other tangible form; and (d) whether or not the Invention is related to the general line of business engaged in by the Company, but do not apply to
Inventions that (x) I develop entirely on my own time or after the date of this Agreement without using the Company’s equipment, supplies, facilities or Proprietary Information; (y) do not relate to the Company’s business, or
actual or demonstrably anticipated research or development of the Company at the time of conception or reduction to practice of the Invention; and (z) do not result from and are not related to any work performed by me for the Company. I hereby
acknowledge and agree that the 

  
 3 

 
Company has notified me that, if I reside in the state of California, assignments provided for in Section 5.1 do not apply to any Invention which qualifies fully for exemption from
assignment under the provisions of Section 2870 of the California Labor Code (“Section 2870”), a copy of which is attached as Exhibit B. If applicable, at the time of disclosure of an Invention that I
believe qualifies under Section 2870, I shall provide to the Company, in writing, evidence to substantiate the belief that such Invention qualifies under Section 2870. I further understand that, to the extent this Agreement shall be
construed in accordance with the laws of any state which precludes a requirement in an employee agreement to assign certain classes of inventions made by an employee, Section 5.1 shall be interpreted not to apply to any Invention which a court
rules and/or the Company agrees falls within such classes. 
 5.3 Records. I will make and maintain adequate and current written
records of all Inventions covered by Section 5.1. These records may be in the form of notes, sketches, drawings, flow charts, electronic data or recordings, notebooks and any other format. These records shall be and remain the property of the
Company at all times and shall be made available to the Company at all times. 
 5.4 Patents and Other Rights. I agree to assist the
Company in obtaining, maintaining and enforcing patents, invention assignments and copyright assignments, and other proprietary rights in connection with any Invention covered by Section 5.1, and will otherwise assist the Company as reasonably
required by the Company to perfect in the Company the rights, title and other interests in my work product granted to the Company under this Agreement (both in the United States and foreign countries). I further agree that my obligations under this
Section 5.4 shall continue beyond the termination of my employment with the Company, but if I am requested by the Company to render such assistance after the termination of such employment, I shall be entitled to a fair and reasonable rate of
compensation for such assistance, and to reimbursement of any expenses incurred at the request of the Company relating to such assistance. If the Company is unable for any reason, after reasonable effort, to secure my signature on any document
needed in connection with the actions specified above, I hereby irrevocably designate and appoint the Company and its duly authorized officers and agents as my agent and attorney in fact, which appointment is coupled with an interest, to act for and
in my behalf to execute, verify and file any such documents and to do all other lawfully permitted acts to further the purposes of this Section 5.4 with the same legal force and effect as if executed by me. 

5.5 Prior Contracts and Inventions; Information Belonging to Third Parties. I represent and warrant that, except as set forth on
Exhibit C, I am not required, and I have not been required during the course of work for the Company or its predecessors, to assign Inventions under any other contracts that are now or were previously in existence between me and any other
person or entity. I further represent that (i) I am not obligated under any consulting, employment or other agreement that would affect the Company’s rights or my duties under this Agreement, and I shall not enter into any such agreement
or obligation during the period of my employment by the Company, (ii) there is no action, investigation, or proceeding pending or threatened, or any basis therefor known to me involving my prior employment or any consultancy or the use of any
information or techniques alleged to be proprietary to any former employer, and (iii) the performance of my duties as an employee of the Company do not and will not breach, or constitute a default under any agreement to which I am bound,
including any agreement limiting the use or disclosure of proprietary information acquired in confidence prior 

  
 4 

 
to engagement by the Company or if applicable, any agreement to refrain from competing, directly or indirectly, with the business of such previous employer or any other party or to refrain from
soliciting employees, customers or suppliers of such previous employer or other party. I will not, in connection with my employment by the Company, use or disclose to the Company any confidential, trade secret or other proprietary information of any
previous employer or other person to which I am not lawfully entitled. As a matter of record, I attach as Exhibit C a brief description of all Inventions made or conceived by me prior to my employment with the Company which I desire to be
excluded from this Agreement (“Background Technology”). If full disclosure of any Background Technology would breach or constitute a default under any agreement to which I am bound, including any agreement limiting the use or
disclosure of proprietary information acquired in confidence prior to engagement by the Company, I understand that I am to describe such Background Technology in Exhibit C at the most specific level possible without
violating any such prior agreement. Without limiting my obligations or representations under this Section 5.5, if I use any Background Technology in the course of my employment or incorporate any Background Technology in any product, service or
other offering of the Company, I hereby grant the Company a non-exclusive, royalty-free, perpetual and irrevocable, worldwide right to use and sublicense the use of Background Technology for the purpose of
developing, marketing, selling and supporting Company technology, products and services, either directly or through multiple tiers of distribution, but not for the purpose of marketing Background Technology separately from Company products or
services. 
 5.6 Works Made for Hire. I acknowledge that all original works of authorship which are made by me (solely or jointly with
others) within the scope of my employment with the Company and which are eligible for copyright protection are “works made for hire” as that term is defined in the United States Copyright Act (17 U.S.C., Section 101). 

6. Restrictive Covenants. 

6.1 Non-Competition. During the term of my employment by the Company and, to the extent permitted under applicable law, for a period of
one year thereafter, I will not without the prior written approval of an executive officer of the Company, (a) engage in any other professional employment or consulting, or (b) directly or indirectly participate in or assist any business
(whether as owner, partner, officer, director, employee, consultant, investor, lender or otherwise, except as the holder of not more than 1% of the outstanding stock of a publicly-held company), in each case, which is a current or potential
supplier, customer or competitor of the Company, including but not limited to any business or enterprise that develops, manufactures, markets, licenses, sells or provides any product or service that competes with any product or service developed,
manufactured, marketed, licensed, sold or provided, or planned to be developed, manufactured, marketed, licensed, sold or provided, by the Company while I was employed by the Company. If I am an executive officer of the Company, any of the foregoing
activities shall require the approval of the Board of Directors of the Company. Notwithstanding the foregoing, this section shall not apply to services rendered by me in California after the date my employment by the Company terminates. 

6.2 Non-Solicitation. During the term of my employment with the Company and for a period of one
year thereafter, I will not (i) solicit, encourage, induce or attempt to induce or assist others to solicit, encourage, induce or attempt to induce any employees, consultants or independent contractors of the Company to terminate their
employment or other engagement with the Company or (ii) hire, or recruit or attempt to hire, or engage or attempt to 

  
 5 

 
engage as an independent contractor, any person who was employed or otherwise engaged by the Company at any time during the term of the my employment with the Company; provided, that this
clause (ii) shall not apply to the recruitment or hiring or other engagement of any individual whose employment or other engagement with the Company has been terminated for a period of six months or longer. During the term of my employment with
the Company and for a period of one year thereafter, I will not solicit, divert or take away, or attempt to divert or take away, the business of any customer or client of the Company (served by the Company during the
12-month period prior to the termination of my employment with the Company) on my own behalf or on behalf of any person or entity other than the Company; provided, however, that this restriction
shall not apply to services rendered by me in California after the date my employment by the Company terminates. 
 6.3 Extension.
Without limiting the Company’s ability to seek other remedies available in law or equity, if I violate the provisions of Sections 6.1 or 6.2 (the “Restrictive Covenants”), I shall continue to be bound by the restrictions
set forth in such sections until a period of one year has expired without any violation of such provisions. 
 6.4 Interpretation. If
any restriction set forth in the Restrictive Covenants is found by any court of competent jurisdiction to be unenforceable because it extends for too long a period of time or over too great a range of activities or in too broad a geographic area, it
shall be interpreted to extend only over the maximum period of time, range of activities or geographic area as to which it may be enforceable. 

7. Notification to Other Parties. In the event of termination of my employment with the Company, I hereby consent to notification by the
Company to my new employer or other party for whom I work about my rights and obligations under this Agreement. 
 8. Employment at
Will. I understand and agree that my employment with the Company is at will. Accordingly, my employment can be terminated, without cause or notice, at my option or the Company’s option. The at-will
nature of my employment also means that I can be transferred or demoted, and my job title, compensation, benefits and other terms and conditions of employment can be reduced, without cause. I acknowledge that such changes shall not affect the
enforceability of the Restrictive Covenants. This at-will status of my employment relationship with the Company shall remain in force and effect throughout my employment with the Company and the Restrictive
Covenants will remain in effect for the periods specified in this Agreement, unless such status or Restrictive Covenants are modified by a further written agreement signed by both an authorized officer of the Company and me which expressly alters
such status or such Restrictive Covenants. 
 9. Miscellaneous. 

9.1 The parties’ rights and obligations under this Agreement will bind and inure to the benefit of their respective successors, heirs,
executors, and administrators and permitted assigns. I will not assign this Agreement or my obligations hereunder without the prior written consent of the Company, which consent may be withheld in the Company’s sole discretion, and any such
purported assignment without consent shall be null and void from the beginning. I agree that the Company may freely assign or otherwise transfer this Agreement to any affiliate or successor in interest (whether by way of merger, sale, acquisition or
corporate re-organization or any substantially similar process) of the Company. 

  
 6 

 9.2 This Agreement constitutes the parties’ final, exclusive and complete understanding and
agreement with respect to the subject matter hereof, and supersedes all prior and contemporaneous understandings and agreements, whether oral or written, relating to its subject matter. 

9.3 Any subsequent change or changes in my duties, obligations, rights or compensation will not affect the validity or scope of this Agreement.
This Agreement may not be waived, modified or amended unless mutually agreed upon in writing by both parties. No delay or omission by the Company in exercising any right under this Agreement will operate as a waiver of that or any other right. A
waiver or consent given by the Company on any one occasion is effective only in that instance and will not be construed as a bar to or waiver of any right on any other occasion. 

9.4 If any provision of this Agreement is found by a proper authority to be unenforceable or invalid such unenforceability or invalidity shall
not render this Agreement unenforceable or invalid as a whole and in such event, such provision shall be changed and interpreted so as to best accomplish the objectives of such unenforceable or invalid provision within the limits of applicable law
or applicable court decisions and the remainder of the terms, provisions, covenants and restrictions of this Agreement shall remain in full force and effect and shall in no way be affected, impaired or invalidated. 

9.5 I acknowledge that the Company will suffer substantial damages not readily ascertainable or compensable in terms of money in the event of
the breach of any of my obligations under this Agreement. I therefore agree that the Company shall be entitled (without limitation of any other rights or remedies otherwise available to the Company) to obtain an injunction from any court of
competent jurisdiction prohibiting the continuance or recurrence of any breach of this Agreement. 
 9.6 The rights and obligations of the
parties under this Agreement shall be governed in all respects by the laws of the Commonwealth of Massachusetts exclusively, without reference to any conflict laws rule that would result in the application of the laws of any other jurisdiction. I
agree that upon the Company’s request, all disputes arising hereunder shall be adjudicated in the state and federal courts having jurisdiction over disputes arising in the Commonwealth of Massachusetts, and I hereby agree to consent to the
personal jurisdiction of such courts. The Company and I each hereby irrevocably waive any right to a trial by jury in any action, suit or other legal proceeding arising under or relating to any provision of this Agreement. 

9.7 Any notices required or permitted hereunder shall be given to the appropriate party at the address specified on the signature page to this
Agreement or at such other address as the party shall specify in writing. Such notice shall be deemed given upon personal delivery, or sent by certified or registered mail, postage prepaid, three (3) days after the date of mailing. 

9.8 Except as otherwise provided herein, the provisions of this Agreement shall survive the termination of my employment with the Company for
any reason. 

  
 7 

 9.9 This Agreement may be executed in any number of counterparts, each of which shall be deemed
to be an original and all of which together shall be deemed to be one and the same instrument. 
 I ACKNOWLEDGE THAT I HAVE HAD THE OPPORTUNITY TO SEEK
THE ADVICE OF INDEPENDENT LEGAL COUNSEL AND THAT I HAVE READ THIS AGREEMENT CAREFULLY AND I UNDERSTAND AND ACCEPT THE OBLIGATIONS WHICH IT IMPOSES UPON ME WITHOUT RESERVATION. NO PROMISES OR REPRESENTATIONS HAVE BEEN MADE TO ME TO INDUCE ME TO SIGN
THIS AGREEMENT. I SIGN THIS AGREEMENT VOLUNTARILY AND FREELY, IN DUPLICATE, WITH THE UNDERSTANDING THAT THE COMPANY WILL RETAIN ONE COUNTERPART AND THE OTHER COUNTERPART WILL BE RETAINED BY ME. 

(Signature Page Follows) 

  
 8 

 IN WITNESS WHEREOF, I have executed this document as of
                    , 2018. 
  

			
	  

		
	Employee:	 	  

		
	Address:	 	  

		
		 	  

 Date employment first commenced:
                     ,         

AGREED AND ACKNOWLEDGED: 
  

			
	HOMOLOGY MEDICINES, INC.
		
	By:	 	  

	Name:	 	  

	Title:	 	  

 

			
	Address:	 	45 Wiggins Avenue
		 	Bedford, MA 01730

 [Signature Page to Proprietary Information and Invention Assignment Agreement] 

 Exhibit A 

Termination Certificate 

I, the undersigned, hereby certify that I do not have in my possession, nor have I failed to return, any documents or materials relating to
the business of Homology Medicines, Inc. or its affiliates (the “Company”), or copies thereof, including, without limitation, any item of Proprietary Information listed in Section 4 of the Company’s Employee Proprietary
Information and Inventions Assignment Agreement (the “Agreement”) to which I am a party, but not including copies of my own employment records. 

I further certify that I have complied with all of the terms of the Agreement signed by me, including the reporting of any Inventions (as
defined in the Agreement) covered by the Agreement. 
 I further agree that in compliance with the Agreement, I will preserve as
confidential any information relating to the Company or any of it business partners, clients, consultants or licensees which has been disclosed to me in confidence during the course of my employment by the Company unless authorized in writing to
disclose such information (i) by an executive officer of the Company, in the event that I am not an executive officer of the Company, or (ii) by the Board of Directors of the Company, in the event that I am an executive officer of the
Company. I understand that nothing herein is intended to or shall prevent me from communicating directly with, cooperating with, or providing information to, any federal, state or local government regulator, including, but not limited to, the U.S.
Securities and Exchange Commission, the U.S. Commodity Futures Trading Commission, or the U.S. Department of Justice. 
  

					
	Date:                     	 	  
	 	
			
		 	(Employee’s Signature)	 	
			
		 	  
	 	
			
		 	(Printed or Typed Name of Employee)	 	

 Exhibit B 

California Labor Code 
 California
Labor Code § 2870. Application of provision providing that employee shall assign or offer to assign rights in invention to employer. 
  

	(a)	Any provision in an employment agreement which provides that an employee shall assign, or offer to assign, any of his or her rights in an invention to his or her employer shall not apply to an invention that the
employee developed entirely on his or her own time without using the employer’s equipment, supplies, facilities, or trade secret information except for those inventions that either: 

 

	 	(1)	Relate at the time of conception or reduction to practice of the invention to the employer’s business, or actual or demonstrably anticipated research or development of the employer; or 

 

	 	(2)	Result from any work performed by the employee for the employer. 

  

	(b)	To the extent a provision in an employment agreement purports to require an employee to assign an invention otherwise excluded from being required to be assigned under subdivision (a), the provision is against the
public policy of this state and is unenforceable. 

 Exhibit C 

Background Technology 
 List here prior
contracts to assign Inventions that are now in existence between any other person or entity and you. 
 [    ] 

List here previous Inventions which you desire to have specifically excluded from the operation of this Agreement. Continue on reverse side if necessary. 

[    ]

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