Document:

EX-4.1

 Exhibit 4.1 

AMENDED AND RESTATED INVESTORS’ RIGHTS AGREEMENT 

THIS AMENDED AND RESTATED INVESTORS’ RIGHTS AGREEMENT (this “Agreement”), is made as of July 28, 2017, by and among
Homology Medicines, Inc., a Delaware corporation (the “Company”), each of the investors listed on Schedule A hereto (each, an “Investor,” and together with any subsequent investors, or transferees, who become
parties hereto as “Investors” in accordance with the terms hereof, the “Investors”), and, solely for purposes of Section 2 (other than Subsections 2.1 and 2.10), Subsection 4.1 and Section 6 (other than
Subsection 6.6), California Institute of Technology (“Caltech”). 
 RECITALS 

WHEREAS, certain of the Investors (the “Existing Investors”) possess registration rights, information rights, rights
of first offer, and other rights pursuant to an Investors’ Rights Agreement, dated as of December 22, 2015, between the Company and such Investors (as amended, the “Prior Agreement”); 

WHEREAS, the Company and the Existing Investors desire to amend and restate the Prior Agreement in its entirety and to accept the
rights created pursuant to this Agreement in lieu of the rights granted to them under the Prior Agreement; 
 WHEREAS, concurrently
with the execution of this Agreement, the Company and certain of the Investors are entering into a Series B Preferred Stock Purchase Agreement (as the same may be amended and/or restated from time to time, the “Purchase Agreement”),
pursuant to which such Investors have agreed to purchase shares of Series B Preferred Stock (as defined below); 
 WHEREAS, the
Company and Caltech are parties to a License Agreement, dated as of September 14, 2016 (as amended, the “Caltech Agreement”), pursuant to which the Company issued to Caltech 533,695 shares of Common Stock and the Company agreed
to grant certain registration and participation rights; and 
 NOW, THEREFORE, the Company and the Existing Investors hereby agree
that the Prior Agreement is amended and restated in its entirety as set forth herein, and all of the parties hereto further agree as follows: 
 1.
Definitions. For purposes of this Agreement: 
 1.1 “Affiliate” means, with respect to any specified Person,
any other Person who, directly or indirectly, controls, is controlled by, or is under common control with such Person, including without limitation any general partner, managing member, officer or director of such Person or any venture capital fund
now or hereafter existing that is controlled by one or more general partners or managing members of, or shares the same management company with, such Person. 

1.2 “Caltech Registrable Securities” means (i) the 533,695 shares of Common Stock held by Caltech on the
date hereof, and (ii) any Common Stock issued as (or issuable upon the conversion or exercise of any warrant, right, or other security that is issued as) a dividend or other distribution with respect to, or in exchange for or in replacement of
such shares. 

 1.3 “Common Stock” means shares of the Company’s common stock, $0.0001 par
value per share. 
 1.4 “Damages” means any loss, damage, claim or liability (joint or several) to which a party hereto may
become subject under the Securities Act, the Exchange Act, or other federal or state law, insofar as such loss, damage, claim or liability (or any action in respect thereof) arises out of or is based upon: (i) any untrue statement or alleged
untrue statement of a material fact contained in any registration statement of the Company, including any preliminary prospectus or final prospectus contained therein or any amendments or supplements thereto; (ii) an omission or alleged
omission to state therein a material fact required to be stated therein, or necessary to make the statements therein not misleading; or (iii) any violation or alleged violation by the indemnifying party (or any of its agents or
Affiliates) of the Securities Act, the Exchange Act, any state securities law, or any rule or regulation promulgated under the Securities Act, the Exchange Act, or any state securities law. 

1.5 “Derivative Securities” means any securities or rights convertible into, or exercisable or exchangeable for (in each
case, directly or indirectly), Common Stock, including options and warrants. 
 1.6 “Exchange Act” means the
Securities Exchange Act of 1934, as amended, and the rules and regulations promulgated thereunder. 
 1.7 “Excluded
Registration” means (i) a registration relating to the sale of securities to employees of the Company or a subsidiary pursuant to a stock option, stock purchase, or similar plan; (ii) a registration relating to an SEC Rule 145
transaction; (iii) a registration on any form that does not include substantially the same information as would be required to be included in a registration statement covering the sale of the Registrable Securities; or (iv) a registration
in which the only Common Stock being registered is Common Stock issuable upon conversion of debt securities that are also being registered. 

1.8 “Form S-1” means such form under the Securities Act as in effect on the
date hereof or any successor registration form under the Securities Act subsequently adopted by the SEC. 
 1.9 “Form S-3” means such form under the Securities Act as in effect on the date hereof or any registration form under the Securities Act subsequently adopted by the SEC that permits incorporation of substantial
information by reference to other documents filed by the Company with the SEC. 
 1.10 “Founders” means Saswati Chatterjee
and Laura Smith. 
 1.11 “GAAP” means generally accepted accounting principles in the United States. 

1.12 “Holder” means any holder of Registrable Securities who is a party to this Agreement. 

1.13 “Immediate Family Member” means a child, stepchild, grandchild, parent, stepparent, grandparent, spouse, sibling, mother-in-law, father-in-law,
son-in-law, daughter-in-law, brother-in-law, or sister-in-law, including, adoptive relationships, of a natural person referred to herein. 

  
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 1.14 “Initiating Holders” means, collectively, Holders who properly initiate a
registration request under this Agreement. 
 1.15 “IPO” means the Company’s first underwritten public offering of its
Common Stock under the Securities Act. 
 1.16 “Key Employee” means any executive-level employee (including, division
director and vice president-level positions) as well as any employee who, either alone or in concert with others, develops, invents, programs, or designs any Company Intellectual Property (as defined in the Purchase Agreement). 

1.17 “Major Investor” means (i) any Investor that, individually or together with such Investor’s Affiliates, holds
at least 7,000,000 shares of Registrable Securities after the Closing under the Purchase Agreement (as adjusted for any stock split, stock dividend, combination, or other recapitalization or reclassification effected after the date hereof) and
(ii) for purposes of Section 4.1 only, Caltech for so long as they continue to hold all of the Caltech Registrable Securities. 

1.18 “New Securities” means, collectively, equity securities of the Company, whether or not currently authorized, as well as
rights, options, or warrants to purchase such equity securities, or securities of any type whatsoever that are, or may become, convertible or exchangeable into or exercisable for such equity securities. 

1.19 “Person” means any individual, corporation, partnership, trust, limited liability company, association or other entity.

 1.20 “Preferred Director” means any director of the Company that the holders of record of the Preferred
Stock, exclusively and voting together as a single class, are entitled to elect pursuant to the Restated Certificate. 
 1.21
“Preferred Stock” means, collectively, the Series A Preferred Stock and the Series B Preferred Stock. 
 1.22
“Qualified IPO” means an IPO in which the Company sells shares of its Common Stock with gross proceeds to the Company of at least $50,000,000 and the shares of Common Stock are listed for trading on the New York Stock Exchange or
the NASDAQ National Market. 
 1.23 “Registrable Securities” means (i) the Common Stock issuable or issued upon
conversion of the Preferred Stock; (ii) the Caltech Registrable Securities, provided, however, that such Caltech Registrable Securities shall not be deemed Registrable Securities and Caltech shall not be deemed a
Holder for the purposes of Subsections 2.1, 2.10, 3.1, 3.2 and 6.6; and (ii) any Common Stock issued as (or issuable upon the conversion or exercise of any warrant, right, or other security that is issued as)
a dividend or other distribution with respect to, or in exchange for or in replacement of, the shares referenced in clauses (i) and (ii) above; excluding in all cases, however, any Registrable Securities sold by a
Person in a transaction in which the applicable rights under this Agreement are not assigned pursuant to Subsection 6.1, and excluding for purposes of Section 2 any shares for which registration rights have
terminated pursuant to Subsection 2.13 of this Agreement. 

  
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 1.24 “Registrable Securities then outstanding” means the number of shares
determined by adding the number of shares of outstanding Common Stock that are Registrable Securities and the number of shares of Common Stock issuable (directly or indirectly) pursuant to then exercisable and/or convertible securities that
are Registrable Securities. 
 1.25 “Requisite Investors” means the holders of shares of Preferred Stock representing at
least seventy-one and one-half percent (71.5%) of the voting power of the outstanding shares of Preferred Stock. 

1.26 “Restated Certificate” means the Company’s Amended and Restated Certificate of Incorporation, as amended
and/or restated from time to time. 
 1.27 “Restricted Securities” means the securities of the Company required to be
notated with the legend set forth in Subsection 2.12(b) hereof. 
 1.28 “SEC” means the Securities and Exchange
Commission. 
 1.29 “SEC Rule 144” means Rule 144 promulgated by the SEC under the Securities Act. 

1.30 “SEC Rule 145” means Rule 145 promulgated by the SEC under the Securities Act. 

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

 1.32 “Selling Expenses” means all underwriting discounts, selling commissions, and stock transfer taxes applicable to
the sale of Registrable Securities, and fees and disbursements of counsel for any Holder, except for the fees and disbursements of the Selling Holder Counsel borne and paid by the Company as provided in Subsection 2.6. 

1.33 “Series A Preferred Stock” means shares of the Company’s Series A Preferred Stock, $0.0001 par value per share.

 1.34 “Series B Preferred Stock” means shares of the Company’s Series B Preferred Stock, $0.0001 par value per
share. 
 1.35 “Voting Agreement” means the Amended and Restated Voting Agreement of even date herewith by and among the
Company, the Investors and the Key Holders, as amended and/or restated from time to time. 

  
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 2. Registration Rights. The Company covenants and agrees as follows: 

2.1 Demand Registration. 

(a) Form S-1 Demand. If (i) at any time after the fourth (4th) anniversary of the date of this Agreement, the Company receives a request from Holders of at least thirty percent (30%) of the Registrable Securities then outstanding, that the Company file a Form S-1 registration statement with respect to at least twenty percent (20%) of the Registrable Securities then outstanding and having an anticipated aggregate offering price, net of Selling Expenses, which would
exceed $20 million, or (ii) at any time or from time to time after one hundred eighty (180) days after the effective date of the registration statement for the IPO, the Company receives a request from Holders of Registrable Securities
that the Company file a Form S-1 registration statement with respect to Registrable Securities having an expected aggregate offering price, net of Selling Expenses, which would exceed $5,000,000, then
the Company shall (x) within ten (10) days after the date such request is given, give notice thereof (the “Demand Notice”) to all Holders other than the Initiating Holders; and (y) as soon as practicable, and in any
event within ninety (90) days after the date such request is given by the Initiating Holders, file a Form S-1 registration statement under the Securities Act covering all Registrable Securities that the
Initiating Holders requested to be registered and any additional Registrable Securities requested to be included in such registration by any other Holders, as specified by notice given by each such Holder to the Company within twenty
(20) days of the date the Demand Notice is given, and in each case, subject to the limitations of Subsections 2.1(c) and 2.3. 

(b) Form S-3 Demand. If at any time when it is eligible to use a Form S-3 registration statement, the Company receives a request from a Holder or Holders of Registrable Securities then outstanding that the Company file a Form S-3 registration
statement with respect to outstanding Registrable Securities of such Holder or Holders having an anticipated aggregate offering price, net of Selling Expenses, of at least $5,000,000, then the Company shall (i) within ten (10) days after
the date such request is given, give a Demand Notice to all Holders other than the Initiating Holders; and (ii) as soon as practicable, and in any event within thirty (30) days after the date such request is given by the Initiating
Holders, file a Form S-3 registration statement under the Securities Act covering all Registrable Securities requested to be included in such registration by any other Holders, as specified by notice given by
each such Holder to the Company within twenty (20) days of the date the Demand Notice is given, and in each case, subject to the limitations of Subsections 2.1(c) and 2.3. 

(c) Notwithstanding the foregoing obligations, if the Company furnishes to Holders requesting a registration pursuant to this Subsection
2.1 a certificate signed by the Company’s chief executive officer stating that in the good faith judgment of the Company’s Board of Directors it would be materially detrimental to the Company and its stockholders for such registration
statement to either become effective or remain effective for as long as such registration statement otherwise would be required to remain effective, because such action would (i) materially interfere with a significant acquisition, corporate
reorganization, or other similar transaction involving the Company; (ii) require premature disclosure of material information that the Company has a bona fide business purpose for preserving as confidential; or (iii) render the Company
unable to comply with requirements under the Securities Act or Exchange Act, then the Company shall have the right to defer taking action with respect to such filing, and any time periods with respect to filing or effectiveness thereof shall be
tolled correspondingly, for a period of not more than one hundred (100) days after the request of the Initiating Holders is given; provided, however, that the Company may not invoke this right more than once in any twelve
(12) month period; and provided further that the Company shall not register any securities for its own account or that of any other stockholder during such one hundred (100) day period other than an Excluded Registration.

  
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 (d) The Company shall not be obligated to effect, or to take any action to effect, any
registration pursuant to Subsection 2.1(a) (i) during the period that is sixty (60) days before the Company’s good faith estimate of the date of filing of, and ending on a date that is one hundred eighty (180) days after
the effective date of, a Company-initiated registration, provided that the Company is actively employing in good faith commercially reasonable efforts to cause such registration statement to become effective; (ii) after the Company has
effected two registrations pursuant to Subsection 2.1(a); or (iii) if the Initiating Holders propose to dispose of shares of Registrable Securities that may be immediately registered on Form S-3
pursuant to a request made pursuant to Subsection 2.1(b). The Company shall not be obligated to effect, or to take any action to effect, any registration pursuant to Subsection 2.1(b) (i) during the period that is thirty
(30) days before the Company’s good faith estimate of the date of filing of, and ending on a date that is ninety (90) days after the effective date of, a Company-initiated registration, provided that the Company is actively
employing in good faith commercially reasonable efforts to cause such registration statement to become effective; or (ii) if the Company has effected two registrations pursuant to Subsection 2.1(b) in any twelve (12) month period. A
registration shall not be counted as “effected” for purposes of this Subsection 2.1(d) until such time as the applicable registration statement has been declared effective by the SEC, unless the Initiating Holders withdraw their
request for such registration, elect not to pay the registration expenses therefor, and forfeit their right to one demand registration statement pursuant to Subsection 2.6, in which case such withdrawn registration statement shall be counted
as “effected” for purposes of this Subsection 2.1(d). 
 2.2 Company Registration. If the Company proposes to
register (including, for this purpose, a registration effected by the Company for stockholders other than the Holders) any of its Common Stock under the Securities Act in connection with the public offering of such securities solely for cash (other
than in an Excluded Registration), the Company shall, at such time, promptly give each Holder notice of such registration. Upon the request of each Holder given within twenty (20) days after such notice is given by the Company, the Company
shall, subject to the provisions of Subsection 2.3, cause to be registered all of the Registrable Securities that each such Holder has requested to be included in such registration. The Company shall have the right to terminate or withdraw
any registration initiated by it under this Subsection 2.2 before the effective date of such registration, whether or not any Holder has elected to include Registrable Securities in such registration. The expenses (other than Selling
Expenses) of such withdrawn registration shall be borne by the Company in accordance with Subsection 2.6. 
 2.3 Underwriting
Requirements. 
 (a) If, pursuant to Subsection 2.1, the Initiating Holders intend to distribute the Registrable Securities
covered by their request by means of an underwriting, they shall so advise the Company as a part of their request made pursuant to Subsection 2.1, and the Company shall include such information in the Demand Notice. The underwriter(s) will be
selected by the Company and shall be reasonably acceptable to a majority in interest of the Initiating Holders. In such event, the right of any Holder to include such Holder’s Registrable Securities in such

  
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registration shall be conditioned upon such Holder’s participation in such underwriting and the inclusion of such Holder’s Registrable Securities in the underwriting to the extent
provided herein. All Holders proposing to distribute their securities through such underwriting shall (together with the Company as provided in Subsection 2.4(e)) enter into an underwriting agreement in customary form with the underwriter(s)
selected for such underwriting. Notwithstanding any other provision of this Subsection 2.3, if the underwriter(s) advise(s) the Initiating Holders in writing that marketing factors require a limitation on the number of shares to be
underwritten, then the Initiating Holders shall so advise all Holders of Registrable Securities that otherwise would be underwritten pursuant hereto, and the number of Registrable Securities that may be included in the underwriting shall be
allocated among such Holders of Registrable Securities, including the Initiating Holders, in proportion (as nearly as practicable) to the number of Registrable Securities owned by each Holder or in such other proportion as shall mutually be agreed
to by all such selling Holders; provided, however, that the number of Registrable Securities held by the Holders to be included in such underwriting shall not be reduced unless all other securities are first entirely excluded from the
underwriting. To facilitate the allocation of shares in accordance with the above provisions, the Company or the underwriters may round the number of shares allocated to any Holder to the nearest one hundred (100) shares. 

(b) In connection with any offering involving an underwriting of shares of the Company’s capital stock pursuant to Subsection 2.2,
the Company shall not be required to include any of the Holders’ Registrable Securities in such underwriting unless the Holders accept the terms of the underwriting as agreed upon between the Company and its underwriters, and then only in such
quantity as the underwriters in their sole discretion determine will not jeopardize the success of the offering by the Company. If the total number of securities, including Registrable Securities, requested by stockholders to be included in such
offering exceeds the number of securities to be sold (other than by the Company) that the underwriters in their reasonable discretion determine is compatible with the success of the offering, then the Company shall be required to include in the
offering only that number of such securities, including Registrable Securities, which the underwriters and the Company in their sole discretion determine will not jeopardize the success of the offering. If the underwriters determine that less than
all of the Registrable Securities requested to be registered can be included in such offering, then the Registrable Securities that are included in such offering shall be allocated among the selling Holders in proportion (as nearly as practicable
to) the number of Registrable Securities owned by each selling Holder or in such other proportions as shall mutually be agreed to by all such selling Holders. To facilitate the allocation of shares in accordance with the above provisions, the
Company or the underwriters may round the number of shares allocated to any Holder to the nearest one hundred (100) shares. Notwithstanding the foregoing, in no event shall (i) the number of Registrable Securities included in the offering
be reduced unless all other securities (other than securities to be sold by the Company) are first entirely excluded from the offering, or (ii) the number of Registrable Securities included in the offering be reduced below twenty-five percent
(25%) of the total number of securities included in such offering, unless such offering is the IPO, in which case the selling Holders may be excluded further if the underwriters make the determination described above and no other stockholder’s
securities are included in such offering. For purposes of the provision in this Subsection 2.3(b) concerning apportionment, for any selling Holder that is a partnership, limited liability company, or corporation, the partners, members,
retired partners, retired members, stockholders, and Affiliates of such Holder, or the estates and Immediate Family Members of any such partners, 

  
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retired partners, members, and retired members and any trusts for the benefit of any of the foregoing Persons, shall be deemed to be a single “selling Holder,” and any pro rata
reduction with respect to such “selling Holder” shall be based upon the aggregate number of Registrable Securities owned by all Persons included in such “selling Holder,” as defined in this sentence. 

(c) For purposes of Subsection 2.1, a registration shall not be counted as “effected” if, as a result of an exercise of the
underwriter’s cutback provisions in Subsection 2.3(a), fewer than fifty percent (50%) of the total number of Registrable Securities that Holders have requested to be included in such registration statement are actually included. 

2.4 Obligations of the Company. Whenever required under this Section 2 to effect the registration of any
Registrable Securities, the Company shall, as expeditiously as reasonably possible: 
 (a) prepare and file with the SEC a registration
statement with respect to such Registrable Securities and use its commercially reasonable efforts to cause such registration statement to become effective and, upon the request of the Holders of
at least seventy-one and one-half percent (71.5%) of the Registrable Securities registered thereunder, keep such registration statement effective for a period of up to
one hundred twenty (120) days or, if earlier, until the distribution contemplated in the registration statement has been completed; provided, however, that (i) such one hundred twenty (120) day period shall be extended
for a period of time equal to the period the Holder refrains, at the request of an underwriter of Common Stock (or other securities) of the Company, from selling any securities included in such registration, and (ii) in the case of any
registration of Registrable Securities on Form S-3 that are intended to be offered on a continuous or delayed basis, subject to compliance with applicable SEC rules, such one hundred twenty (120) day
period shall be extended for up to sixty (60) days, if necessary, to keep the registration statement effective until all such Registrable Securities are sold; 

(b) prepare and file with the SEC such amendments and supplements to such registration statement, and the prospectus used in connection with
such registration statement, as may be necessary to comply with the Securities Act in order to enable the disposition of all securities covered by such registration statement; 

(c) furnish to the selling Holders such numbers of copies of a prospectus, including a preliminary prospectus, as required by the Securities
Act, and such other documents as the Holders may reasonably request in order to facilitate their disposition of their Registrable Securities; 

(d) use its commercially reasonable efforts to register and qualify the securities covered by such registration statement under such other
securities or blue-sky laws of such jurisdictions as shall be reasonably requested by the selling Holders; provided that the Company shall not be required to qualify to do business or to file a general
consent to service of process in any such states or jurisdictions, unless the Company is already subject to service in such jurisdiction and except as may be required by the Securities Act; 

  
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 (e) in the event of any underwritten public offering, enter into and perform its obligations
under an underwriting agreement, in usual and customary form, with the underwriter(s) of such offering; 
 (f) use its commercially
reasonable efforts to cause all such Registrable Securities covered by such registration statement to be listed on a national securities exchange or trading system and each securities exchange and trading system (if any) on which similar securities
issued by the Company are then listed; 
 (g) provide a transfer agent and registrar for all Registrable Securities registered pursuant to
this Agreement and provide a CUSIP number for all such Registrable Securities, in each case not later than the effective date of such registration; 

(h) promptly make available for inspection by the selling Holders, any underwriter(s) participating in any disposition pursuant to such
registration statement, and any attorney or accountant or other agent retained by any such underwriter or selected by the selling Holders, all financial and other records, pertinent corporate documents, and properties of the Company, and cause the
Company’s officers, directors, employees, and independent accountants to supply all information reasonably requested by any such seller, underwriter, attorney, accountant, or agent, in each case, as necessary or advisable to verify the accuracy
of the information in such registration statement and to conduct appropriate due diligence in connection therewith; 
 (i) notify each
selling Holder, promptly after the Company receives notice thereof, of the time when such registration statement has been declared effective or a supplement to any prospectus forming a part of such registration statement has been filed; and 

(j) after such registration statement becomes effective, notify each selling Holder of any request by the SEC that the Company amend or
supplement such registration statement or prospectus. 
 In addition, the Company shall ensure that, at all times after any registration
statement covering a public offering of securities of the Company under the Securities Act shall have become effective, its insider trading policy shall provide that the Company’s directors may implement a trading program under Rule 10b5-1 of the Exchange Act. 
 2.5 Furnish Information. It shall be a condition precedent to the
obligations of the Company to take any action pursuant to this Section 2 with respect to the Registrable Securities of any selling Holder that such Holder shall furnish to the Company such information regarding itself, the
Registrable Securities held by it, and the intended method of disposition of such securities as is reasonably required to effect the registration of such Holder’s Registrable Securities. 

2.6 Expenses of Registration. All expenses (other than Selling Expenses) incurred in connection with registrations, filings, or
qualifications pursuant to Section 2, including all registration, filing, and qualification fees; printers’ and accounting fees; fees and disbursements of counsel for the Company; and the reasonable fees and
disbursements, not to exceed $35,000 per registration, of one counsel for the selling Holders selected by the Holders of at least seventy-one 

  
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and one-half percent (71.5%) of the Registrable Securities included in such registration (“Selling Holder Counsel”), shall be borne and
paid by the Company; provided, however, that the Company shall not be required to pay for any expenses of any registration proceeding begun pursuant to Subsection 2.1 if the registration request is subsequently withdrawn at the
request of the Holders of at least seventy-one and one-half percent (71.5%) of the Registrable Securities to be registered (in which case all selling Holders shall bear
such expenses pro rata based upon the number of Registrable Securities that were to be included in the withdrawn registration), unless the Holders of at least seventy-one and
one-half percent (71.5%) of the Registrable Securities agree to forfeit their right to one registration pursuant to Subsections 2.1(a) or 2.1(b), as the case may be; provided
further that if, at the time of such withdrawal, the Holders shall have learned of a material adverse change in the condition, business, or prospects of the Company from that known to the Holders at the time of their request and have
withdrawn the request with reasonable promptness after learning of such information then the Holders shall not be required to pay any of such expenses and shall not forfeit their right to one registration pursuant to Subsections 2.1(a) or
2.1(b). All Selling Expenses relating to Registrable Securities registered pursuant to this Section 2 shall be borne and paid by the Holders pro rata on the basis of the number of Registrable Securities registered on
their behalf. 
 2.7 Delay of Registration. No Holder shall have any right to obtain or seek an injunction restraining or otherwise
delaying any registration pursuant to this Agreement as the result of any controversy that might arise with respect to the interpretation or implementation of this Section 2. 

2.8 Indemnification. If any Registrable Securities are included in a registration statement under this
Section 2: 
 (a) To the extent permitted by law, the Company will indemnify and hold harmless each selling Holder,
and the partners, members, officers, directors, and stockholders of each such Holder; legal counsel and accountants for each such Holder; any underwriter (as defined in the Securities Act) for each such Holder; and each Person, if any, who controls
such Holder or underwriter within the meaning of the Securities Act or the Exchange Act, against any Damages, and the Company will pay to each such Holder, underwriter, controlling Person, or other aforementioned Person any legal or other expenses
reasonably incurred thereby in connection with investigating or defending any claim or proceeding from which Damages may result, as such expenses are incurred; provided, however, that the indemnity agreement contained in this
Subsection 2.8(a) shall not apply to amounts paid in settlement of any such claim or proceeding if such settlement is effected without the consent of the Company, which consent shall not be unreasonably withheld, nor shall the Company be
liable for any Damages to the extent that they arise out of or are based upon actions or omissions made in reliance upon and in conformity with written information furnished by or on behalf of any such Holder, underwriter, controlling Person, or
other aforementioned Person expressly for use in connection with such registration. 
 (b) To the extent permitted by law, each selling
Holder, severally and not jointly, will indemnify and hold harmless the Company, and each of its directors, each of its officers who has signed the registration statement, each Person (if any), who controls the Company within the meaning of the
Securities Act, legal counsel and accountants for the Company, any underwriter (as defined in the Securities Act), any other Holder selling securities in such 

  
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registration statement, and any controlling Person of any such underwriter or other Holder, against any Damages, in each case only to the extent that such Damages arise out of or are based upon
actions or omissions made in reliance upon and in conformity with written information furnished by or on behalf of such selling Holder expressly for use in connection with such registration; and each such selling Holder will pay to the Company and
each other aforementioned Person any legal or other expenses reasonably incurred thereby in connection with investigating or defending any claim or proceeding from which Damages may result, as such expenses are incurred; provided,
however, that the indemnity agreement contained in this Subsection 2.8(b) shall not apply to amounts paid in settlement of any such claim or proceeding if such settlement is effected without the consent of the Holder, which consent
shall not be unreasonably withheld; and provided further that in no event shall the aggregate amounts payable by any Holder by way of indemnity or contribution under Subsections 2.8(b) and 2.8(d) exceed the proceeds from
the offering received by such Holder (net of any Selling Expenses paid by such Holder), except in the case of fraud or willful misconduct by such Holder. 

(c) Promptly after receipt by an indemnified party under this Subsection 2.8 of notice of the commencement of any action
(including any governmental action) for which a party may be entitled to indemnification hereunder, such indemnified party will, if a claim in respect thereof is to be made against any indemnifying party under this Subsection 2.8, give the
indemnifying party notice of the commencement thereof. The indemnifying party shall have the right to participate in such action and, to the extent the indemnifying party so desires, participate jointly with any other indemnifying party to which
notice has been given, and to assume the defense thereof with counsel mutually satisfactory to the parties; provided, however, that an indemnified party (together with all other indemnified parties that may be represented without
conflict by one counsel) shall have the right to retain one separate counsel, with the fees and expenses to be paid by the indemnifying party, if representation of such indemnified party by the counsel retained by the indemnifying party would be
inappropriate due to actual or potential differing interests between such indemnified party and any other party represented by such counsel in such action. The failure to give notice to the indemnifying party within a reasonable time of the
commencement of any such action shall relieve such indemnifying party of any liability to the indemnified party under this Subsection 2.8, to the extent that such failure materially prejudices the indemnifying party’s ability to defend
such action. The failure to give notice to the indemnifying party will not relieve it of any liability that it may have to any indemnified party otherwise than under this Subsection 2.8. 

(d) To provide for just and equitable contribution to joint liability under the Securities Act in any case in which either: (i) any party
otherwise entitled to indemnification hereunder makes a claim for indemnification pursuant to this Subsection 2.8 but it is judicially determined (by the entry of a final judgment or decree by a court of competent jurisdiction and the
expiration of time to appeal or the denial of the last right of appeal) that such indemnification may not be enforced in such case, notwithstanding the fact that this Subsection 2.8 provides for indemnification in such case, or
(ii) contribution under the Securities Act may be required on the part of any party hereto for which indemnification is provided under this Subsection 2.8, then, and in each such case, such parties will contribute to the aggregate
losses, claims, damages, liabilities, or expenses to which they may be subject (after contribution from others) in such proportion as is appropriate to reflect the relative fault of each of the indemnifying party and the indemnified party

  
 11 

 
in connection with the statements, omissions, or other actions that resulted in such loss, claim, damage, liability, or expense, as well as to reflect any other relevant equitable considerations.
The relative fault of the indemnifying party and of the indemnified party shall be determined by reference to, among other things, whether the untrue or allegedly untrue statement of a material fact, or the omission or alleged omission of a material
fact, relates to information supplied by the indemnifying party or by the indemnified party and the parties’ relative intent, knowledge, access to information, and opportunity to correct or prevent such statement or omission; provided,
however, that, in any such case (x) no Holder will be required to contribute any amount in excess of the public offering price of all such Registrable Securities offered and sold by such Holder pursuant to such registration statement,
and (y) no Person guilty of fraudulent misrepresentation (within the meaning of Section 11(f) of the Securities Act) will be entitled to contribution from any Person who was not guilty of such fraudulent misrepresentation; and
provided further that in no event shall a Holder’s liability pursuant to this Subsection 2.8(d), when combined with the amounts paid or payable by such Holder pursuant to Subsection 2.8(d), exceed the proceeds from
the offering received by such Holder (net of any Selling Expenses paid by such Holder), except in the case of willful misconduct or fraud by such Holder. 

(e) Notwithstanding the foregoing, to the extent that the provisions on indemnification and contribution contained in the underwriting
agreement entered into in connection with the underwritten public offering are in conflict with the foregoing provisions, the provisions in the underwriting agreement shall control. 

(f) Unless otherwise superseded by an underwriting agreement entered into in connection with the underwritten public offering, the obligations
of the Company and Holders under this Subsection 2.8 shall survive the completion of any offering of Registrable Securities in a registration under this Section 2, and otherwise shall survive the termination of this
Agreement. 
 2.9 Reports Under Exchange Act. With a view to making available to the Holders the benefits of SEC Rule 144 and any
other rule or regulation of the SEC that may at any time permit a Holder to sell securities of the Company to the public without registration or pursuant to a registration on Form S-3, the Company shall: 

(a) make and keep available adequate current public information, as those terms are understood and defined in SEC Rule 144, at all times after
the effective date of the registration statement filed by the Company for the IPO; 
 (b) use commercially reasonable efforts to file with
the SEC in a timely manner all reports and other documents required of the Company under the Securities Act and the Exchange Act (at any time after the Company has become subject to such reporting requirements); and 

(c) furnish to any Holder, so long as the Holder owns any Registrable Securities, forthwith upon request (i) to the extent accurate, a
written statement by the Company that it has complied with the reporting requirements of SEC Rule 144 (at any time after ninety (90) days after the effective date of the registration statement filed by the Company for the IPO), the Securities
Act, and the Exchange Act (at any time after the Company has become subject to such reporting requirements), or that it qualifies as a registrant whose securities may be resold pursuant 

  
 12 

 
to Form S-3 (at any time after the Company so qualifies); and (ii) such other information as may be reasonably requested in availing any Holder of any
rule or regulation of the SEC that permits the selling of any such securities without registration (at any time after the Company has become subject to the reporting requirements under the Exchange Act) or pursuant to Form S-3 (at any time after the Company so qualifies to use such form). 
 2.10 Limitations on Subsequent
Registration Rights. From and after the date of this Agreement, the Company shall not, without the prior written consent of the Holders of at least seventy-one and
one-half percent (71.5%) of the Registrable Securities then outstanding, enter into any agreement with any holder or prospective holder of any securities of the Company that (i) would allow such holder or
prospective holder to include such securities in any registration unless, under the terms of such agreement, such holder or prospective holder may include such securities in any such registration only to the extent that the inclusion of such
securities will not reduce the number of the Registrable Securities of the Holders that are included; or (ii) allow such holder or prospective holder to initiate a demand for registration of any securities held by such holder or prospective
holder; provided that this limitation shall not apply to any additional Investor who becomes a party to this Agreement in accordance with Subsection 6.9. 

2.11 “Market Stand-off” Agreement. Each Holder hereby agrees
that it will not, without the prior written consent of the managing underwriter, during the period commencing on the date of the final prospectus relating to the IPO, and ending on the date specified by the Company and the managing underwriter (such
period not to exceed one hundred eighty (180) days, or such other period as may be requested by the Company or an underwriter to accommodate regulatory restrictions on (1) the publication or other distribution of research reports, and
(2) analyst recommendations and opinions, including, but not limited to, the restrictions contained in FINRA Rule 2241 or NYSE Rule 472(f)(4), or any successor provisions or amendments thereto), (i) lend; offer; pledge; sell; contract to sell;
sell any option or contract to purchase; purchase any option or contract to sell; grant any option, right, or warrant to purchase; or otherwise transfer or dispose of, directly or indirectly, any shares of Common Stock or any securities convertible
into or exercisable or exchangeable (directly or indirectly) for Common Stock held immediately before the effective date of the registration statement for the IPO or (ii) enter into any swap or other arrangement that transfers to another, in
whole or in part, any of the economic consequences of ownership of such securities, whether any such transaction described in clause (i) or (ii) above is to be settled by delivery of Common Stock or other securities, in cash, or otherwise. The
foregoing provisions of this Subsection 2.11 shall apply only to the IPO and shall not apply to the sale of any shares to an underwriter pursuant to an underwriting agreement, any securities of the Company purchased in the Company’s IPO,
any securities of the Company purchased in open market transactions, or the transfer of any shares to any trust for the direct or indirect benefit of the Holder or the immediate family of the Holder, provided that the trustee of the trust
agrees to be bound in writing by the restrictions set forth herein, and provided further that any such transfer shall not involve a disposition for value, and shall be applicable to the Holders only if all officers and directors are
subject to the same restrictions and the Company uses commercially reasonable efforts to obtain a similar agreement from all stockholders individually owning more than one percent (1%) of the Company’s outstanding Common Stock (after giving
effect to conversion into Common Stock of all outstanding Preferred Stock). The underwriters in connection with such registration are intended third-party beneficiaries of this Subsection 2.11 and shall
have the right, power and 

  
 13 

 
authority to enforce the provisions hereof as though they were a party hereto. Each Holder further agrees to execute such agreements as may be reasonably requested by the underwriters in
connection with such registration that are consistent with this Subsection 2.11 or that are necessary to give further effect thereto. Any discretionary waiver or termination of the restrictions of any or all of such agreements by the Company
or the underwriters shall apply pro rata to all Holders subject to such agreements, based on the number of shares subject to such agreements. 

2.12 Restrictions on Transfer. 

(a) The Preferred Stock and the Registrable Securities shall not be sold, pledged, or otherwise transferred, and the Company shall not
recognize and shall issue stop-transfer instructions to its transfer agent with respect to any such sale, pledge, or transfer, except upon the conditions specified in this Agreement, which conditions are intended to ensure compliance with the
provisions of the Securities Act. A transferring Holder will cause any proposed purchaser, pledgee, or transferee of the Preferred Stock and the Registrable Securities held by such Holder to agree to take and hold such securities subject to the
provisions and upon the conditions specified in this Agreement. 
 (b) Each certificate, instrument, or book entry representing (i) the
Preferred Stock, (ii) the Registrable Securities, and (iii) any other securities issued in respect of the securities referenced in clauses (i) and (ii), upon any stock split, stock dividend, recapitalization, merger, consolidation, or
similar event, shall (unless otherwise permitted by the provisions of Subsection 2.12(c)) be notated with a legend substantially in the following form: 

THE SHARES REPRESENTED BY THIS CERTIFICATE HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED, AND HAVE BEEN ACQUIRED FOR
INVESTMENT AND NOT WITH A VIEW TO, OR IN CONNECTION WITH, THE SALE OR DISTRIBUTION THEREOF. NO SUCH TRANSFER MAY BE EFFECTED WITHOUT AN EFFECTIVE REGISTRATION STATEMENT RELATED THERETO OR A VALID EXEMPTION FROM THE REGISTRATION AND PROSPECTUS
DELIVERY REQUIREMENTS OF THE SECURITIES ACT OF 1933, AS AMENDED. 
 THE SECURITIES REPRESENTED HEREBY MAY BE TRANSFERRED ONLY IN ACCORDANCE
WITH THE TERMS OF AN AGREEMENT BETWEEN THE COMPANY AND THE STOCKHOLDER, AS AMENDED FROM TIME TO TIME, A COPY OF WHICH IS ON FILE WITH THE SECRETARY OF THE COMPANY. 

The Holders consent to the Company making a notation in its records and giving instructions to any transfer agent of the Restricted Securities in order to
implement the restrictions on transfer set forth in this Subsection 2.12. 

  
 14 

 (c) The holder of such Restricted Securities, by acceptance of ownership thereof, agrees to
comply in all respects with the provisions of this Section 2. Before any proposed sale, pledge, or transfer of any Restricted Securities, unless there is in effect a registration statement under the Securities Act covering
the proposed transaction, the Holder thereof shall give notice to the Company of such Holder’s intention to effect such sale, pledge, or transfer. Each such notice shall describe the manner and circumstances of the proposed sale, pledge, or
transfer in sufficient detail and, if reasonably requested by the Company, shall be accompanied at such Holder’s expense by either (i) a written opinion of legal counsel who shall, and whose legal opinion shall, be reasonably satisfactory
to the Company, addressed to the Company, to the effect that the proposed transaction may be effected without registration under the Securities Act; (ii) a “no action” letter from the SEC to the effect that the proposed sale, pledge,
or transfer of such Restricted Securities without registration will not result in a recommendation by the staff of the SEC that action be taken with respect thereto; or (iii) any other evidence reasonably satisfactory to counsel to the Company
to the effect that the proposed sale, pledge, or transfer of the Restricted Securities may be effected without registration under the Securities Act, whereupon the Holder of such Restricted Securities shall be entitled to sell, pledge, or transfer
such Restricted Securities in accordance with the terms of the notice given by the Holder to the Company. The Company will not require such a legal opinion or “no action” letter (x) in any transaction in compliance with SEC Rule 144;
or (y) in any transaction in which such Holder distributes Restricted Securities to an Affiliate of such Holder for no consideration; provided that each transferee agrees in writing to be subject to the terms of this Subsection
2.12. Each certificate, instrument, or book entry representing the Restricted Securities transferred as above provided shall be notated with, except if such transfer is made pursuant to SEC Rule 144, the appropriate restrictive legend set forth
in Subsection 2.12(b), except that such certificate instrument, or book entry shall not be notated with such restrictive legend if, in the opinion of counsel for such Holder and the Company, such legend is not required in order to establish
compliance with any provisions of the Securities Act. 
 2.13 Termination of Registration Rights. The right of any Holder to request
registration or inclusion of Registrable Securities in any registration pursuant to Subsections 2.1 or 2.2 shall terminate upon the earliest to occur of: 

(a) the closing of a Deemed Liquidation Event, as such term is defined in the Restated Certificate; and 

(b) the third anniversary of the IPO. 
 3.
Information and Observer Rights. 
 3.1 Delivery of Financial Statements. For so long as an aggregate of at least 7,000,000
shares of Registrable Securities remain outstanding (subject to adjustment for stock splits, dividends and the like with respect to the Preferred Stock), the Company shall deliver to each Major Investor, provided that the Board of Directors
has not reasonably determined that such Major Investor is a competitor of the Company: 
 (a) as soon as practicable, but in any event within
one hundred twenty (120) days after the end of each fiscal year of the Company (i) a balance sheet as of the end of such year, (ii) statements of income and of cash flows for such year, and a comparison between (x) the actual

  
 15 

 
amounts as of and for such fiscal year and (y) the comparable amounts for the prior year and as included in the Budget (as defined in Subsection 3.1(e)) for such year, with an
explanation of any material differences between such amounts and a schedule as to the sources and applications of funds for such year, and (iii) a statement of stockholders’ equity as of the end of such year, all such financial statements
audited and certified by independent public accountants of recognized standing selected by the Board of Directors of the Company (or the Audit Committee thereof); 

(b) as soon as practicable, but in any event within thirty (30) days after the end of each fiscal year of the Company, a statement
showing (i) all debt holders and (ii) the number of shares of each class and series of capital stock and securities convertible into or exercisable for shares of capital stock outstanding at the end of the period, the Common Stock issuable
upon conversion or exercise of any outstanding securities convertible or exercisable for Common Stock and the exchange ratio or exercise price applicable thereto, and the number of shares of issued stock options and stock options not yet issued but
reserved for issuance, if any, all in sufficient detail as to permit the Major Investors to calculate their respective percentage equity ownership in the Company; 

(c) as soon as practicable, but in any event within thirty (30) days after the end of each of the first three (3) quarters of each
fiscal year of the Company, (i) unaudited statements of income and cash flows for such fiscal quarter, and a comparison between (x) the actual amounts as of and for such fiscal quarter and (y) the comparable amounts for the prior
quarter and as included in the Budget for such quarter, with an explanation of any material differences between such amounts and a schedule as to the sources and applications of funds for such quarter, and (ii) an unaudited balance sheet as of
the end of such fiscal quarter, all prepared in accordance with GAAP (except that such financial statements may (a) be subject to normal year-end audit adjustments; and (b) not contain all notes
thereto that may be required in accordance with GAAP); 
 (d) as soon as practicable, but in any event within thirty (30) days of the
end of each month, an unaudited income statement for such month, and an unaudited balance sheet as of the end of such month, all prepared in accordance with GAAP (except that such financial statements may (i) be subject to normal year-end audit adjustments and (ii) not contain all notes thereto that may be required in accordance with GAAP); 

(e) as soon as practicable, but in any event thirty (30) days before the end of each fiscal year, a budget and business plan for the next
fiscal year (collectively, the “Budget”), prepared on a monthly basis, including balance sheets, income statements, and statements of cash flow for such months and, promptly after prepared, any other budgets or revised budgets
prepared by the Company; 
 (f) with respect to the financial statements called for in Subsection 3.1(c) and Subsection
3.1(d), an instrument executed by the chief financial officer and chief executive officer of the Company certifying that such financial statements were prepared in accordance with GAAP consistently applied with prior practice for earlier periods
(except as otherwise set forth in Subsection 3.1(c) and Subsection 3.1(d)) and fairly present the financial condition of the Company and its results of operation for the periods specified therein; and 

  
 16 

 (g) such other information relating to the financial condition, business, prospects, or corporate
affairs of the Company as any Major Investor may from time to time reasonably request; provided, however, that the Company shall not be obligated under this Subsection 3.1 to provide information (i) that the Company
reasonably determines in good faith to be a trade secret or confidential information (unless covered by an enforceable confidentiality agreement, in a form acceptable to the Company); or (ii) the disclosure of which would adversely affect the
attorney-client privilege between the Company and its counsel. 
 If, for any period, the Company has any subsidiary whose accounts are
consolidated with those of the Company, then in respect of such period the financial statements delivered pursuant to the foregoing sections shall be the consolidated and consolidating financial statements of the Company and all such consolidated
subsidiaries. 
 Notwithstanding anything else in this Subsection 3.1 to the contrary, the Company may cease providing the
information set forth in this Subsection 3.1 during the period starting with the date sixty (60) days before the Company’s good-faith estimate of the date of filing of a registration statement if it reasonably concludes it must do
so to comply with the SEC rules applicable to such registration statement and related offering; provided that the Company’s covenants under this Subsection 3.1 shall be reinstated at such time as the Company is no longer actively
employing its commercially reasonable efforts to cause such registration statement to become effective. 
 3.2 Inspection. For so
long as an aggregate of at least 7,000,000 shares of Registrable Securities remain outstanding (subject to adjustment for stock splits, dividends and the like with respect to the Preferred Stock), the Company shall permit each Major Investor,
provided that the Board of Directors has not reasonably determined that such Major Investor is a competitor of the Company, at such Major Investor’s expense, to visit and inspect the Company’s properties; examine its books of
account and records; and discuss the Company’s affairs, finances, and accounts with its officers, during normal business hours of the Company as may be reasonably requested by the Major Investor; provided, however, that the
Company shall not be obligated pursuant to this Subsection 3.2 to provide access to any information that it reasonably and in good faith considers to be a trade secret or confidential information (unless covered by an enforceable
confidentiality agreement, in form acceptable to the Company) or the disclosure of which would adversely affect the attorney-client privilege between the Company and its counsel. 

3.3 Observer Rights. The Company shall invite a representative of (i) 5AM Ventures IV, L.P., (ii) Deerfield Healthcare
Innovations Fund, L.P. and Deerfield Private Design Fund, L.P., collectively (together, “Deerfield”) and (iii) TLS Beta Pte. Ltd. to attend all meetings of its Board of Directors and any committee of its Board of
Directors in a nonvoting observer capacity and, in this respect, shall give such representative copies of all notices, minutes, consents, and other materials that it provides to its directors at the same time and in the same manner as provided to
such directors; provided, however, that such representative shall agree to hold in confidence and trust and to act in a fiduciary manner with respect to all information so provided; and provided further, that the Company reserves the right to
withhold any information and to exclude such representative from any meeting or portion thereof if access to such information or attendance at such meeting could adversely affect the attorney-client privilege between the Company and its counsel or
result in disclosure of trade secrets or a conflict of interest, or if such designating Investor or its representative is a competitor of the Company.  

  
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 3.4 Termination of Rights. The covenants set forth in Subsection 3.1, Subsection
3.2 and Subsection 3.3 shall terminate and be of no further force or effect (i) immediately before the consummation of the IPO, (ii) when the Company first becomes subject to the periodic reporting requirements of
Section 12(g) or 15(d) of the Exchange Act, or (iii) upon a Deemed Liquidation Event, whichever event occurs first. 
 3.5
Confidentiality. Each Investor agrees that such Investor will keep confidential and will not disclose, divulge, or use for any purpose (other than to monitor its investment in the Company) any confidential information obtained from the
Company pursuant to the terms of this Agreement (including notice of the Company’s intention to file a registration statement), unless such confidential information (a) is known or becomes known to the public in general (other than as a
result of a breach of this Subsection 3.5 by such Investor), (b) is or has been independently developed or conceived by the Investor without use of the Company’s confidential information, or (c) is or has been made known or
disclosed to the Investor by a third party without a breach of any obligation of confidentiality such third party may have to the Company; provided, however, that an Investor may disclose confidential information (i) to its attorneys,
accountants, consultants, and other professionals to the extent necessary to obtain their services in connection with monitoring its investment in the Company; (ii) to any prospective purchaser of any Registrable Securities from such Investor,
if such prospective purchaser agrees to be bound by the provisions of this Subsection 3.5; (iii) to any existing or prospective Affiliate, partner, member, stockholder, or wholly owned subsidiary of such Investor in the ordinary course
of business, provided that such Investor informs such Person that such information is confidential and directs such Person to maintain the confidentiality of such information; or (iv) as may otherwise be required by law, provided that the
Investor promptly notifies the Company of such disclosure and takes reasonable steps to minimize the extent of any such required disclosure. 
 4. Rights
to Future Stock Issuances. 
 4.1 Right of First Offer. Subject to the terms and conditions of this Subsection 4.1 and
applicable securities laws, if the Company proposes to offer or sell any New Securities, the Company shall first offer such New Securities to each Major Investor. Any Major Investor (other
than Caltech) shall be entitled to apportion the right of first offer hereby granted to it in such proportions as it deems appropriate, among (i) itself and (ii) its Affiliates, provided that each such Affiliate agrees to enter into this
Agreement and each of the Voting Agreement and Right of First Refusal and Co-Sale Agreement of even date herewith among the Company, the Investors and the other parties named therein, as an
“Investor” under each such agreement; provided that, Caltech may assign only its rights and obligations under this Subsection 4.1 in accordance with Subsection 6.1. 

(a) The Company shall give notice (the “Offer Notice”) to each Major Investor, stating (i) its bona fide intention to
offer such New Securities, (ii) the number of such New Securities to be offered, and (iii) the price and terms, if any, upon which it proposes to offer such New Securities. 

(b) By notification to the Company within twenty (20) days after the Offer Notice is given, each Major Investor may elect to purchase or
otherwise acquire, at the price and on the terms specified in the Offer Notice, up to that portion of such New Securities which equals 

  
 18 

 
the proportion that the Common Stock then held by such Major Investor (including all shares of Common Stock then issuable (directly or indirectly) upon conversion and/or exercise, as applicable,
of the Preferred Stock and any other Derivative Securities then held by such holder) bears to the total Common Stock of the Company then outstanding (assuming full conversion and/or exercise, as applicable, of all Preferred Stock and other
Derivative Securities) At the expiration of such twenty (20) day period, the Company shall promptly notify any Major Investor that elects to purchase or acquire all the shares available to it (each, a “Fully Exercising
Investor”) of any other Major Investor’s failure to do likewise. During the ten (10) day period commencing after the Company has given such notice, each Fully Exercising Investor may, by giving notice to the Company, elect to
purchase or acquire, in addition to the number of shares specified above, up to that portion of the New Securities for which the Major Investors were entitled to subscribe but that were not subscribed for by the Major Investors which is equal to the
proportion that the Common Stock issued and held, or issuable (directly or indirectly) upon conversion and/or exercise, as applicable, of Preferred Stock and any other Derivative Securities then held, by such Fully Exercising Investor bears to the
Common Stock issued and held, or issuable (directly or indirectly) upon conversion and/or exercise, as applicable, of the Preferred Stock and any other Derivative Securities then held, by all Fully Exercising Investors who wish to purchase
such unsubscribed shares. The closing of any sale pursuant to this Subsection 4.1(b) shall occur within the later of ninety (90) days of the date that the Offer Notice is given and the
date of initial sale of New Securities pursuant to Subsection 4.1(c). 
 (c) If all New Securities referred to in the Offer Notice
are not elected to be purchased or acquired as provided in Subsection 4.1(b), the Company may, during the ninety (90) day period following the expiration of the periods provided in Subsection 4.1(b), offer and sell the remaining
unsubscribed portion of such New Securities to any Person or Persons at a price not less than, and upon terms no more favorable to the offeree than, those specified in the Offer Notice. If the Company does not enter into an agreement for the sale of
the New Securities within such period, or if such agreement is not consummated within thirty (30) days of the execution thereof, the right provided hereunder shall be deemed to be revived and such New Securities shall not be offered unless
first reoffered to the holders of Preferred Stock in accordance with this Subsection 4.1. 
 (d) The right of first offer in this
Subsection 4.1 shall not be applicable to (i) Exempted Securities (as defined in the Restated Certificate); (ii) shares of Common Stock issued in the IPO; (iii) the issuance of shares of Preferred Stock to pursuant to the Purchase
Agreement; and (iv) the issuance of up to 6,944,445 shares of Series B Preferred Stock to Novartis Institutes For Biomedical Research, Inc. or its affiliates (“Novartis”) in connection with the Company entering into a
collaboration arrangement with Novartis. 
 4.2 Directed IPO. If an IPO is undertaken, the Company will use its reasonable best
efforts to cause the managing underwriter(s) of the IPO to designate a number of shares of the Common Stock to be offered in the IPO with an aggregate offering price of at least $10,000,000 (based on the price of the Common Stock in the IPO) for
sale under a “directed shares program,” and shall instruct such underwriter(s) to allocate such directed shares program to Deerfield. The shares designated by the underwriter(s) for sale under a directed shares program are referred to
herein as “directed shares.” Deerfield acknowledges that, despite the Company’s use of its reasonable best efforts, the underwriter(s) may determine in their sole discretion that it is not

  
 19 

 
advisable to designate all such shares as directed shares in the IPO, in which case the number of directed shares may be reduced or no directed shares may be designated, as applicable. Deerfield
also acknowledges that notwithstanding the terms of this Agreement, the sale of any directed shares to Deerfield pursuant to this Section 4.2 will only be made in compliance with NASD Rules 2110 and 2790 and federal, state, and local laws,
rules, and regulations. In the event the sale of directed shares to Deerfield pursuant to this Section 4.2 is prohibited by federal, state, or local laws, rules or regulations (as determined by either (a) the Company or
(b) Deerfield), the Company shall undertake its reasonable best efforts to approve and execute a private placement of Common Stock with an aggregate amount of proceeds equal to $10,000,000 (with a per share purchase price equal to the public
offering price of the Common Stock in the IPO) with Deerfield concurrent with the closing of the IPO on such terms and conditions as are standard for such transactions and reasonably acceptable to each of the Company and Deerfield. 

4.3 Termination. The covenants set forth in Subsection 4.1 and Subsection 4.2 shall terminate and be of no further force
or effect upon the earliest of (i) immediately before the consummation of the IPO (provided in the case of Subsection 4.2, that the Company complies with such Subsection in connection with such IPO), (ii) when the Company first becomes subject
to the periodic reporting requirements of Section 12(g) or 15(d) of the Exchange Act, or (iii) upon a Deemed Liquidation Event. 
 5.
Additional Covenants. 
 5.1 Insurance. The Company shall obtain, within ninety (90) days of the date hereof, from
financially sound and reputable insurers, Directors and Officers liability insurance, with a limit of liability not less than three million dollars ($3,000,000) in an amount and on terms and conditions satisfactory to the Board of Directors, and
will cause such insurance policy to be maintained until such time as the Board of Directors determines that such insurance should be discontinued and shall not be cancelable by the Company without the prior approval by the Board of Directors. 

5.2 Employee Agreements. The Company will cause (i) each person now or hereafter employed by it or by any subsidiary (or engaged
by the Company or any subsidiary as a consultant/independent contractor) to enter into a nondisclosure and proprietary rights assignment agreement in a form reasonably acceptable to the Board of Directors; and (ii) each Founder and Key Employee
to enter into a one (1) year nonsolicitation agreement, in a form reasonably acceptable to the Board of Directors. In addition, the Company shall not amend, modify, terminate, waive, or otherwise alter, in whole or in part, any of the
above-referenced agreements or any restricted stock agreement between the Company and any employee, without the consent of the Board of Directors. 

5.3 Employee Stock. Unless otherwise approved by the Board of Directors, all future employees and consultants of the Company who
purchase, receive options to purchase, or receive awards of shares of the Company’s capital stock after the date hereof shall be required to execute restricted stock or option agreements, as applicable, providing for (i) vesting of shares
over a four (4) year period, with the first twenty-five percent (25%) of such shares vesting following twelve (12) months of continued employment or service, and the remaining shares vesting in equal monthly installments over the following
thirty-six (36) months, and (ii) a market stand-off 

  
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provision substantially similar to that in Subsection 2.11. In addition, unless otherwise approved by the Board of Directors, the Company shall retain a “right of first
refusal” on employee transfers until the Company’s IPO and shall have the right to repurchase unvested shares at cost upon termination of employment of a holder of restricted stock. 

5.4 Matters Requiring Investor Director Approval. So long as the holders of Preferred Stock are entitled to elect a Preferred Director,
the Company hereby covenants and agrees with each of the Investors that it shall not, without approval of the Board of Directors, which approval must include the affirmative vote of at least two Preferred Directors: 

(a) make, or permit any subsidiary to make, any loan or advance to, or own any stock or other securities of, any subsidiary or other
corporation, partnership, or other entity unless it is wholly owned by the Company; 
 (b) make, or permit any subsidiary to make, any loan
or advance to any Person, including, without limitation, any employee or director of the Company or any subsidiary, except advances and similar expenditures in the ordinary course of business or under the terms of an employee stock or option plan
approved by the Board of Directors; 
 (c) guarantee, directly or indirectly, or permit any subsidiary to guarantee, directly or indirectly,
any indebtedness except for trade accounts of the Company or any subsidiary arising in the ordinary course of business; 
 (d) make any
investment inconsistent with any investment policy approved by the Board of Directors; 
 (e) incur any aggregate indebtedness that is not
already included in a budget approved by the Board of Directors, other than trade credit incurred in the ordinary course of business; 
 (f)
otherwise enter into or be a party to any transaction with any director, officer, or employee of the Company or any “associate” (as defined in Rule 12b-2 promulgated under the Exchange Act) of any
such Person; 
 (g) hire, terminate, or change the compensation of the executive officers, including approving any option grants or stock
awards to executive officers; 
 (h) change the principal business of the Company, enter new lines of business, or exit the current line of
business; 
 (i) sell, assign, license, pledge, or encumber material technology or intellectual property, other than licenses granted in the
ordinary course of business; or 
 (j) enter into any corporate strategic relationship involving the payment, contribution, or assignment by
the Company or to the Company of money or assets greater than $500,000. 

  
 21 

 5.5 Board Matters. Unless otherwise determined by the vote of a majority of the directors
then in office, the Board of Directors shall meet at least quarterly in accordance with an agreed-upon schedule. The Company shall reimburse the non-employee directors and observers for all reasonable out-of-pocket travel expenses incurred (consistent with the Company’s travel policy) in connection with attending meetings of the Board of Directors. The Company shall
cause to be established, as soon as practicable after such request, and will maintain, an audit committee (the “Audit Committee”) and compensation committee (the “Compensation Committee”), each of which shall
consist of three (3) non-management directors. The Audit Committee will have the authority to approve the Budget and all non-budgeted capital expenditures. The
Compensation Committee will have the authority to approve all compensation plans, including the issuance of options, stock and other incentive compensation for employees earning more than $200,000 per year. Each
non-employee director shall be entitled in such person’s discretion to be a member of any Board committee. Each committee of the Board of Directors shall have at least one Preferred Director as a member.

 5.6 Successor Indemnification. If the Company or any of its successors or assignees consolidates with or merges into any other
Person and is not the continuing or surviving corporation or entity of such consolidation or merger, then to the extent necessary, proper provision shall be made so that the successors and assignees of the Company assume the obligations of the
Company with respect to indemnification of members of the Board of Directors as in effect immediately before such transaction, whether such obligations are contained in the Company’s Bylaws, its Certificate of Incorporation, or elsewhere, as
the case may be. 
 5.7 Expenses of Counsel. In the event of a transaction which is a Sale of the Company (as defined in the Voting
Agreement of even date herewith among the Investors and the Company), the reasonable fees and disbursements, not to exceed $50,000, of one counsel for the Investors (“Investor Counsel”), in their capacities as stockholders, shall be
borne and paid by the Company. At the outset of considering a transaction which, if consummated would constitute a Sale of the Company, the Company shall obtain the ability to share with the Investor Counsel (and such counsel’s clients) and
shall share the confidential information (including, without limitation, the initial and all subsequent drafts of memoranda of understanding, letters of intent and other transaction documents and related noncompete, employment, consulting and other
compensation agreements and plans) pertaining to and memorializing any of the transactions which, individually or when aggregated with others would constitute the Sale of the Company. The Company shall be obligated to share (and cause the
Company’s counsel and investment bankers to share) such materials when distributed to the Company’s executives and/or any one or more of the other parties to such transaction(s). In the event that Investor Counsel deems it appropriate, in
its reasonable discretion, to enter into a joint defense agreement or other arrangement to enhance the ability of the parties to protect their communications and other reviewed materials under the attorney client privilege, the Company shall, and
shall direct its counsel to, execute and deliver to Investor Counsel and its clients such an agreement in form and substance reasonably acceptable to Investor Counsel. In the event that one or more of the other party or parties to such transactions
require the clients of Investor Counsel to enter into a confidentiality agreement and/or joint defense agreement in order to receive such information, then the Company shall share whatever information can be shared without entry into such agreement
and shall, at the same time, in good faith work expeditiously to enable Investor Counsel and its clients to negotiate and enter into the appropriate agreement(s) without undue burden to the clients of Investor Counsel. 

  
 22 

 5.8 Indemnification Matters. The Company hereby acknowledges that one (1) or more of
the directors nominated to serve on the Board of Directors by the Investors (each a “Fund Director”) may have certain rights to indemnification, advancement of expenses and/or insurance provided by one or more of the Investors and
certain of their affiliates (collectively, the “Fund Indemnitors”). The Company hereby agrees (a) that it is the indemnitor of first resort (i.e., its obligations to any such Fund Director are primary and any
obligation of the Fund Indemnitors to advance expenses or to provide indemnification for the same expenses or liabilities incurred by such Fund Director are secondary), (b) that it shall be required to advance the full amount of expenses incurred by
such Fund Director and shall be liable for the full amount of all expenses, judgments, penalties, fines and amounts paid in settlement by or on behalf of any such Fund Director to the extent legally permitted and as required by the Company’s
Certificate of Incorporation or Bylaws of the Company (or any agreement between the Company and such Fund Director), without regard to any rights such Fund Director may have against the Fund Indemnitors, and, (c) that it irrevocably waives,
relinquishes and releases the Fund Indemnitors from any and all claims against the Fund Indemnitors for contribution, subrogation or any other recovery of any kind in respect thereof. The Company further agrees that no advancement or payment by
the Fund Indemnitors on behalf of any such Fund Director with respect to any claim for which such Fund Director has sought indemnification from the Company shall affect the foregoing and the Fund Indemnitors shall have a right of contribution and/or
be subrogated to the extent of such advancement or payment to all of the rights of recovery of such Fund Director against the Company. 

5.9 Right to Conduct Activities. The Company hereby agrees and acknowledges that certain of the Investors and
certain of their respective Affiliates are professional venture capital and private equity investment funds (collectively, the “Funds”), and as such invest in numerous portfolio companies, some of which may be deemed competitive
with the Company’s business (as currently conducted or as may be conducted in the future). The parties agree that no Fund or any Fund Affiliate investment fund or any of their Affiliates, or any of their or their Affiliates’ partners,
officers or representatives which manage or advise any such investment funds shall be considered a competitor of the Company solely as a result of such investment, management or advisory activities for purposes of this Agreement and the Company
agrees that, to the extent permitted under applicable law, neither the Funds nor their Affiliates shall be liable to the Company for any claim solely arising out of, or solely based upon, (i) the investment by a Fund or any of their Affiliates
in any entity competitive with the Company, or (ii) actions taken by any partner, officer or other representative of a Fund or Fund Affiliate to assist any such competitive company, whether or not such action was taken as a member of the board
of directors of such competitive company or otherwise, and whether or not such action has a detrimental effect on the Company; provided, however, that the foregoing shall not relieve (x) any of the Funds from liability associated with the
unauthorized disclosure of the Company’s confidential information obtained pursuant to this Agreement, or (y) any director or officer of the Company from any liability associated with his or her fiduciary duties to the Company. 

  
 23 

 5.10 Tax Reporting. The Company will comply with any obligation imposed on the Company to
make any filing (including any filing on Internal Revenue Service Form 5471) as a result of any interest that the Company holds in a non-U.S. Person or any activities that the Company conducts outside of the
U.S. and shall include in such filing any information necessary to obviate (to the extent possible) any similar obligation to which any shareholder would otherwise be subject with respect to such interest or such activity. The Company shall promptly
provide each Investor with a copy of any such filing. 
 5.11 Restrictions on Publicity. Each Investor agrees not to discuss the
Purchase Agreement (including the transactions contemplated thereunder), use the name or logo of the Company or its Affiliates, or refer to the Company or its Affiliates, directly or indirectly, in connection with the sale of the Preferred Stock, in
any advertisement, press release, professional or trade publication, or in any other manner without the approval of the Board of Directors. 

5.12 Termination of Covenants. The covenants set forth in this Section 5, except for Subsections 5.6,
through 5.9, shall terminate and be of no further force or effect (i) immediately before the consummation of the IPO, (ii) when the Company first becomes subject to the periodic reporting requirements of Section 12(g) or 15(d)
of the Exchange Act, or (iii) upon a Deemed Liquidation Event, whichever event occurs first. 
 6. Miscellaneous. 

6.1 Successors and Assigns. The rights under this Agreement may be assigned (but only with all related obligations) by a Holder
to a transferee of Registrable Securities that (i) is an Affiliate of a Holder; (ii) is a Holder’s Immediate Family Member or trust for the benefit of an individual Holder or one or more of such Holder’s Immediate Family Members;
or (iii) after such transfer, holds at least 20% of such Holder’s shares of Registrable Securities immediately prior to such assignment or transfer; provided, however, that (x) the Company is, within a reasonable time
after such transfer, furnished with written notice of the name and address of such transferee and the Registrable Securities with respect to which such rights are being transferred; and (y) such transferee agrees in a written instrument
delivered to the Company to be bound by and subject to the terms and conditions of this Agreement, including the provisions of Subsection 2.11. For the purposes of determining the number of shares of Registrable Securities held by a
transferee, the holdings of a transferee (1) that is an Affiliate or stockholder of a Holder; (2) who is a Holder’s Immediate Family Member; or (3) that is a trust for the benefit of an individual Holder or such Holder’s
Immediate Family Member shall be aggregated together and with those of the transferring Holder; provided further that all transferees who would not qualify individually for assignment of rights shall have a single attorney-in-fact for the purpose of exercising any rights, receiving notices, or taking any action under this Agreement. Notwithstanding the foregoing, Caltech may not assign
or otherwise transfer any of its rights under this Agreement without the prior written consent of the Company; provided, however, that Caltech may assign its rights under Subsection 4.1 (together with its obligations) to Osage
University Partners after giving the Company prior written notice of such transfer or to any other entity approved in writing in advance by the Company, and such assignee agrees in a written instrument delivered to the Company to be bound by and
subject to the terms and conditions of this Agreement, including without limitation the provisions of Subsection 2.11, and any other agreements between the Company and its stockholders applicable to the securities purchased by such assignee.
The terms and conditions of this Agreement inure to the benefit of and are binding upon the respective successors and permitted assignees of the parties. Nothing in this Agreement, express or implied, is intended to confer upon any party other than
the parties hereto or their respective successors and permitted assignees any rights, remedies, obligations or liabilities under or by reason of this Agreement, except as expressly provided herein. 

  
 24 

 6.2 Governing Law. This Agreement shall be governed by the internal law of the State of
Delaware, without regard to any conflicts of laws principles that could result in the application of laws of any other jurisdiction. 
 6.3
Counterparts. This Agreement may be executed in two (2) or more counterparts, each of which shall be deemed an original, but all of which together shall constitute one and the same instrument. Counterparts may be delivered via facsimile,
electronic mail (including pdf or any electronic signature complying with the U.S. federal ESIGN Act of 2000, e.g., www.docusign.com) or other transmission method and any counterpart so delivered shall be deemed to have been duly and validly
delivered and be valid and effective for all purposes. 
 6.4 Titles and Subtitles. The titles and subtitles used in this Agreement
are for convenience only and are not to be considered in construing or interpreting this Agreement. 
 6.5 Notices. All notices and
other communications given or made pursuant to this Agreement shall be in writing and shall be deemed effectively given upon the earlier of actual receipt or (i) personal delivery to the party to be notified; (ii) when sent, if sent by
electronic mail or facsimile during the recipient’s normal business hours, and if not sent during normal business hours, then on the recipient’s next business day; (iii) five (5) days after having been sent by registered or certified
mail, return receipt requested, postage prepaid; or (iv) one (1) business day after the business day of deposit with a nationally recognized overnight courier, freight prepaid, specifying next-day
delivery, with written verification of receipt. All communications shall be sent to the respective parties at their addresses as set forth on Schedule A hereto (including any address designated to receive a copy of such communication, or to
the principal office of the Company and to the attention of the Chief Executive Officer, in the case of the Company, or to such email address, facsimile number, or address as subsequently modified by written notice given in accordance with this
Subsection 6.5. If notice is given to the Company, a copy shall also be sent to Peter N. Handrinos, Latham & Watkins LLP, 200 Clarendon Street, Boston, Massachusetts 02116, facsimile no.: (617)
948-6001, electronic mail: peter.handrinos@lw.com. If any notice or other communication given or made pursuant to this Agreement is required to be given to a group of parties pursuant to the terms hereof, such
notice shall be sent to the members of such group substantially simultaneously (and in any event with a 24 hour period). 
 6.6
Amendments and Waivers. Any term of this Agreement may be amended and the observance of any term of this Agreement may be waived (either generally or in a particular instance, and either retroactively or prospectively) only with the written
consent of the Company and the holders of at least seventy-one and one-half percent (71.5%) of the Registrable Securities then outstanding; provided that the
Company may in its sole discretion waive compliance with Subsection 2.12(c) (and the Company’s failure to object promptly in writing after notification of a proposed assignment allegedly in violation of Subsection 2.12(c) shall be
deemed to be a waiver); provided further that any provision hereof may be waived by any waiving party on such party’s own behalf, without the consent of any other party; and provided further that the Company may
update Schedule A hereto at any time to reflect any transfers of shares of the Company’s capital 

  
 25 

 
stock or parties to be added to this Agreement, effected in accordance with the terms hereof, and to correct any errors in the information set forth therein, without the consent of the other
parties hereto. Notwithstanding the foregoing, this Agreement may not be amended or terminated and the observance of any term hereof may not be waived with respect to any Investor without the written consent of such Investor, unless such amendment,
termination, or waiver applies to all Investors in the same fashion (it being agreed that a waiver of the provisions of Section 4 with respect to a particular transaction shall be deemed to apply to all Investors in the
same fashion if such waiver does so by its terms, notwithstanding the fact that certain Investors may nonetheless, by agreement with the Company, purchase securities in such transaction). The Company shall give prompt notice of any amendment or
termination hereof or waiver hereunder to any party hereto that did not consent in writing to such amendment, termination, or waiver. Any amendment, termination, or waiver effected in accordance with this Subsection 6.6 shall be binding on
all parties hereto, regardless of whether any such party has consented thereto. No waivers of or exceptions to any term, condition, or provision of this Agreement, in any one or more instances, shall be deemed to be or construed as a further or
continuing waiver of any such term, condition, or provision. 
 6.7 Severability. In case any one or more of the provisions contained
in this Agreement is for any reason held to be invalid, illegal or unenforceable in any respect, such invalidity, illegality, or unenforceability shall not affect any other provision of this Agreement, and such invalid, illegal, or unenforceable
provision shall be reformed and construed so that it will be valid, legal, and enforceable to the maximum extent permitted by law. 
 6.8
Aggregation of Stock. All shares of Registrable Securities held or acquired by Affiliates (other than Affiliates of Caltech) shall be aggregated together for the purpose of determining the availability of any rights under this Agreement and
such Affiliated persons may apportion such rights as among themselves in any manner they deem appropriate. 
 6.9 Entire Agreement.
This Agreement (including any Schedules and Exhibits hereto) constitutes the full and entire understanding and agreement among the parties with respect to the subject matter hereof, and any other written or oral agreement relating to the subject
matter hereof existing between the parties is expressly canceled. Upon the effectiveness of this Agreement, the Prior Agreement shall be deemed amended and restated and superseded and replaced in its entirety by this Agreement, and shall be of no
further force or effect. 
 6.10 Dispute Resolution. Any unresolved controversy or claim arising out of or relating to this
Agreement, except as (i) otherwise provided in this Agreement, or (ii) any such controversies or claims arising out of the Company’s intellectual property rights for which a provisional remedy or equitable relief is sought, shall be
submitted to arbitration by one arbitrator mutually agreed upon by the parties, and if no agreement can be reached within thirty (30) days after names of potential arbitrators have been proposed by the American Arbitration Association (the
“AAA”), then by one arbitrator having reasonable experience in corporate finance transactions of the type provided for in this Agreement and who is chosen by the AAA. The arbitration shall take place in Boston, Massachusetts, in
accordance with the AAA rules then in effect, and judgment upon any award rendered in such arbitration will be binding and may be entered in any court having jurisdiction thereof. There shall be limited discovery prior to the arbitration hearing as
follows: (a) exchange of witness lists and copies of documentary evidence and documents relating to or arising out of the issues to be arbitrated, (b) depositions of all party witnesses, and

  
 26 

 
(c) such other depositions as may be allowed by the arbitrators upon a showing of good cause. Depositions shall be conducted in accordance with the Delaware Code of Civil Procedure, the
arbitrator shall be required to provide in writing to the parties the basis for the award or order of such arbitrator, and a court reporter shall record all hearings, with such record constituting the official transcript of such proceedings. Each
party will bear its own costs in respect of any disputes arising under this Agreement. Each of the parties to this Agreement consents to personal jurisdiction for any equitable action sought in the U.S. District Court for the District of Delaware or
the Court of Chancery of the State of Delaware. 
 WAIVER OF JURY TRIAL: EACH PARTY HEREBY WAIVES ITS RIGHTS TO A JURY TRIAL OF ANY CLAIM OR CAUSE OF
ACTION BASED UPON OR ARISING OUT OF THIS AGREEMENT, THE OTHER TRANSACTION DOCUMENTS, THE SECURITIES OR THE SUBJECT MATTER HEREOF OR THEREOF. THE SCOPE OF THIS WAIVER IS INTENDED TO BE ALL-ENCOMPASSING OF ANY
AND ALL DISPUTES THAT MAY BE FILED IN ANY COURT AND THAT RELATE TO THE SUBJECT MATTER OF THIS TRANSACTION, INCLUDING, WITHOUT LIMITATION, CONTRACT CLAIMS, TORT CLAIMS (INCLUDING NEGLIGENCE), BREACH OF DUTY CLAIMS, AND ALL OTHER COMMON LAW AND
STATUTORY CLAIMS. THIS SECTION HAS BEEN FULLY DISCUSSED BY EACH OF THE PARTIES HERETO AND THESE PROVISIONS WILL NOT BE SUBJECT TO ANY EXCEPTIONS. EACH PARTY HERETO HEREBY FURTHER WARRANTS AND REPRESENTS THAT SUCH PARTY HAS REVIEWED THIS WAIVER WITH
ITS LEGAL COUNSEL, AND THAT SUCH PARTY KNOWINGLY AND VOLUNTARILY WAIVES ITS JURY TRIAL RIGHTS FOLLOWING CONSULTATION WITH LEGAL COUNSEL. 

6.11 Delays or Omissions. No delay or omission to exercise any right, power, or remedy accruing to any party under this Agreement, upon
any breach or default of any other party under this Agreement, shall impair any such right, power, or remedy of such nonbreaching or nondefaulting party, nor shall it be construed to be a waiver of or acquiescence to any such breach or default, or
to any similar breach or default thereafter occurring, nor shall any waiver of any single breach or default be deemed a waiver of any other breach or default theretofore or thereafter occurring. All remedies, whether under this Agreement or by law
or otherwise afforded to any party, shall be cumulative and not alternative. 
 6.12 Acknowledgment. The Company acknowledges that
the Investors are in the business of venture capital investing and therefore review the business plans and related proprietary information of many enterprises, including enterprises which may have products or services which compete directly or
indirectly with those of the Company. Nothing in this Agreement shall preclude or in any way restrict the Investors from investing or participating in any particular enterprise whether or not such enterprise has products or services which compete
with those of the Company. 
 6.13 Additional Investors. Notwithstanding anything to the contrary contained herein, if the Company
issues additional shares of Preferred Stock after the date hereof, any purchaser of such shares of Preferred Stock may become a party to this Agreement by executing and delivering an additional counterpart signature page to this Agreement, and
thereafter shall be deemed an “Investor” for all purposes hereunder. No action or consent by the Investors shall be required for such joinder to this Agreement by such additional Investor, so long as such additional Investor has agreed in
writing to be bound by all of the obligations as an “Investor” hereunder. 

  
 27 

 6.14 Caltech. Caltech hereby agrees that the rights granted to Caltech hereunder shall be
deemed to fully satisfy the obligations of the Company under Sections 5.13 and 5.14 of the Caltech Agreement, and neither Section 5.13 nor Section 5.14 of the Caltech Agreement shall have any further force or effect. 

[Remainder of Page Intentionally Left Blank] 

  
 28 

 IN WITNESS WHEREOF, the parties have executed this Amended and Restated Investors’ Rights
Agreement as of the date first written above. 
  

			
	HOMOLOGY MEDICINES, INC.
		
	By:	 	/s/ Arthur Tzianabos
	Name:	 	Arthur Tzianabos, Ph.D.
	Title:	 	President and Chief Executive Officer

  
 [Signature Page to
Investors’ Rights Agreement] 

 
			
	INVESTORS:
	  
 5AM VENTURES IV, L.P.

	
	 by 5AM Partners IV, LLC
 its General
Partner

		
	By:	 	/s/ Scott Rocklage
	Name:	 	Scott Rocklage
	Title:	 	Managing Member

  

			
	5AM CO-INVESTORS IV, L.P.
	
	 by 5AM Partners IV, LLC
 its General
Partner

		
	By:	 	/s/ Scott Rocklage
	Name:	 	Scott Rocklage
	Title:	 	Managing Member

  
 [Signature Page to
Investors’ Rights Agreement] 

 
			
	INVESTORS:
	
	ARCH VENTURE FUND VIII, L.P.
	
	By: ARCH Venture Partners VIII, L.P., its General Partner
	
	By: ARCH Venture Partners VIII, LLC, its General Partner
		
	By:	 	/s/ Mark McDonnell 
	Name:	 	Mark McDonnell
	Title:	 	Managing Director

  

			
	ARCH VENTURE FUND VIII OVERAGE, L.P.
	
	By: ARCH Venture Partners VIII, LLC, its General Partner
		
	By:	 	/s/ Mark McDonnell
	Name:	 	Mark McDonnell
	Title:	 	Managing Director

  
 [Signature Page to
Investors’ Rights Agreement] 

 
			
	INVESTORS:
	
	TLS BETA PTE. LTD.
		
	By:	 	/s/ Khoo Shih
	Name:	 	Khoo Shih
	Title:	 	Authorized Signatory

  
 [Signature Page to
Investors’ Rights Agreement] 

 
			
	INVESTORS:
	
	DEERFIELD HEALTHCARE INNOVATIONS FUND, L.P.
	
	By: Deerfield Mgmt HIF, L.P., its General Partner
	
	By: J.E. Flynn Capital HIF LLC, its General Partner
		
	By:	 	/s/ David J. Clark
	Name:	 	David J. Clark
	Title:	 	Authorized Signatory

  

			
	DEERFIELD PRIVATE DESIGN FUND III, L.P.
	
	By: Deerfield Mgmt III, L.P., its General Partner
	
	By: J.E. Flynn Capital III LLC, its General Partner
		
	By:	 	/s/ David J. Clark
	Name:	 	David J. Clark
	Title:	 	Authorized Signatory

  
 [Signature Page to
Investors’ Rights Agreement] 

 
			
	INVESTORS:
	
	 FIDELITY GROWTH COMPANY
 COMMINGLED
POOL

	By:	 	Fidelity Management & Trust Co.
		
	By:	 	/s/ Stacie M. Smith 

 
			
	Name: Stacie M. Smith
	Title: Authorized Signatory
	
	 FIDELITY MT. VERNON STREET TRUST:

FIDELITY SERIES GROWTH COMPANY FUND

		
	By:	 	/s/ Stacie M. Smith 

 
			
	Name: Stacie M. Smith
	Title: Authorized Signatory
	
	 FIDELITY MT. VERNON STREET TRUST:

FIDELITY GROWTH COMPANY FUND

		
	By:	 	/s/ Stacie M. Smith

 
			
	Name: Stacie M. Smith
	Title: Authorized Signatory

 [Signature Page to Investors’ Rights Agreement] 

 
			
	INVESTORS:
	
	ROCK SPRINGS CAPITAL MASTER FUND LP
		
	By:	 	Rock Springs General Partner LLC
		
	By:	 	/s/ Kris Jenner
	Name: Kris Jenner
	Title: Managing Member

 [Signature Page to Investors’ Rights Agreement] 

 
			
	INVESTORS:
	
	OSAGE UNIVERSITY PARTNERS II, L.P.
		
	By:	 	Osage University GP II, LP, its general partner
		
	By:	 	Osage Partners, LLC, its general partner
		
	By:	 	/s/ William Harrington
	Name: William Harrington
	Title: Member

 [Signature Page to Investors’ Rights Agreement] 

 
			
	INVESTORS:
	
	VIDA VENTURES, LLC
		
	By:	 	VV Manager, LLC., its Managing Member
		
	By:	 	/s/ Clive D. Bode
	Name: Clive D. Bode
	Title: Manager

 [Signature Page to Investors’ Rights Agreement] 

 
			
	INVESTORS:
	
	ALEXANDRIA VENTURE INVESTMENTS, LLC,
	a Delaware limited liability company
		
	By:	 	Alexandria Real Estate Equities, Inc.,
	a Maryland corporation, managing member
		
	By:	 	/s/ Aaron Jacobson
	Name: Aaron Jacobson
	Title: VP – Corporate Counsel

 [Signature Page to Investors’ Rights Agreement] 

 
			
	INVESTORS:
	
	 NOVARTIS INSTITUTES FOR BIOMEDICAL

RESEARCH, INC.

		
	By:	 	/s/ Scott A. Brown
	Name: Scott A. Brown
	Title: VP, General Counsel

 [Signature Page to Investors’ Rights Agreement] 

 
			
	INVESTORS:
	
	VIVO PANDA FUND, L.P.
		
	By:	 	Vivo Panda, LLC, its general partner
		
	By:	 	/s/ Mahendra Shah
	Name: Mahendra Shah
	Title: Managing Member

 [Signature Page to Investors’ Rights Agreement] 

 
			
	INVESTORS:
	
	 HBM HEALTHCARE INVESTMENTS
 (CAYMAN)
LTD.

		
	By:	 	/s/ Jean Marc LeSieur
	Name: Jean Marc LeSieur
	Title: Director

 [Signature Page to Investors’ Rights Agreement] 

 
			
	INVESTORS:
	
	 MAVERICK PRIVATE OPPORTUNITIES

FUND, L.P.

	
	By: Maverick Capital Ventures, LLC, its General Partner
	
	By: Maverick Capital Advisors, L.P., its Manager
		
	By:	 	/s/ Ginessa A. Avila
	Name: Ginessa A. Avila
	Title: Authorized Representative
	
	MAVERICK ADVISORS FUND, L.P.
	
	By: Maverick Capital Ventures, LLC, its General Partner
	
	By: Maverick Capital Advisors, L.P., its Manager
		
	By:	 	/s/ Ginessa A. Avila
	Name: Ginessa A. Avila
	Title: Authorized Representative

 [Signature Page to Investors’ Rights Agreement] 

 
			
	For purposes of Section 6.14 and the rights and obligations referenced therein:
	
	 CALIFORNIA INSTITUTE OF TECHNOLOGY

(Caltech)

		
	By:	 	/s/ Fred Farina
	Name: Fred Farina
	Title: Chief Innovation & Corporate Partnership Officer

 [Signature Page to Investors’ Rights Agreement] 

 SCHEDULE A 

Investors 
 5AM Ventures IV, L.P. 

5AM Co-Investors IV, L.P. 

Address: 
 2200 Sand Hill
Road, Suite 110 
 Menlo Park, CA 94025 

Alexandria Venture Investments, LLC 

Address: 
 385 E. Colorado
Blvd., Suite 299 
 Pasadena, CA 91101 
 ARCH
Venture Fund VIII, L.P. 
 ARCH Venture Fund VIII Overage, L.P. 

Address: 
 c/o ARCH Venture
Partners VIII, L.P. 
 8755 W. Higgins Road, Suite 1025 

Chicago, IL 60631 
 Attn: Mark
McDonnell 
 Phone: [***] 
 Fax:
[***] 
 Email: [***] 
 With
a copy, which shall not constitute notice, to: 
 Proskauer Rose LLP 

One International Place 
 Boston,
MA 02110 
 Attn: [***] 
 Phone:
[***] 
 Fax: [***] 
 Email:
[***] 
 Deerfield Healthcare Innovations Fund, L.P. 

Deerfield Private Design Fund III, L.P. 

Address: 
 Deerfield
Management Company, L.P. 
 780 Third Avenue, 37th Flr. 

New York, NY 10017 
 Attention:
Lawrence Atinsky 
 Tel: [***]Fax: [***] 

With a copy, which shall not constitute notice, to: 

Goodwin Procter LLP 
 100 Northern
Ave. 
 Boston, MA 02210 
 Attn: [***]Phone:
[***] 

 Fidelity Growth Company Commingled Pool 

Address: 
 Mag &
Co. 
 c/o Brown Brothers Harriman & Co. 

Attn: Corporate Actions /Vault 

140 Broadway 
 New York, NY 10005

 Email: [***] 
 Fidelity Mt. Vernon Street
Trust: Fidelity Series Growth Company Fund 
 Address: 

State Street Bank & Trust 

PO Box 5756 
 Boston,
Massachusetts 02206 
 Attn: WAVELENGTH + CO Fidelity Mt. Vernon Street Trust: Fidelity Series Growth Company Fund 

Email: [***]Fax number: [***] 
 Fidelity Mt.
Vernon Street Trust: Fidelity Growth Company Fund 
 Address: 

BNY Mellon 
 Attn: Stacey Wolfe

 525 William Penn Place Rm 0400 

Pittsburgh, PA 15259 
 Email:
[***] 
 Fax number: [***] 
 HBM Healthcare
Investments (Cayman) Ltd. 
 Address: 

Governors Square, Suite #4-212-2 

23 Lime Tree Bay Avenue 
 West
Bay, Grand Cayman 
 Cayman Islands 
 Maverick
Private Opportunities Fund, L.P. 
 Maverick Advisors Fund, L.P. 

Address: 
 Maverick
Capital, Ltd. 
 300 Crescent Court, 18th Floor 

Dallas, TX 75201 
 Phone:
[***]Fax: [***] 
 Email: [***] 

 Novartis Institutes For BioMedical Research, Inc. 

Address: 
 Novartis
Institutes for BioMedical Research, Inc. 
 250 Massachusetts Avenue 

Cambridge, MA 02139 
 Attn:
General Counsel 
 With a copy, which shall not constitute notice, to: 

Hogan Lovells US LLP 
 4085
Campbell Avenue 
 Suite 100 

Menlo Park, CA 94025 
 Attn: [***]

 Phone: [***] 
 Fax: [***]

 Email: [***] 
 Osage University Partners II,
L.P. 
 Address: 
 50
Monument Road, Suite 201 
 Bala Cynwyd, PA 19004 

Attn: Beth Grafstrom 
 Rock Springs Capital
Master Fund LP 
 Address: 

650 South Exeter Street 
 Suite
1070 
 Baltimore, Maryland 21202 

Attention: General Counsel 

Email: [***] 
 TLS Beta Pte. Ltd. 

Address: 
 Attn: Khoo Shih

 60B Orchard Road 
 #06-18 Tower 2 
 The Atrium@Orchard 

Singapore 238891 
 Vida Ventures, LLC 

Address: 
 Tarrant
Management, LLC 
 TPG Family Office 

Attn: Sherri Conn 
 301 Commerce
Street, Suite 3150 

 Fort Worth, TX 76102 

[***]With a copy, which shall not constitute notice, to: 

Vida Ventures 
 31 St James
Avenue, Boston, MA 02116 
 Attn: Arjun Goyal 

[***] 
 VIVO PANDA FUND, L.P. 

Address: 
 505 Hamilton
Avenue, Suite 207 
 Palo Alto, CA 94301 

Attn: Mahendra ShahEX-10.1

 Exhibit 10.1 

HOMOLOGY MEDICINES 

2015 Stock Incentive Plan 

1. Purpose. 
 The purpose
of this plan (the “Plan”) is to secure for Homology Medicines, Inc., a Delaware corporation (the “Company”) and its shareholders the benefits arising from capital stock ownership by employees, officers and directors of, and
consultants or advisors to, the Company and its parent and subsidiary corporations who are expected to contribute to the Company’s future growth and success. Under the Plan recipients may be awarded both (i) Options (as defined in
Section 2.1) to purchase the Company’s common stock, par value $0.0001 per share (“Common Stock”) and (ii) shares of Common Stock (“Restricted Stock Awards”). Except where the context otherwise requires, the term
“Company” shall include any parent and all present and future subsidiaries of the Company as defined in Sections 424(e) and 424(f) of the Internal Revenue Code of 1986, as amended or replaced from time to time (the “Code”). Those
provisions of the Plan which make express reference to Section 422 of the Code shall apply only to Incentive Stock Options (as that term is defined below). Appendix A to this Plan shall apply only to participants in the Plan who are
residents of the State of California.  
 2. Types of Awards and Administration. 

2.1 Options. Options granted pursuant to the Plan (“Options”) shall be authorized by action of the Board of Directors of the
Company (the “Board” or “Board of Directors”) and may be either incentive stock options (“Incentive Stock Options”) meeting the requirements of Section 422 of the Code or
non-statutory Options which are not intended to meet the requirements of Section 422. All Options when granted are intended to be non-statutory Options, unless the
applicable Option Agreement (as defined in Section 5.1) explicitly states that the Option is intended to be an Incentive Stock Option. The vesting of Options may be conditioned upon the completion of a specified period of employment with the
Company and/or such other conditions or events as the Board may determine. The Board may also provide that Options are immediately exercisable subject to certain repurchase rights in the Company dependent upon the continued employment of the
optionee and/or such other conditions or events as the Board may determine. 
 2.1.1 Incentive Stock Options. Incentive Stock
Options may only be granted to employees of the Company. For so long as the Code shall so provide, Options granted to any employee under the Plan (and any other incentive stock option plans of the Company) which are intended to constitute Incentive
Stock Options shall not constitute Incentive Stock Options to the extent that such Options, in the aggregate, become exercisable for the first time in any one calendar year for shares of Common Stock with an aggregate fair market value (determined
as of the respective date or dates of grant) of more than $100,000. If an Option is intended to be an Incentive Stock Option, and if for any reason such Option (or any portion thereof) shall not qualify as an Incentive Stock Option, then, to the
extent of such nonqualification, such Option (or portion thereof) shall be regarded as a non-statutory Option appropriately granted under the Plan provided that such Option (or portion thereof) otherwise meets
the Plan’s requirements relating to non-statutory Options. 

 2.2 Restricted Stock Awards. The Board in its discretion may grant Restricted Stock
Awards, entitling the recipient to acquire, for a purchase price determined by the Board, shares of Common Stock subject to such restrictions and conditions as the Board may determine at the time of grant (“Restricted Stock”), including
continued employment and/or achievement of pre-.established performance goals and objectives. 
 2.3
Administration. The Plan shall be administered by the Board, whose construction and interpretation of the terms and provisions of the Plan shall be final and conclusive. The Board may in its sole discretion authorize issuance of Restricted
Stock, the grant of Options and the issuance of shares upon exercise of such Options as provided in the Plan. The Board shall have authority, subject to the express provisions of the Plan, to construe Restricted Stock Agreements, Option Agreements
and the Plan, to prescribe, amend and rescind rules and regulations relating to the Plan, to determine the terms and provisions of Restricted Stock Agreements and Option Agreements, and to make all other determinations in the judgment of the Board
necessary or desirable for the administration of the Plan. The Board may correct any defect or supply any omission or reconcile any inconsistency in the Plan or in any Restricted Stock Agreement or Option Agreement in the manner and to the extent it
shall deem expedient to carry the Plan into effect and it shall be the sole and final judge of such expediency. No director or person acting pursuant to authority delegated by the Board shall be liable for any action or determination under the Plan
made in good faith. The Board may, to the full extent permitted by or consistent with applicable laws or regulations, delegate any or all of its powers under the Plan to a committee (the “Committee”) appointed by the Board, and if the
Committee is so appointed, to the extent of such delegation, all references to the Board in the Plan shall mean and relate to such Committee, other than references to the Board in this sentence and in Section 18 (as to amendment or termination
of the Plan) and Section 22. 
 3. Eligibility. 

Options may be granted, and Restricted Stock may be issued, to persons who are, at the time of such grant or issuance, employees, officers or
directors of, or consultants or advisors to, the Company; provided, that the class of persons to whom Incentive Stock Options may be granted shall be limited to employees of the Company. 

3.1 10% Shareholder. If any employee to whom an Incentive Stock Option is to be granted is, at the time of the grant of such Option,
the owner of stock possessing more than 10% of the total combined voting power of all classes of stock of the Company (after taking into account the attribution of stock ownership rules of Section 424(d) of the Code) (a “Greater Than 10%
Shareholder”), any Incentive Stock Option granted to such individual must: (i) have an exercise price per share of not less than 110% of the fair market value of one share of Common Stock at the time of grant; and (ii) expire by its
terms not more than five years from the date of grant. 
 4. Stock Subject to Plan. 

Subject to adjustment as provided in Section 14.2 below, the maximum number of shares of Common Stock which may be issued under the Plan
is 16,975,000 shares, all of which may be issued with respect to Incentive Stock Options. If an Option shall expire or terminate for any 

  
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reason without having been exercised in full, the unpurchased shares subject to such Option shall again be available for subsequent Option grants or Restricted Stock Awards under the Plan. If
shares of Restricted Stock shall be forfeited to, or otherwise repurchased by, the Company pursuant to a Restricted Stock Agreement, such repurchased shares shall again be available for subsequent Option grants or Restricted Stock Awards under the
Plan. If shares otherwise issuable upon exercise of an Option are withheld by the Company in payment of the exercise price of an Option or to satisfy tax withholding obligations with respect to such exercise, such withheld shares shall again be
available for subsequent Option grants or Restricted Stock Awards under the Plan. 
 5. Forms of Restricted Stock Agreements and Option
Agreements. 
 5 .1 Option Agreement. Each recipient of an Option shall execute an option agreement (“Option Agreement”)
in such form not inconsistent with the Plan as may be approved by the Board of Directors. Such Option Agreements may differ among recipients. 

5.2 Restricted Stock Agreement. Each recipient of a grant of Restricted Stock shall execute an agreement (“Restricted Stock
Agreement”) in such form not inconsistent with the Plan as may be approved by the Board of Directors. Such Restricted Stock Agreements may differ among recipients. 

5.3 ‘‘Lock-Up” Agreement. Unless the Board specifies otherwise, each Restricted Stock Agreement and Option Agreement
shall provide that upon the request of the Company or the managing underwriter(s) of any offering of securities of the Company that is the subject of a registration statement filed under the United States Securities Act of 1933, as amended from time
to time (the “Act”), the holder of any Option or the purchaser of any Restricted Stock shall, in connection therewith, agree in writing (in such form as the Company or such managing underwriter(s) shall request) to the general effect that
for a period of time (not to exceed 180 days, plus such additional number of days (not to exceed 35) as may reasonably be requested to enable the underwriter(s) of such offering to comply with Rule 271l(f) of the Financial Industry Regulatory
Authority or any amendment or successor thereto) from the effective date of the registration statement under the Act for such offering, the holder or purchaser will not sell, make any short sale of, loan, grant any option for the purchase of, or
otherwise dispose of any shares of the common stock of the Company owned or controlled by him or her. 
 6. Purchase Price. 

6.1 General. The purchase price per share of Restricted Stock and per share of stock deliverable upon the exercise of an Option shall be
determined by the Board, provided, however, that in the case of any Option, the exercise price shall not be less than 100% of the fair market value of such stock, as determined by the Board, at the time of grant of such Option, or less than 110% of
such fair market value in the case of any Incentive Stock Option granted to a Greater Than 10% Shareholder. 

  
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 6.2 Payment of Purchase Price. Option Agreements may provide for the payment of the
exercise price by delivery of cash or a check to the order of the Company in an amount equal to the exercise price of such Options, or, to the extent provided in the applicable Option Agreement, ·by one of the following methods: 

(i) with the consent of the Board, by delivery to the Company of shares of Common Stock; such surrendered shares shall have a
fair market value equal in amount to the exercise price of the Options being exercised, 
 (ii) with the consent of the
Board, a personal recourse note issued by the optionee to the Company in a principal amount equal to such aggregate exercise price and with such other terms, including interest rate and maturity, as the Company may determine in its discretion;
provided, however, that the interest rate borne by such note shall not be less than the lowest applicable federal rate, as defined in Section 1274(d) of the Code, 

(iii) with the consent of the Board, if the class of Common Stock is registered under the Securities Exchange Act of 1934 at
such time, subject to rules as may be established by the Board, by delivery to the Company of a properly executed exercise notice along with irrevocable instructions to a broker to promptly deliver to the Company cash or a check payable and
acceptable to the Company for the purchase price, 
 (iv) with the consent of the Board, by reducing the number of Option
shares otherwise issuable to the optionee upon exercise of the Option by a number of shares of Common Stock having a fair market value equal to such aggregate exercise price, 

(v) with the consent of the Board, by any combination of such methods of payment. 

The fair market value of any shares of Common Stock or other non-cash consideration which may be
delivered upon exercise of an Option shall be determined by the Board of Directors. Restricted Stock Agreements may provide for the payment of any purchase price in any manner approved by the Board of Directors at the time of authorizing the
issuance thereof. 
 7. Option Period. 

Notwithstanding any other provision of the Plan or any Option Agreement, each Option and all rights thereunder shall expire on the date
specified in the applicable Option Agreement, provided that such date shall not be later than ten years after the date on which the Option is granted ( or five years in the case of an Incentive Stock Option granted to a Greater Than 10%
Shareholder), and in either case, shall be subject to earlier termination as provided in the Plan or Option Agreement. 
 8. Exercise of
Options. 
 8.1 General. Each Option shall be exercisable either in full or in installments at such time or times and during such
period as shall be set forth in the Option Agreement evidencing such Option, subject to the provisions of the Plan. To the extent not exercised, installments shall accumulate and be exercisable, in whole or in part, at any time after becoming
exercisable, but not later than the date the Option expires. 

  
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 8.2 Notice of Exercise. An Option may be exercised by the optionee by delivering to the
Company on any business day a written notice specifying the number of shares of Common Stock the optionee then desires to purchase and specifying the address to which the certificates for such shares are to be mailed (the “Notice”),
accompanied by payment for such shares. In addition, the Company may require any individual to whom an Option is granted, as a condition of exercising such Option, to give written assurances (the “Investment Letter”) in a substance and
form satisfactory to the Company to the effect that such individual is acquiring the Common Stock subject to the Option for his or her own account for investment and not with a view to the resale or distribution thereof, and to such other effects as
the Company deems necessary or advisable in order to comply with any securities law(s). 
 8.3 Delivery. As promptly as practicable
after receipt of the Notice, the Investment Letter (if required) and payment, the Company shall deliver or cause to be delivered to the optionee certificates for the number of shares with respect to which such Option has been so exercised, issued in
the optionee’s name; provided, however, that such delivery shall be deemed effected for all purposes when the Company or a stock transfer agent shall have deposited such certificates in the United States mail, addressed to the optionee, at the
address specified in the Notice. 
 9. Nontransferability of Options. 

No Option shall be assignable or transferable by the person to whom it is granted, either voluntarily or by operation of law, except by will or
the laws of descent and distribution. During the life of an optionee, an Option shall be exercisable only by the optionee. 
 10.
Termination of Employment; Disability; Death. Except as may be otherwise expressly provided in the terms and conditions of the Option Agreement, Options shall terminate on the earliest to occur of: 

 

	 	(i)	the date of expiration thereof; 

  

	 	(ii)	0 days after termination of the optionee’s employment with, or provision of services to, the Company by the Company for Cause (as hereinafter defined); 

 

	 	(iii)	90 days after the date of voluntary termination of the optionee’s employment with, or provision of services to, the Company by the optionee (other than for death or permanent disability as defined below); or

  

	 	(iv)	90 days after the date of termination of the optionee’s employment with, or provision of services to, the Company by the Company without Cause (other than for death or permanent disability as defined below).

  
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 Until the date on which the Option so expires, the optionee may exercise that portion of his or her Option which
is exercisable at the time of termination of the employment or service relationship. 
 An employment or service relationship between the
Company and the optionee shall be deemed to exist during any period during which the optionee is employed by or providing services to the Company. Whether an authorized leave of absence or an absence due to military or government service shall
constitute termination of the employment relationship between the Company and the optionee shall be determined by the Board at the time thereof. 

For purposes of this Section 10, the term “Cause” shall mean (a) any material breach by the optionee of any agreement to
which the optionee and the Company are both parties, (b) any act (other than retirement) or omission to act by the optionee which may have a material and adverse effect on the Company’s business or on the optionee’s ability to perform
services for the Company, including, without limitation, the commission of any crime (other than minor traffic violations), or (c) any material misconduct or material neglect of duties by the optionee in connection with the business or affairs
of the Company. An optionee’s employment shall be deemed to have been terminated for Cause if the Company determines within thirty (30) days of the termination of employment (whether such termination was voluntary or involuntary) that
termination for Cause was warranted. 
 In the event of the permanent and total disability or death of an optionee while in an employment or
other relationship with the Company, any Option held by such optionee shall terminate on the earlier of the date of expiration of the Option or one year following the date of such disability or death. After disability or death, the optionee (or in
the case of death, his or her executor, administrator or any person or persons to whom this option may be transferred by will or by laws of descent and distribution) shall have the right, at any time prior to such termination of an Option, to
exercise the Option to the extent the optionee was entitled to exercise such Option as of the date of his or her disability or death. An optionee is permanently and totally disabled if he or she is unable to engage in any substantial gainful
activity by reason of any medically determinable physical or mental impairment which can be expected to last for a continuous period of not less than 12 months; permanent and total disability shall be determined in accordance with
Section 22(e)(3) of the Code and the regulations issued thereunder. 
 11. Rights as a Shareholder. The holder of an Option
shall have no rights as a shareholder with respect to any shares covered by the Option (including, without limitation, any rights to receive dividends or non-cash distributions with respect to such shares)
until the date of issue of a stock certificate to him or her for such shares. No adjustment shall be made for dividends or other rights for which the record date is prior to the date such stock certificate is issued. 

12. Additional Provisions. The Board of Directors may, in its sole discretion, include additional provisions in Restricted Stock
Agreements and Option Agreements, including, without limitation, restrictions on transfer, rights of the Company to repurchase shares of Restricted Stock or shares of Common Stock acquired upon exercise of Options, commitments to pay cash bonuses,
to make, arrange for or guaranty loans or to transfer other property to optionees upon exercise of Options, or such other provisions as shall be determined by the Board 

  
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of Directors; provided that such additional provisions shall not be inconsistent with any other term or condition of the Plan and such additional provisions shall not be such as to cause
any Incentive Stock Option to fail to qualify as an Incentive Stock Option within the meaning of Section 422 of the Code. 
 13.
Acceleration, Extension, Etc. The Board of Directors may, in its sole discretion, (i) accelerate the date or dates on which all or any particular Option or Options may be exercised or (ii) extend the period or periods of time during
which all, or any particular, Option or Options may be exercised. 
 14. Adjustment Upon Changes in Capitalization 

14.1 No Effect of Options upon Certain Corporate Transactions. The existence of outstanding Options shall not affect in any way the
right or power of the Company to make or authorize any or all adjustments, recapitalizations, reorganizations or other changes in the Company’s capital structure or its business, or any merger or consolidation, or any issue of Common Stock, or
any issue of bonds, debentures, preferred or prior preference stock ahead of or affecting the Common Stock or the rights thereof, or the dissolution or liquidation of the Company, or any sale or transfer of all or any part of its assets or business,
or any other corporate act or proceeding, whether of a similar character or otherwise. 
 14.2 Adjustment Provisions. If, through or
as a result of any merger, consolidation, sale of all or substantially all of the assets of the Company, reorganization, recapitalization, reclassification, stock dividend, stock split, reverse stock split or other similar transaction, (i) the
outstanding shares of Common Stock are increased, decreased or exchanged for a different number or kind of shares or other securities of the Company, or (ii) additional shares, or new or different shares or other securities of the Company or
other non-cash assets are distributed with respect to such shares of Common Stock or other securities, an appropriate and proportionate adjustment shall be made in (x) the maximum number and kind of
shares reserved for issuance under the Plan, (y) the number and kind of shares or other securities subject to any then outstanding Options, and (z) the price for each share or other security subject to any then outstanding Options, so that
upon exercise of such Options, in lieu of the shares of Common Stock for which such Options were then exercisable, the relevant optionee shall be entitled to receive, for the same aggregate consideration, the same total number and kind of shares or
other securities, cash or property that the owner of an equal number of outstanding shares of Common Stock immediately prior to the event requiring adjustment would own as a result of the event. If any such event shall occur, appropriate adjustment
shall also be made in the application of the provisions of this Section 14 and Section 15 with respect to Options and the rights of optionees after the event so that the provisions of such Sections shall be applicable after the event and
be as nearly equivalent as practicable in operation after the event as they were before the event. 
 14.3 No Adjustment in Certain
Cases. Except as hereinbefore expressly provided, the issue by the Company of shares of stock of any class, or securities convertible into shares of stock of any class, for cash or property or for labor or services, either upon direct sale or
upon the exercise of rights or warrants to subscribe therefor, or upon conversion of shares or obligations of the Company convertible into such shares or other securities, shall not affect, and no adjustment by reason thereof shall be made with
respect to, the number or price of shares of Common Stock then subject to outstanding options. 

  
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 14.4 Board Authority to Make Adjustments. Any adjustments under this Section 14 will
be made by the Board of Directors, whose determination as to what adjustments, if any, will be made and the extent thereof will be final, binding and conclusive. No fractional shares will be issued under the Plan on account of any such adjustments.

 15. Effect of Certain Transactions 

15 .1 General. Except as provided in any Option Agreement or Restricted Stock Agreement to the contrary, if the Company is merged with
or into or consolidated with another corporation under circumstances where the stockholders of the Company immediately prior to such merger or consolidation do not own after such merger or consolidation shares representing at least fifty percent
(50%) of the voting power of the Company or the surviving or resulting corporation, as the case may be, or if shares representing fifty percent (50%) or more of the voting power of the Company are transferred to an Unrelated Third Party, as
hereinafter defined, or if the Company is liquidated, or sells or otherwise disposes of all or substantially all its assets (each such transaction is referred to herein as a “Change in Control Transaction”), the Board, or the board of
directors of any corporation assuming the obligations of the Company, may, in its discretion, take any one or more of the following actions, as to some or all outstanding Options or Restricted Stock Awards (and need not take the same action as to
each such Option or Restricted Stock Award): (i) provide that such Options shall be assumed, or equivalent Options shall be substituted, by the acquiring or succeeding corporation (or an affiliate thereof) provided that any such Options
substituted for Incentive Stock Options shall meet the requirements of Section 424(a) of the Code, (ii) upon written notice to the optionees, provide that all unexercised Options (whether vested or unvested) will terminate immediately
prior to the consummation of the Change in Control Transaction unless exercised by the optionee to the extent otherwise then exercisable within a specified period following the date of such notice, (iii) upon written notice to the grantees,
provide that all unvested shares of Restricted Stock shall be repurchased at cost, (iv) make or provide for a cash payment to the optionees equal to the difference between (A) the fair market value of the per share consideration (whether
cash, securities or other property or any combination of the above) the holder of a share of Common Stock will receive upon consummation of the Change in Control Transaction (the “Per Share Transaction Price”) times the number of shares of
Common Stock subject to outstanding vested Options (to the extent then exercisable at prices not equal to or in excess of the Per Share Transaction Price) and (B) the aggregate exercise price of such outstanding vested Options, in exchange for
the termination of such Options, or (v) provide that all or any outstanding Options shall become exercisable and all or any outstanding Restricted Stock Awards shall vest in part or in full immediately prior to such event. To the extent that
any Options are exercisable at a price equal to or in excess of the Per Share Transaction Price, the Board may provide that such Options shall terminate immediately upon the consummation of the Change in Control Transaction without any payment being
made to the holders of such Options. “Unrelated Third Party” shall mean any person who is not, on the date of adoption of this Plan by the Board, a holder of stock of any class or preference or any stock option of the Company. 

  
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 15.2 Substitute Options. The Company may grant Options in substitution for options held by
employees, officers or directors of, or consultants or advisors to, another corporation who become employees, officers or directors of, or consultants or advisors to, the Company, as the result of a merger or consolidation of the employing
corporation with the Company or as a result of the acquisition by the Company of property or stock of the employing corporation. The Company may direct that substitute Options be granted on such terms and conditions as the Board considers
appropriate in the circumstances. 
 15.3 Restricted Stock. In the event of a business combination or other transaction of the type
detailed in Section 15.1, any securities, cash or other property received in exchange for shares of Restricted Stock shall continue to be governed by the provisions of any Restricted Stock Agreement pursuant to which they were issued, including
any provision regarding vesting, and such securities, cash, or other property may be held in escrow on such terms as the Board of Directors may direct, to insure compliance with the terms of any such Restricted Stock Agreement. 

16. No Special Employment Rights. Nothing contained in the Plan or in any Option Agreement or Restricted Stock Agreement shall confer
upon any optionee or holder of Restricted Stock any right with respect to the continuation of his or her employment by the Company or interfere in any way with the right of the Company at any time to terminate such employment or to increase or
decrease his or her compensation. 
 17. Other Employee Benefits. The amount of any compensation deemed to be received by an employee
as a result of the issuance of shares of Restricted Stock or the grant or exercise of an Option or the sale of shares received upon issuance of a Restricted Stock Award or exercise of an Option will not constitute compensation with respect to which
any other employee benefits of such employee are determined, including, without limitation, benefits under any bonus, pension, profit-sharing, life insurance or salary continuation plan, except as otherwise specifically determined by the Board of
Directors. 
 18. Amendment of the Plan. 

18.1 The Board may at any time, and from time to time, modify or amend in any respect or terminate the Plan. If shareholder approval is not
obtained within twelve months after any amendment increasing the number of shares authorized under the Plan or changing the class of persons eligible to receive Options under the Plan, no Options granted pursuant to such amendments shall be deemed
to be Incentive Stock Options and no Incentive Stock Options shall be issued pursuant to such amendments thereafter. 
 18.2 The termination
or any modification or amendment of the Plan shall not, without the consent of an optionee or the holder of Restricted Stock, adversely affect his or her rights under an Option or Restricted Stock Award previously granted to him or her. With the
consent of the recipient of Restricted Stock or optionee affected, the Board may amend outstanding Restricted Stock Agreements or Option Agreements in a manner not inconsistent with the Plan. 

  
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 19. Withholding. The Company shall have the right to deduct from payments of any kind
otherwise due to the optionee or recipient of Restricted Stock, any federal, state or local taxes of any kind required by law to be withheld with respect to issuance of any shares of Restricted Stock or shares issued upon exercise of Options. Prior
to delivery of any Common Stock pursuant to the terms of this Plan, the Board has the right to require that the optionee or recipient of Restricted Stock remit to the Company an amount sufficient to satisfy any minimum tax withholding obligation.
Subject to the prior approval of the Company, which may be withheld by the Company in its sole discretion, the obligor may elect to satisfy any minimum withholding obligations, in whole or in part, (i) by causing the Company to withhold shares
of Common Stock otherwise issuable, or (ii) by delivering to the Company a sufficient number of shares of Common Stock. The shares so withheld shall have a fair market value equal to such minimum withholding obligation. The fair market value of
the shares used to satisfy such minimum withholding obligation shall be determined by the Company as of the date that the amount of tax to be withheld is to be determined. A person who has made an election pursuant to this Section 19 may
only satisfy his or her withholding obligation with shares of Common Stock which are not subject to any repurchase, forfeiture, unfulfilled vesting or other similar restrictions. 

20. Effective Date and Duration of the Plan. 

20.1 Effective Date. The Plan shall become effective when adopted by the Board of Directors. If shareholder approval is not obtained
within twelve months after the date of the Board’s adoption of the Plan, no Options previously granted under the Plan shall be deemed to be Incentive Stock Options and no Incentive Stock Options shall be granted thereafter. Amendments to the
Plan not requiring shareholder approval shall become effective when adopted by the Board. Amendments requiring shareholder approval shall become effective when adopted by the Board, but if shareholder approval is not obtained within twelve months of
the Board’s adoption of such amendment, any Incentive Stock Options granted pursuant to such amendment shall be deemed to be non-statutory Options provided that such Options are authorized by the Plan.
Subject to this limitation, Options may be granted under the Plan at any time after the effective date and before the date fixed for termination of the Plan. 

20.2 Termination. Unless sooner terminated by action of the Board of Directors, the Plan shall terminate upon the close of business on
the day next preceding the tenth anniversary of the date of its adoption by the Board of Directors. 
 21. Provision for Foreign
Participants. The Board of Directors may, without amending the Plan, modify the terms of Option Agreements or Restricted Stock Agreements to differ from those specified in the Plan with respect to participants who are foreign nationals or
employed outside the United States to recognize differences in laws, rules, regulations or customs of such foreign jurisdictions with respect to tax, securities, currency, employee benefit or other matters. 

  
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 22. Requirements of Law. The Company shall not be required to sell or issue any shares
under any Option or Restricted Stock Award if the issuance of such shares shall constitute a violation by the optionee, the Restricted Stock Award recipient, or by the Company of any provision of any law or regulation of any governmental authority.
In addition, in connection with the Act, the Company shall not be required to issue any shares upon exercise of any Option unless the Company has received evidence satisfactory to it to the effect that the holder of such Option will not transfer
such shares except pursuant to a registration statement in effect under the Act or unless an opinion of counsel satisfactory to the Company has been received by the Company to the effect that such registration is not required in connection with any
such transfer. Any determination in this connection by the Board shall be final, binding and conclusive. In the event the shares issuable on exercise of an Option are not registered under the Act or under the securities laws of each relevant state
or other jurisdiction, the Company may imprint on the certificate(s) appropriate legends that counsel for the Company considers necessary or advisable to comply with the Act or any such state or other securities law. The Company may register, but in
no event shall be obligated to register, any securities covered by the Plan pursuant to the Act; and in the event any shares are so registered the Company may remove any legend on certificates representing such shares. The Company shall not be
obligated to take any affirmative action in order to cause the exercise of an Option, the grant of any Restricted Stock Award or the issuance of shares pursuant thereto to comply with any law or regulation of any governmental authority. 

23. Conversion of Incentive Stock Options into Non-Qualified Options; Termination. The Board of
Directors, with the consent of any optionee, may in its discretion take such actions as may be necessary to convert such optionee’s Incentive Stock Options (or any installments or portions of installments thereof) that have not been exercised
on the date of conversion into non-statutory Options at any time prior to the expiration of such Incentive Stock Options, regardless of whether the optionee is an employee of the Company or a parent or
subsidiary of the Company at the time of such conversion. At the time of such conversion, the Board of Directors (with the consent of the optionee) may impose such conditions on the exercise of the resulting
non-statutory Options as the Board of Directors in its discretion may determine, provided that such conditions shall not be inconsistent with this Plan. Nothing in this Plan shall be deemed to give any
optionee the right to have such optionee’s Incentive Stock Options converted into non-statutory Options, and no such conversion shall occur until and unless the Board of Directors takes appropriate
action. The Board of Directors, with the consent of the optionee, may also terminate any portion of any Incentive Stock Option that has not been exercised at the time of such termination. 

24. Non-Exclusivity of this Plan; Non-Uniform
Determinations. Neither the adoption of this Plan by the Board of Directors nor the approval of this Plan by the stockholders of the Company shall be construed as creating any limitations on the power of the Board of Directors to adopt such
other incentive arrangements as it may deem desirable, including, without limitation, the granting of stock options otherwise than under this Plan, and such arrangements may be either applicable generally or only in specific cases. 

The determinations of the Board of Directors under this Plan need not be uniform and may be made by it selectively among persons who receive
or are eligible to receive Options or Restricted Stock Awards under this Plan (whether or not such persons are similarly situated). Without limiting the generality of the foregoing, the Board of Directors shall be entitled, among

  
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other things, to make non-uniform and selective determinations, and to enter into non-uniform and selective Option
Agreements and Restricted Stock Agreements, as to (a) the persons to receive Options or Restricted Stock Awards under this Plan, (b) the terms and provisions of Options or Restricted Stock A wards, ( c) the exercise by the Board of
Directors of its discretion in respect of the exercise of Options pursuant to the terms of this Plan, and ( d) the treatment of leaves of absence pursuant to Section 10 hereof. 

25. Governing Law. This Plan and each Option or Restricted Stock Award shall be governed by the laws of Delaware, without regard to its
principles of conflicts of law. 

  
 - 12 - 

 APPENDIX A 

TO HOMOLOGY MEDICINES, INC. 2015 STOCK INCENTIVE PLAN 

FOR CALIFORNIA RESIDENTS ONLY 

This Appendix to the Homology Medicines, Inc. 2015 Stock Incentive Plan (the “Plan”) shall have application only to participants in
the Plan who are residents of the State of California. Capitalized terms contained herein shall have the same meanings given to them in the Plan, unless otherwise provided in this Appendix. Notwithstanding any provision contained in the Plan to
the contrary and to the extent required by applicable law, the following terms and conditions shall apply to all Options and Restricted Stock Awards (collectively “Awards”) granted to residents of the State of California, until such time
as the Common Stock becomes subject to registration under the Securities Act of 1933:  
 1. Awards shall be nontransferable other than
by will or the laws of descent and distribution. Notwithstanding the foregoing, and to the extent permitted by Section 422 of the Code, the Board, in its discretion, may permit distribution of an Award to an inter vivos or testamentary trust in
which the Award is to be passed to beneficiaries upon the death of the trustor (settler), or by gift to “immediate family” as that term is defined in Rule 16a-l(e) of the United States Exchange Act of 1934. 

2. Unless employment is terminated for Cause, the right to exercise an Option in the event of termination of employment, to the extent that
the optionee is otherwise entitled to exercise an Option on the date employment terminates, shall be 
 (a) at least six months from the
date of termination of employment if termination was caused by death or permanent disability; and 
 (b) at least 30 days from the date of
termination if termination of employment was caused by other than death or permanent disability; 
 (c) but in no event later than the
remaining term of the Option. 
 3. Any Award exercised before shareholder approval is obtained shall be rescinded if shareholder approval
is not obtained within 12 months of the Board’s adoption of the Plan. 

 INCENTIVE STOCK OPTION 

Granted by 
 Homology Medicines,
Inc. (the “Company”) 
 Under the 2015 Stock Incentive Plan 

This Option is and shall be subject in every respect to the provisions of the Company’s 2015 Stock Incentive Plan, as amended from time
to time (the “Plan”), which is incorporated herein by reference and made a part hereof. The holder of this Option (the “Holder”) hereby accepts this Option subject to all the terms and provisions of the Plan and agrees that
(a) in the event of any conflict between the terms hereof and those of the Plan, the latter shall prevail, and (b) all decisions under and interpretations of the Plan by the Board or the Committee shall be final, binding and conclusive
upon the Holder and his or her heirs and legal representatives. 
  

	1.	Name of Holder: 

  

	2.	Date of Grant: 

  

	3.	Vesting Start Date: 

  

	4.	Maximum number of shares for 

	  	which this Option is exercisable: 

  

	5.	Exercise (purchase) price per share: [Note: must be at least fair market value, or 110% of fair market value in case of ISO granted to Greater Than 10% Shareholder] 

 

	6.	Method of Exercise: This Option may be exercised by the delivery of written notice to the Company setting forth the number of shares with respect to which the Option is to be exercised, together with payment by
one of the following methods: 

 cash or a personal, certified or bank check or postal money order payable to the order of the
Company for an amount equal to the exercise price of the shares being purchased; or 
 with the consent of the Company, any of the other
methods set forth in the Plan. 
 As an additional condition to exercise of this Option, the Holder shall deliver to the
Company an investment letter in form and substance satisfactory to the Company and its counsel. No such investment letter shall be required as a condition to such exercise at any time when there shall be an effective registration statement under the
Securities Act of 1933, as amended (the “Act”) covering the shares for which this Option may be exercised. 
  

	7.	Expiration Date of Option: [Note: for ISO, cannot be longer than 10 years from date of grant, or 5 years in case of a Greater Than 10% Shareholder] 

	8.	Vesting Schedule: [Note: Company to elect vesting schedule; following is an example of a standard vesting provision] This Option shall become exercisable for 25% of the
maximum number of shares granted on the first anniversary of the Vesting Start Date, and shall become exercisable for an additional 2.0833% of the maximum number of shares granted on the last day of each one month period
thereafter; so that the Option shall be fully vested on the fourth anniversary of the Vesting Start Date. All vesting shall cease upon the date of termination of employment. 

In addition to the foregoing, upon the Holder’s election at any time after the Date of Grant of this Option, the Holder shall be entitled
to exercise this Option immediately and in full for the maximum number of shares as set forth herein, whether or not fully vested, provided that, upon such exercise, the Holder shall execute a stock restriction agreement containing a “reverse
vesting” schedule effectively equivalent to the Vesting Schedule set forth herein, pursuant to which the Holder agrees to sell back any unvested shares at cost should he or she leave the employ of the Company prior to full vesting. Early
exercise of this Option in accordance with the preceding sentence may have adverse tax implications, including the loss of potential tax benefits otherwise available to holders of incentive stock options, and the Holder is advised to consult his or
her personal tax advisor prior to making any such election. 
  

	9.	Termination of Employment. This Option shall terminate on the earliest to occur of: 

  

	 	(i)	the date of expiration hereof; 

  

	 	(ii)	0 days after termination of the Holder’s employment with the Company by the Company for Cause (as defined in the Plan); 

  

	 	(iii)	90 days after the date of voluntary termination of employment by the Holder (other than for death or permanent and total disability as defined in the Plan); 

 

	 	(iv)	90 days after the date of termination of the Holder’s employment with the Company by the Company without Cause (other than for death or permanent and total disability as defined in the Plan); or 

 

	 	(v)	one year after the “permanent and total disability” (as defined at Section 10 of the Plan) or death of the Holder. 

  

	10.	Company’s Right of First Refusal. Prior to the effective date of a registration statement under the Act, any shares of stock issued pursuant to exercise of this Option shall be subject to the Company’s
right of first refusal as set forth at Appendix A. 

  

	11.	 Lock-Up Agreement. The Holder agrees that upon the request of the
Company or the managing underwriter(s) of any offering of securities of the Company that is the subject of a registration statement filed under the Act, for a period of time (not to exceed 180 days, plus such additional number of days (not to exceed
35) as may reasonably be requested to enable the underwriter(s) of such offering to comply with Rule 2711(f) of the Financial Industry Regulatory Authority or any amendment or successor thereto)

  
 2 

	 	
from the effective date of the registration statement under the Act for such offering, the Holder will not sell, make any short sale of, loan, grant any option for the purchase of, or otherwise
dispose of any shares of Common Stock issued pursuant to the exercise of this Option, without the prior written consent of the Company and such underwriters. 

  

	12.	Incentive Stock Option; Disqualifying Disposition. Although this Option is intended to qualify as an incentive stock option under the Internal Revenue Code of 1986 (the “Code”), the Company makes no
representation as to the tax treatment upon exercise of this Option or sale or other disposition of the shares covered by this Option, and the Holder is advised to consult a personal tax advisor. Upon a Disqualifying Disposition of shares received
upon exercise of this Option, the Holder will forfeit the favorable income tax treatment otherwise available with respect to the exercise of this Option. A “Disqualifying Disposition” shall have the meaning specified in Section 421(b)
of the Code; as of the date of grant of this Option a Disqualifying Disposition is any disposition (including any sale) of such shares before the later of (a) the second anniversary of the date of grant of this Option and (b) the first
anniversary of the date on which the Holder acquired such shares by exercising this Option, provided that such holding period requirements terminate upon the death of the Holder. The Holder shall notify the Company in writing immediately upon
making a Disqualifying Disposition of any shares of Common Stock received pursuant to the exercise of this Option, and shall provide the Company with any information that the Company shall request concerning any such Disqualifying Disposition.

  

	13.	Notice. Any notice to be given to the Company hereunder shall be deemed sufficient if addressed to the Company and delivered to the office of the Company, Homology Medicines, Inc., c/o 5AM Ventures, Waltham Woods
Corporate Center, 890 Winter Street, Suite 140, Waltham, MA 02451, attention of the president, or such other address as the Company may hereafter designate. 

Any notice to be given to the Holder hereunder shall be deemed sufficient if addressed to and delivered in person to the Holder at his or her
address furnished to the Company or when deposited in the mail, postage prepaid, addressed to the Holder at such address. 
 * * * * *

 IN WITNESS WHEREOF, the parties have executed this Option, or caused this Option to be executed, as of the Date of Grant. 

 

			
	Homology Medicines, Inc.

 
			
		
	By:	 	 

  
 3 

 The undersigned Holder hereby acknowledges receipt of a copy of the Plan and this Option (including Appendix A
hereto), and agrees to the terms of this Option and the Plan. 
  

			
	  

	Holder:	 	

  
 4 

 APPENDIX A 

Right of First Refusal 

1. General. Prior to the effective date of a registration statement under the Securities Act of 1933, as amended (the “Act”),
covering any shares of the Company’s Common Stock and until such time as the Company shall have effected a public offering of its Common Stock registered under the Act, in the event that, at any time when the Holder (which term for purposes of
this section shall mean the Holder and his or her executors, administrators and any other person to whom this Option may be transferred by will or the laws of descent and distribution) is permitted to do so, the Holder desires to sell, assign or
otherwise transfer any of the shares issued upon the exercise of this Option, the Holder shall first offer such shares to the Company by giving written notice of the Holder’s desire so to sell, assign or transfer such shares. 

2. Notice of Intended Transfer. The notice shall state the number of shares offered, the name of the person or persons to whom it is
proposed to sell, assign or transfer such shares and the price at which such shares are intended to be sold, assigned or transferred. Such notice shall constitute an offer to the Company for the Company to purchase the number of shares set forth in
the notice at a price per share equal to the price stated therein. 
 3. Company to Accept or Decline Within 30 Days. The Company may
accept the offer as to all, but not less than all, such shares by notifying the Holder in writing within 30 days after receipt of such notice of its acceptance of the offer. If the offer is accepted, the Company shall have 60 days after such
acceptance within which to purchase the offered shares at a price per share as aforesaid. If within the applicable time periods the Holder does not receive notice of the Company’s intention to purchase the offered shares, or if payment in full
of the purchase price is not made by the Company, the offer shall be deemed to have been rejected and the Holder may transfer title to such shares within 90 days from the date of the Holder’s written notice to the Company of the Holder’s
intention to sell, but such transfer shall be made only to the proposed transferee and at the proposed price as stated in such notice and after compliance with any other provisions of this Option applicable to the transfer of such shares. 

4. Transferred Shares to Remain Subject to Right of First Refusal. Shares that are so transferred to such transferee shall remain
subject to the rights of the Company set forth in this Appendix A. As a condition to such transfer, such transferee shall execute and deliver all such documents as the Company may require to evidence the binding agreement of
such transferee so to remain subject to the rights of the Company. 
 5. Remedies of Company. No sale, assignment, pledge or
other transfer of any of the shares covered by this Option shall be effective or given effect on the books of the Company unless all of the applicable provisions of this Appendix A have been duly complied with, and the Company may inscribe on the
face of any certificate representing any of such shares a legend referring to the provisions of this Appendix A. If any transfer of shares is made or attempted in violation of the foregoing restrictions, or if shares are not offered to the Company
as required hereby, the Company shall have the right to purchase such shares from the owner thereof or his transferee at any time before or after the transfer, as herein provided. In addition to 

  
 5 

 
any other legal or equitable remedies which it may have, the Company may enforce its rights by actions for specific performance (to the extent permitted by law) and may refuse to recognize any
transferee as one of its stockholders for any purpose, including, without limitation, for purposes of dividend and voting rights, until all applicable provisions hereof have been complied with. 

6. Shares Subject to Right of First Refusal. For purposes of the Right of First Refusal pursuant to this Appendix A, the term
“shares” shall mean any and all new, substituted or additional securities or other property issued to the Holder, by reason of his or her ownership of Common Stock pursuant to the exercise of this Option, in connection with any stock
dividend, liquidating dividend, stock split or other change in the character or amount of any of the outstanding securities of the Company, or any consolidation, merger or sale of all or substantially all of the assets of the Company. 

7. Legends on Stock Certificates. Any certificate representing shares of stock subject to the provisions of this Appendix A may have
endorsed thereon one or more legends, substantially as follows: 
  

	 	(i)	“Any disposition of any interest in the securities represented by this certificate is subject to restrictions, and the securities represented by this certificate are subject to certain options, contained in a
certain agreement between the record holder hereof and the Company, a copy of which will be mailed to any holder of this certificate without charge upon receipt by the Company of a written request therefor.” 

 

	 	(ii)	“The shares of stock represented by this certificate have not been registered under the Securities Act of 1933 or under the securities laws of any state and may not be pledged, hypothecated, sold or otherwise
transferred unless such shares have been registered under the Act or unless the Company has received an opinion of counsel satisfactory to the Company, in form and substance satisfactory to the Company, that such registration is not required.”

 8. Right of First Refusal to Lapse Upon Registration. The restrictions imposed by this Appendix A shall terminate in
all respects upon the effective date of a registration statement under the Act covering any of the Company’s Common Stock. 

  
 6 

 NON-STATUTORY STOCK OPTION 

Granted by 
 Homology Medicines,
Inc. (the “Company”) 
 Under the 2015 Stock Incentive Plan 

This Option is and shall be subject in every respect to the provisions of the Company’s 2015 Stock Incentive Plan, as amended from time
to time (the “Plan”), which is incorporated herein by reference and made a part hereof. The holder of this Option (the “Holder”) hereby accepts this Option subject to all the terms and provisions of the Plan and agrees that
(a) in the event of any conflict between the terms hereof and those of the Plan, the latter shall prevail, and (b) all decisions under and interpretations of the Plan by the Board or the Committee shall be final, binding and conclusive
upon the Holder and his or her heirs and legal representatives. 
  

	1.	Name of Holder: 

  

	2.	Date of Grant: 

  

	3.	Vesting Start Date: 

  

	4.	Maximum number of shares for which this Option is exercisable: 

  

	5.	Exercise (purchase) price per share: [must be at least fair market value] 

  

	6.	Method of Exercise: This Option may be exercised by the delivery of written notice to the Company setting forth the number of shares with respect to which the Option is to be exercised, together with payment by
one of the following methods: 

 cash or a personal, certified or bank check or postal money order payable to the order of the
Company for an amount equal to the exercise price of the shares being purchased; or 
 with the consent of the Company, any of the other
methods set forth in the Plan. 
 As an additional condition to exercise of this Option, the Holder shall deliver to the
Company an investment letter in form and substance satisfactory to the Company and its counsel. No such investment letter shall be required as a condition to such exercise at any time when there shall be an effective registration statement under the
Securities Act of 1933, as amended (the “Act”) covering the shares for which this Option may be exercised. 

	7.	Expiration Date of Option: 

  

	8.	Vesting Schedule: [Note: Company to elect vesting schedule; following is an example of a standard vesting provision] This Option shall become exercisable for 25% of the maximum number of shares
granted on the first anniversary of the Vesting Start Date, and shall become exercisable for an additional 2.0833% of the maximum number of shares granted on the last day of each one month period thereafter; so that the Option shall be fully vested
on the fourth anniversary of the Vesting Start Date. All vesting shall cease upon the date of termination of employment with or provision of services to the Company. 

In addition to the foregoing, upon the Holder’s election at any time after the Date of Grant of this Option, the Holder shall be entitled
to exercise this Option immediately and in full for the maximum number of shares as set forth herein, whether or not fully vested, provided that, upon such exercise, the Holder shall execute a stock restriction agreement containing a “reverse
vesting” schedule effectively equivalent to the Vesting Schedule set forth herein, pursuant to which the Holder agrees to sell back any unvested shares at cost should he or she leave the service of the Company prior to full vesting. 

 

	9.	Termination of Employment with or Services to the Company. This Option shall terminate on the earliest to occur of: 

  

	 	(i)	the date of expiration thereof; 

  

	 	(ii)	0 days after termination of the Holder’s employment with or services to the Company by the Company for Cause (as defined in the Plan); 

 

	 	(iii)	90 days after the date of voluntary termination of employment with or services to the Company by the Holder (other than for death or permanent and total disability as defined in the Plan); 

 

	 	(iv)	90 days after the date of termination of the Holder’s employment with or services to the Company by the Company without Cause (other than for death or permanent and total disability as defined in the Plan); or

  

	 	(v)	one year after the “permanent and total disability” (as defined at Section 10 of the Plan) or death of the Holder. 

  

	10.	Company’s Right of First Refusal. Prior to the effective date of a registration statement under the Act, any shares of stock issued pursuant to exercise of this Option shall be subject to the Company’s
right of first refusal as set forth at Appendix A. 

  

	11.	Lock-Up Agreement. The Holder agrees that upon the request of the Company or the managing underwriter(s) of any offering of securities of the Company that is the subject of
a registration statement filed under the Act, for a period of time (not to exceed 180 days, plus such additional number of days (not to exceed 35) as may reasonably be requested to enable the underwriter(s) of such offering to comply with Rule
2711(f) of the Financial Industry Regulatory Authority or any amendment or successor thereto) 

  
 2 

	 	
from the effective date of the registration statement under the Act for such offering, the Holder will not sell, make any short sale of, loan, grant any option for the purchase of, or otherwise
dispose of any shares of Common Stock issued pursuant to the exercise of this Option, without the prior written consent of the Company and such underwriters. 

  

	12.	Tax Withholding. The Company’s obligation to deliver shares shall be subject to the Holder’s satisfaction of any federal, state and local income and employment tax withholding requirements.

  

	13.	Notice. Any notice to be given to the Company hereunder shall be deemed sufficient if addressed to the Company and delivered to the office of the Company, Homology Medicines, Inc., c/o 5AM Ventures, Waltham Woods
Corporate Center, 890 Winter Street, Suite 140, Waltham, MA 02451, attention of the president, or such other address as the Company may hereafter designate. 

Any notice to be given to the Holder hereunder shall be deemed sufficient if addressed to and delivered in person to the Holder at his or her
address furnished to the Company or when deposited in the mail, postage prepaid, addressed to the Holder at such address. 
 * * * * *

 IN WITNESS WHEREOF, the parties have executed this Option, or caused this Option to be executed, as of the Date of Grant. 

 

			
	Homology Medicines, Inc.
		
	By:	 	
                                         
                                   

 The undersigned Holder hereby acknowledges receipt of a copy of the Plan and this Option (including Appendix A hereto),
and agrees to the terms of this Option and the Plan. 
  

	
	  
 Holder:

  
 3 

 APPENDIX A 

Right of First Refusal 

1. General. Prior to the effective date of a registration statement under the Securities Act of 1933, as amended (the
“Act”), covering any shares of the Company’s Common Stock and until such time as the Company shall have effected a public offering of its Common Stock registered under the Act, in the event that, at any time when the Holder (which
term for purposes of this section shall mean the Holder and his or her executors, administrators and any other person to whom this Option may be transferred by will or the laws of descent and distribution) is permitted to do so, the Holder desires
to sell, assign or otherwise transfer any of the shares issued upon the exercise of this Option, the Holder shall first offer such shares to the Company by giving written notice of the Holder’s desire so to sell, assign or transfer such shares.

 2. Notice of Intended Transfer. The notice shall state the number of shares offered, the name of the person or persons to
whom it is proposed to sell, assign or transfer such shares and the price at which such shares are intended to be sold, assigned or transferred. Such notice shall constitute an offer to the Company for the Company to purchase the number of shares
set forth in the notice at a price per share equal to the price stated therein. 
 3. Company to Accept or Decline Within 30 Days.
The Company may accept the offer as to all, but not less than all, such shares by notifying the Holder in writing within 30 days after receipt of such notice of its acceptance of the offer. If the offer is accepted, the Company shall have 60
days after such acceptance within which to purchase the offered shares at a price per share as aforesaid. If within the applicable time periods the Holder does not receive notice of the Company’s intention to purchase the offered shares, or if
payment in full of the purchase price is not made by the Company, the offer shall be deemed to have been rejected and the Holder may transfer title to such shares within 90 days from the date of the Holder’s written notice to the Company of the
Holder’s intention to sell, but such transfer shall be made only to the proposed transferee and at the proposed price as stated in such notice and after compliance with any other provisions of this Option applicable to the transfer of such
shares. 
 4. Transferred Shares to Remain Subject to Right of First Refusal. Shares that are so transferred to such transferee shall
remain subject to the rights of the Company set forth in this Appendix A. As a condition to such transfer, such transferee shall execute and deliver all such documents as the Company may require to evidence the binding agreement of such transferee
so to remain subject to the rights of the Company. 
 5. Remedies of Company. No sale, assignment, pledge or other transfer of
any of the shares covered by this Option shall be effective or given effect on the books of the Company unless all of the applicable provisions of this Appendix A have been duly complied with, and the Company may inscribe on the face of any
certificate representing any of such shares a legend referring to the provisions of this Appendix A. If any transfer of shares is made or attempted in violation of the foregoing restrictions, or if shares are not offered to the Company as required
hereby, the Company shall have the right to purchase such shares from the owner thereof or his transferee at any time before or after the transfer, as herein provided. In addition to 

  
 4 

 
any other legal or equitable remedies which it may have, the Company may enforce its rights by actions for specific performance (to the extent permitted by law) and may refuse to recognize any
transferee as one of its stockholders for any purpose, including, without limitation, for purposes of dividend and voting rights, until all applicable provisions hereof have been complied with. 

6. Shares Subject to Right of First Refusal. For purposes of the Right of First Refusal pursuant to this Appendix A, the term
“shares” shall mean any and all new, substituted or additional securities or other property issued to the Holder, by reason of his or her ownership of Common Stock pursuant to the exercise of this Option, in connection with any stock
dividend, liquidating dividend, stock split or other change in the character or amount of any of the outstanding securities of the Company, or any consolidation, merger or sale of all or substantially all of the assets of the Company. 

7. Legends on Stock Certificates. Any certificate representing shares of stock subject to the provisions of this Appendix A may
have endorsed thereon one or more legends, substantially as follows: 
  

	 	(i)	“Any disposition of any interest in the securities represented by this certificate is subject to restrictions, and the securities represented by this certificate are subject to certain options, contained in a
certain agreement between the record holder hereof and the Company, a copy of which will be mailed to any holder of this certificate without charge upon receipt by the Company of a written request therefor.” 

 

	 	(ii)	“The shares of stock represented by this certificate have not been registered under the Securities Act of 1933 or under the securities laws of any state and may not be pledged, hypothecated, sold or otherwise
transferred unless such shares have been registered under the Act or unless the Company has received an opinion of counsel satisfactory to the Company, in form and substance satisfactory to the Company, that such registration is not required.”

 8. Right of First Refusal to Lapse Upon Registration. The restrictions imposed by this Appendix A shall
terminate in all respects upon the effective date of a registration statement under the Act covering any of the Company’s Common Stock. 

  
 5

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