EX-4.2





 Exhibit 4.2 

AMENDED AND RESTATED 

INVESTORS’ RIGHTS AGREEMENT 

THIS AMENDED AND RESTATED INVESTORS’ RIGHTS AGREEMENT (this “Agreement”), is made as of the 12th day of April, 2018, by and among Synthorx, Inc., a Delaware corporation (the “Company”), and each of the investors listed on Schedule A attached hereto, each of which is
referred to in this Agreement as an “Investor” and any Additional Investor (as defined in Section 6.9 hereof) that becomes a party to this Agreement in accordance with Section 6.9
hereof. 
 RECITALS 

WHEREAS, certain of the Investors (the “Existing Investors”) hold shares of the Company’s Series
A Preferred Stock and/or Series B Preferred Stock and are parties to that certain Amended and Restated Investor Rights Agreement dated as of July 14, 2016 by and among the Company and the persons and entities listed on Exhibit A thereto (the
“Prior Agreement”); 
 WHEREAS, the Existing Investors are holders of a majority of the outstanding
Registrable Securities (as defined in the Prior Agreement), and 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; and 
 WHEREAS, certain of the Investors are parties to that certain Series C Preferred Stock Purchase
Agreement of even date herewith by and among the Company and the Investors (as the same may be amended from time to time, the “Purchase Agreement”), under which certain of the Company’s and such Investors’ obligations are
conditioned upon the execution and delivery of this Agreement. 
 NOW, THEREFORE, the Existing Investors hereby
agree that the Prior Agreement shall be amended and restated in its entirety by this Agreement, and the parties to this Agreement 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 or private equity 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          “Common Stock” means shares
of the Company’s common stock, par value $0.001 per share. 

1.3          “Certificate of
Incorporation” means the Company’s Amended and Restated Certificate of Incorporation, as the same may be amended from time to time. 

  
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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          “Defaulting
Purchaser” shall have the same meaning ascribed to such term in the Purchase Agreement. 

1.6          “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. 

1.7          “Early Converted Common
Stock” shall have the meaning ascribed to such term in the Purchase Agreement. 

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

1.9          “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. 

1.10        “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. 

1.11        “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. 

1.12        “GAAP” means generally accepted
accounting principles in the United States. 

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

1.14        “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. 

1.15        “Initiating Holders” means,
collectively, Holders who properly initiate a registration request under this Agreement. 

1.16        “IPO” means the Company’s first
underwritten public offering of its Common Stock under the Securities Act. 

1.17        “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). 
 1.18        “Major Investor” means
any Investor that, individually or together with such Investor’s Affiliates, holds at least 200,000 shares of Registrable Securities (as adjusted for any stock split, stock dividend, combination, or other recapitalization or reclassification
effected after the date hereof). 
 1.19        “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 (including but not
limited to debt instruments) that are, or may become, convertible or exchangeable into or exercisable for such equity securities. 

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

1.21        “Preferred Directors” means any director
of the Company that the holders of record of the Series A Preferred Stock, Series B Preferred Stock or Series C Preferred Stock are entitled to elect as separate classes pursuant to the Certificate of Incorporation. 

1.22        “Preferred Stock” means, collectively,
shares of the Company’s Series A Preferred Stock, Series B Preferred Stock and Series C Preferred Stock. 

1.23        “Registrable Securities” means
(i) the Common Stock issuable or issued upon conversion of the Preferred Stock, excluding any Early Converted Common Stock, 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 clause (i) above; excluding in all cases, however, any Registrable
Securities sold by a Person in a transaction 

  
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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. 

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        “Restricted Securities” means the
securities of the Company required to be notated with the legend set forth in Subsection 2.12(b) hereof. 

1.26        “SEC” means the Securities and Exchange
Commission. 
 1.27        “SEC Rule 144” means
Rule 144 promulgated by the SEC under the Securities Act. 

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

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

1.30        “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. 

1.31        “Series A Preferred Stock” means shares
of the Company’s Series A Preferred Stock, par value $0.001 per share. 

1.32        “Series B Director” means any director
of the Company that the holders of record of the Series B Preferred Stock are entitled to elect pursuant to the Certificate of Incorporation. 

1.33        “Series B Preferred Stock” means shares
of the Company’s Series B Preferred Stock, par value $0.001 per share. 

1.34        “Series C Preferred Stock” means shares
of the Company’s Series C Preferred Stock, par value $0.001 per share. 

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

(a)        Form S-1 Demand. If at any
time after the earlier of (i) five (5) years after the date of this Agreement or (ii) one hundred eighty (180) days following the effective date 

  
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of the registration statement for the IPO, the Company receives a request from Holders of thirty percent (30%) of the Registrable Securities then outstanding that the Company file a Form S-1 registration statement with respect to a majority of the Registrable Securities then outstanding (or a lesser percent if the anticipated aggregate offering price, net of Selling Expenses, would exceed
$10,000,000 million), then the Company shall (x) within thirty (30) 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 sixty (60) 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 fifteen (15) 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 Holders of at least ten percent (10%) of the Registrable Securities then outstanding that the Company file a Form S-3 registration statement with respect to outstanding Registrable Securities of such Holders having an anticipated aggregate offering price, net of Selling Expenses, of at least $1 million, 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 sixty (60) 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 fifteen (15) 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 (the “Board”) 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 ninety (90) 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. 

(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): (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


  
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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) within the twelve (12) month period immediately preceding the date of such request. 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 securities 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 fifteen (15) 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 Holders of a majority of the Registrable Securities to be included in such underwriting, subject to the approval of the Company (which underwriter or underwriters shall be reasonably
acceptable to the Company). In such event, the right of any Holder to include such Holder’s Registrable Securities in such 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))


  
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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 managing
underwriter(s) advise(s) the Company in writing that marketing factors require a limitation on the number of shares to be underwritten, then the Company 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, pro rata based on the number of Registrable
Securities owned by each Holder; 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. 
 (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.
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, 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 underwriters cutback provisions in 

  
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Subsection 2.3(a), fewer than fifty percent (50%) of the total number of the Registrable Securities that the Holders have requested to be included in such registration
are not 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 a majority of the Registrable Securities registered thereunder, keep such
registration statement effective for a period of up to thirty (30) days or, if earlier, until the distribution contemplated in the registration statement has been completed; provided, however, that (i) such one thirty
(30) 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;

 (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; 

(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; 

  
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 (h)      promptly make available for
inspection by the selling Holders, any managing 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 $50,000 of one counsel for the selling Holders (“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 a majority
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 a
majority 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 

  
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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; 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 under the Securities Act, the Exchange Act or other federal or state law, insofar as such Damages arise out of or are based upon any of the following statements, omissions or
violations (collectively a “Violation”) by the Company: (i) any untrue statement or alleged untrue statement of a material fact contained in such registration statement or incorporated reference therein, including any
preliminary prospectus or final prospectus contained therein or any amendments or supplements thereto, (ii) the 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 Company 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 in connection with the offering covered by such registration statement, and the Company will pay to each 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 a Violation which occurs 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 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


  
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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 a written 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 not relieve such indemnifying party of any liability to the indemnified party under this Subsection 2.8, except to the extent that such failure is
prejudicial to 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 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 

  
 11. 













 
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(b), 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 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


  
 12. 













 
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 a majority of the Registrable Securities then outstanding, enter into any agreement with any holder or prospective holder of any securities of the
Company that would grant such holder rights to demand the registration of shares of the Company’s capital stock, or to include such shares in a registration statement that would reduce the number of shares includable by the Holders;
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 registration by the Company of shares of its Common Stock or any other equity securities under the Securities Act on a registration statement on Form S-1 or Form S-3,
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, not to exceed eighteen
(18) days, 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
2711(f)(4) or NYSE Rule 472(f)(4), or any successor provisions or amendments thereto (the “Lock-Up Period”): (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 such offering; 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, shall not apply to the sale of any shares to an underwriter pursuant to an underwriting agreement, 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), and shall not apply to transactions relating to
shares of Common Stock acquired by a Holder in the IPO or in open market transactions subsequent to the IPO date, provided that (a) no filing under Section 13 or Section 16(a) of the Exchange Act reporting a reduction in beneficial
ownership of shares of Common Stock shall be required and (b) no filing under Section 13 

  
 13. 













 
or Section 16(a) of the Exchange Act or other public announcement shall be voluntarily made by the Holder, in the case of both clauses (a) and (b), during the Lock-Up Period. 
 The underwriters in connection with such registration are intended third-party beneficiaries of this Subsection 2.11 and shall have the right, power and 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, except that, notwithstanding the
foregoing, the Company and the underwriters may, in their sole discretion, waive or terminate these restrictions with respect to up to one percent (1%) shares of the Common Stock. 

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 SECURITIES REPRESENTED HEREBY HAVE BEEN ACQUIRED FOR INVESTMENT AND HAVE NOT BEEN REGISTERED UNDER THE
SECURITIES ACT OF 1933. SUCH SHARES MAY NOT BE SOLD, PLEDGED, OR TRANSFERRED IN THE ABSENCE OF SUCH REGISTRATION OR A VALID EXEMPTION FROM THE REGISTRATION AND PROSPECTUS DELIVERY REQUIREMENTS OF SAID ACT. 

THE SECURITIES REPRESENTED HEREBY MAY BE TRANSFERRED ONLY IN ACCORDANCE WITH THE TERMS OF AN AGREEMENT
BETWEEN THE COMPANY AND THE STOCKHOLDER, A COPY OF WHICH IS ON FILE WITH THE SECRETARY OF THE COMPANY. 

  
 14. 













 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. 

(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 Certificate
of Incorporation, pursuant to which all proceeds received by such Holder are cash or immediately freely tradeable equity securities, or pursuant to which the Holders receive registration rights substantially similar to those set forth herein. 

(b)      such time as Rule 144 or another similar exemption under the Securities Act is
available for the sale of all of such Holder’s shares without limitation during a three-month period without registration; and 

(c)      the fifth anniversary of the IPO. 

  
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 3.      Information and Observer
Rights. 
 3.1      Delivery of Financial Statements. The Company
shall deliver to each Major Investor, provided that the Board 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 fifty (150) 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 (iii) a statement of stockholders’ equity as of the end of such year,
all of which are prepared in accordance with GAAP consistently applied, and, commencing with the fiscal year ending December 31, 2018, audited and certified by independent public accountants of recognized national standing selected by the
Company; provided that the Board, including a majority of the Preferred Directors may waive in writing the delivery of audited financial statements for a fiscal year, in which case the Company shall deliver unaudited financial statements within 90
days of the end of each such fiscal year; 
 (b)      as soon as practicable, but in any
event within forty-five (45) days after the end of each of the first three (3) quarters of each fiscal year of the Company, unaudited statements of income and cash flows for such fiscal quarter, and
an unaudited balance sheet as of the end of such fiscal quarter, each 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); 

(c)      as soon as practicable, but in any event within forty-five (45) days after the
end of each quarter of each fiscal year of the Company, a statement showing 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, and certified by the chief financial officer or chief
executive officer of the Company as being true, complete, and correct; 
 (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, each 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; 

  
 16. 













 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. The Company shall permit each Major Investor
(provided that the Board 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 the disclosure of which would adversely affect the
attorney-client privilege between the Company and its counsel. In no event will any Major Investor that is an investment fund be deemed to be a competitor for any purpose under this Agreement merely because it or any of its Affiliates invest in
entities that may be deemed competitive with the Company. 

3.3      Observer Rights. 

(a)      For so long as RA Capital Healthcare Fund, L.P. or any of its Affiliates
(collectively, “RA Capital”) hold any shares of Series B Preferred Stock or Series C Preferred Stock (or any amount of Common Stock issued upon conversion thereof) and is not a Defaulting Purchaser, RA Capital shall have the right
to designate one (1) observer who, at any time during which (a) RA Capital does not have a representative designated to serve on the Board or (b) RA Capital’s Board representative is unable to attend a meeting of the Board, shall
be entitled to attend all meetings of its Board in a nonvoting observer capacity and, in this respect, receive copies of all notices, minutes, consents, and other materials provided to the directors at the same time and in the same manner as
provided to such directors; provided, however, that such observer shall agree to hold in confidence and trust all information so provided; and provided further, that the Company reserves the right to withhold any
information and to exclude such observer 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 the observer is an Affiliate of a competitor of the Company. 

  
 17. 













 (b)      For so long as Medicxi Ventures I LP
or any of its Affiliates (collectively, “Medicxi”) hold any shares of Series C Preferred Stock (or any amount of Common Stock issued upon conversion thereof) and is not a Defaulting Purchaser, Medicxi shall have the right to
designate one (1) observer who shall be entitled to attend all meetings of its Board in a nonvoting observer capacity and, in this respect, receive copies of all notices, minutes, consents, and other materials provided to the directors at the
same time and in the same manner as provided to such directors; provided, however, that such observer shall agree to hold in confidence and trust all information so provided; and provided further, that the Company
reserves the right to withhold any information and to exclude such observer 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 the observer is an Affiliate of a competitor of the Company. 

3.4      Termination of Information and Observer 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) upon the consummation of the IPO, or (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 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. 
 3.6      Right to Conduct Activities.
The Company hereby acknowledges that OrbiMed Private Investments VII, LP and its Affiliates (collectively, “OrbiMed”) are investment funds and as such invest in numerous portfolio companies and have affiliates, some of which may be
deemed competitive with the Company’s business. Neither OrbiMed nor its partners, employees, affiliates, advisors or affiliated investment funds shall be liable to the Company for any claim arising out of, or based upon: (i) the investment
by OrbiMed or any 

  
 18. 













 
affiliated investment fund in any entity or activities that may be competitive to the Company, or (ii) actions taken by any partner, officer, advisor or other representative of OrbiMed in
his, her or its capacity as such to assist any such competitive company provided, however, that nothing herein shall relieve OrbiMed or any other party from liability associated with misuse of the Company’s confidential information as set forth
in Section 3.5 above. 
 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. A Major Investor shall be entitled to apportion the
right of first offer hereby granted to it in such proportions as it deems appropriate, among itself and its Affiliates. 

(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 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 Major Investor, but excluding any
Early Converted Common Stock, 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, but excluding all Early Converted Stock
(such Major Investor’s “Pro Rata Amount”). At the expiration of such twenty (20) day period, the Company shall promptly notify each 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 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 (excluding all Early Converted Common Stock), 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 (excluding all Early Converted Common Stock), 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). 

  
 19. 













 (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 Major Investors 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 Certificate of Incorporation), (ii) shares of Common Stock issued in the IPO, and (iii) the issuance of shares of Series C Preferred Stock pursuant to the Purchase Agreement. 

4.2       Termination.     The covenants set forth in
Subsection 4.1 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, as such term is defined in the Certificate of Incorporation, whichever event occurs first. 

5.      Additional Covenants. 

5.1 Insurance.     The Company shall obtain Directors and Officers liability insurance in an
amount and on terms and conditions satisfactory to the Board, including the Preferred Directors, and shall cause such insurance policy to be maintained until such time as the Board determines that such insurance should be discontinued. 

5.2 Employee Agreements.     The Company will cause 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 non-disclosure and proprietary rights assignment agreement substantially in the form
approved by the Board. 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. 
 5.3 Directors’ Liability and Indemnification. The Company shall enter into and maintain
indemnification agreements with each of its directors to indemnify such directors and the Investors who designate such directors to the maximum extent permissible under applicable law. 

5.4 Employee Stock. Unless otherwise approved by the Board, including the approval of a majority of the Preferred
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 

  
 20. 













 
following thirty-six (36) months and (ii) a market stand-off provision substantially similar to that in
Subsection 2.11. 
 5.5    Board Matters. Unless otherwise determined by the vote of a
majority of the directors then in office, the Board shall meet at least quarterly in accordance with an agreed-upon schedule. The Company shall reimburse the nonemployee directors for all reasonable out-of-pocket travel expenses incurred (consistent with the Company’s travel policy) in connection with attending meetings of the Board. Upon request by the Series B Director or the Series C Director,
respectively, each Board committee shall include such Series B Director or such Series C Director, as applicable. 
 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 as in effect immediately before
such transaction, whether such obligations are contained in the Company’s Bylaws, the Certificate of Incorporation, or elsewhere, as the case may be. 

5.7 Indemnification Matters.     The Company hereby acknowledges that one (1) or more of the
directors nominated to serve on the Board 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 Certificate of Incorporation or the
Company’s Bylaws (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. 
 5.8 Authorized
Series C Preferred Stock.     The Company shall not issue any shares of its Series C Preferred Stock other than pursuant to the Purchase Agreement unless agreed to by Board, including the approval of all Preferred Directors.

 5.9 Termination of Covenants.     The covenants set forth in this
Section 5, except for Subsections 5.6 and 5.7, shall terminate and be of no further force or effect (i) upon consummation of the IPO (ii) when the Company first becomes subject to the periodic reporting


  
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requirements of Section 12(g) or 15(d) of the Exchange Act, or (iii) upon a Deemed Liquidation Event, as such term is defined in the Certificate of Incorporation, 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 or (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; 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. 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. 
 6.2      Governing
Law. This Agreement shall be governed by the internal law of the State of Delaware, without regard to conflict of law principals that would result in the application of any law other than the law of the State of Delaware. 

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. 

  
 22. 













 (a)      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, 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. 

(b)      Consent to Electronic Notice. Each Investor and Key Holder consents to the
delivery of any stockholder notice pursuant to the Delaware General Corporation Law (the “DGCL”), as amended or superseded from time to time, by electronic transmission pursuant to Section 232 of the DGCL (or any successor
thereto) at the electronic mail address or the facsimile number as on the books of the Company. To the extent that any notice given by means of electronic transmission is returned or undeliverable for any reason, the foregoing consent shall be
deemed to have been revoked until a new or corrected electronic mail address has been provided, and such attempted electronic notice shall be ineffective and deemed to not have been given. Each Investor and Key Holder agrees to promptly notify the
Company of any change in such stockholder’s electronic mail address, and that failure to do so shall not affect the foregoing. 

6.6      Amendments and Waivers. 

(a)      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 a majority 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); and 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. 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 and is proportional to the Registrable Securities held by such Investor. 

(b)      In the event that the right of first offer in Subsection 4.1 above is waived
pursuant to Subsection 6.6(a) above and any Major Investor that consented to such waiver (a “Waiving Investor”) nevertheless purchases any such New Securities, each Major Investor that is not a Waiving Investor (a “Non-Waiving Major Investor”) shall be entitled to purchase its Adjusted Pro Rata Amount (as defined below) of such New Securities upon the terms and conditions set forth in Subsection 4.1. For
purposes of this Subsection 6.6(b) a Non-Waiving Major Investor’s “Adjusted Pro Rata Amount” of the New Securities subject to the waiver


  
 23. 













 
described herein shall be equal to (i) such Non-Waiving Major Investor’s Pro Rata Amount of such New Securities multiplied by (ii) the
highest percentage (up to 100%) of any Waiving Investor’s Pro Rata Amount that such Waiving Investor actually purchases. Notwithstanding the provisions of Subsection 6.6(a) above, this Subsection 6.6(b) may not be amended or
waived without the prior written consent of all Major Investors. 
 (c)      Notwithstanding
the foregoing, Schedule A hereto may be amended by the Company from time to time to add transferees of any Registrable Securities in compliance with the terms of this Agreement without the consent of the other parties; and Schedule A hereto may also
be amended by the Company after the date of this Agreement without the consent of the other parties to add information regarding any additional Investor who becomes a party to this Agreement in accordance with Subsection 6.9. 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 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      Additional Investors. Notwithstanding anything to the
contrary contained herein, if the Company issues additional shares of the Series C Preferred Stock after the date hereof (each, an “Additional Investor”), any purchaser of such shares of Series C 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. 

6.10      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 

  
 24. 













 
and restated and superseded and replaced in its entirety by this Agreement, and shall be of no further force or effect. 

6.11    Dispute Resolution. The parties (a) hereby irrevocably and
unconditionally submit to the jurisdiction of any state or federal court located in the County of San Diego, State of California for the purpose of any suit, action or other proceeding arising out of or based upon this Agreement, (b) agree not
to commence any suit, action or other proceeding arising out of or based upon this Agreement except in any state or federal court located in the County of San Diego, State of California, and (c) hereby waive, and agree not to assert, by way of
motion, as a defense, or otherwise, in any such suit, action or proceeding, any claim that it is not subject personally to the jurisdiction of the above-named courts, that its property is exempt or immune from attachment or execution, that the suit,
action or proceeding is brought in an inconvenient forum, that the venue of the suit, action or proceeding is improper or that this Agreement or the subject matter hereof may not be enforced in or by such court. 

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. 
 The prevailing party
shall be entitled to reasonable attorney’s fees, costs, and necessary disbursements in addition to any other relief to which such party may be entitled. Each of the parties to this Agreement consents to personal jurisdiction for any equitable
action sought in any state or federal court located in the County of San Diego, State of California. 

6.12    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. 

  
 25. 













 [Remainder of Page Intentionally Left Blank] 

  
 26. 













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

			
	COMPANY:
	
	SYNTHORX, INC.
	
	By:   /s/ Laura Shawver                    
	Name: Laura Shawver
	Title: President and Chief Executive Officer














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

			
	 INVESTOR:

	
	ORBIMED PRIVATE INVESTMENTS VII, LP
		
	 By:
	 	 OrbiMed Capital GP VII LLC,

	 Its: General Partner

		
	 By:
	 	 OrbiMed Advisors LLC,

	 Its: Managing Member

	
	By: /s/ Jonathan Silverstein                
	Name: Jonathan Silverstein
	Title: Member














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

			
	INVESTOR:
	
	RA CAPITAL HEALTHCARE FUND, L.P.
	
	 By: RA Capital Management, LLC

	Its: General Partner
	
	By:   /s/ Rajeev Shah                            
	Name:   Rajeev Shah                            
	Title: Authorized Signatory














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

			
	INVESTOR:
	
	BLACKWELL PARTNERS LLC – SERIES A
	
	By:   /s/ Aboyami A. Adigun                
	Name: Abayomi A. Adigun                  
	Title: Investment Manager
		 	DUMAC, Inc.
		 	Authorized Agent
	
	By: /s/ Jannine M. Lall                        
	Name: Jannine M. Lall                        
	Title: Controller
		 	DUMAC, Inc.
		 	Authorized Agent














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

			
	INVESTORS:
	
	AVALON VENTURES X, L.P.
	
	By:   /s/ Jay
Lichter                                
	Name: Jay
Lichter                                
	Title: Authorized Signatory
	
	AVALON X SPV, L.P.
	
	By:   /s/ Jay
Lichter                              
	Name: Jay
Lichter                                
	Title: Authorized Signatory














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

			
	 INVESTORS:

	
	 MEDICXI VENTURES I LP

	
	 By: its manager Medicxi Ventures Management
      (Jersey) Limited

	
	 By:  /s/ Sherin
Sugeeswaran                

	 Name: Sherin
Sugeeswaran                 

	 Title: Alternate
Director                       

	
	 MEDICXI CO-INVEST I
LP

	
	 By: its manager Medicxi Ventures Management
      (Jersey) Limited

		
	 By:
	 	   /s/ Sherin
Sugeeswaran                  

	 Name: Sherin
Sugeeswaran                  

	 Title: Alternate
Director                        















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

			
	INVESTOR:
	
	CORRELATION VENTURES, L.P.
		 	as nominee for
		 	Correlation Ventures, L.P.
		 	Correlation Ventures Executives Fund, L.P.
	
	By: CORRELATION VENTURES GP, LLC
	
	 By:   /s/ David E. Coats

	Name: David E. Coats
	Title: Managing Member














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

			
	 INVESTOR:

	
	 OSAGE UNIVERSITY PARTNERS II, LP

	
	 By: OSAGE UNIVERSITY GP II, LLC

	 ITS: GENERAL PARTNER

	
	 By:   /s/ William
Harrington                    

	 Name: William
Harrington                      

	 Title:
Member                                        
  















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

			
	INVESTOR:
	
	WIDDER FAMILY LIMITED PARTNERSHIP
		
	By:	 	 /s/ Kenneth J. Widder, M.D.

	Name: Kenneth J. Widder, M.D.
	Title: General Partner