SC 13G

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13G

UNDER THE SECURITIES EXCHANGE ACT OF 1934

(Amendment No.     )*

Synthorx, Inc.

(Name of Issuer)

Common Stock, par value $0.001 per share

(Title of Class of Securities)

87167A 10 3

(CUSIP Number)

December 11, 2018

(Date of Event Which Requires Filing of this Statement)

Check the appropriate box to designate the rule pursuant to which this Schedule is filed:

☐ Rule 13d-1(b)

☒ Rule 13d-1(c)

☐ Rule 13d-1(d)

* The remainder of this cover page shall be filled out for a reporting person’s initial filing on this form with respect to the subject class of securities, and for any subsequent amendment containing information which would alter the disclosures provided in a prior cover page.

The information required in the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934 (“Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

    CUSIP No. 87167A 10 3       SCHEDULE 13G       Page 2 of 10 Pages  

1.	NAMES OF REPORTING PERSONS Medicxi Ventures I LP
2.	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (a)  ☐        (b)  ☒
3.	SEC USE ONLY
4.	CITIZENSHIP OR PLACE OF ORGANIZATION Jersey
NUMBER OF SHARES BENEFICIALLY   OWNED BY EACH REPORTING PERSON WITH		5.	SOLE VOTING POWER 2,252,582
		6.	SHARED VOTING POWER 0
		7.	SOLE DISPOSITIVE POWER 2,252,582
		8.	SHARED DISPOSITIVE POWER 0
9.	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 2,252,582
10.	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES  ☐
11.	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9 7.45% (1)
12.	TYPE OF REPORTING PERSON PN

(1) The percentage of class was calculated based on 30,230,319 shares of the Issuer’s common stock, par value $0.001 per share (“Common Stock”), as set forth in the Issuer’s final prospectus, dated December 6, 2018, and filed with the Securities and Exchange Commission (the “Commission”) on December 7, 2018.

    CUSIP No. 87167A 10 3       SCHEDULE 13G       Page 3 of 10 Pages  

1.	NAMES OF REPORTING PERSONS Medicxi Co-Invest I LP
2.	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (a)  ☐        (b)  ☒
3.	SEC USE ONLY
4.	CITIZENSHIP OR PLACE OF ORGANIZATION Jersey
NUMBER OF SHARES BENEFICIALLY   OWNED BY EACH REPORTING PERSON WITH		5.	SOLE VOTING POWER 28,513
		6.	SHARED VOTING POWER 0
		7.	SOLE DISPOSITIVE POWER 28,513
		8.	SHARED DISPOSITIVE POWER 0
9.	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 28,513
10.	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES  ☐
11.	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9 0.09% (1)
12.	TYPE OF REPORTING PERSON PN

    CUSIP No. 87167A 10 3       SCHEDULE 13G       Page 4 of 10 Pages  

1.	NAMES OF REPORTING PERSONS Medicxi Ventures I GP Limited
2.	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (a)  ☐        (b)  ☒
3.	SEC USE ONLY
4.	CITIZENSHIP OR PLACE OF ORGANIZATION Jersey
NUMBER OF SHARES BENEFICIALLY   OWNED BY EACH REPORTING PERSON WITH		5.	SOLE VOTING POWER 2,281,095
		6.	SHARED VOTING POWER 0
		7.	SOLE DISPOSITIVE POWER 2,281,095
		8.	SHARED DISPOSITIVE POWER 0
9.	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 2,281,095
10.	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES  ☐
11.	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9 7.54% (1)
12.	TYPE OF REPORTING PERSON OO

    CUSIP No. 87167A 10 3       SCHEDULE 13G       Page 5 of 10 Pages  

1.	NAMES OF REPORTING PERSONS Medicxi Ventures Management (Jersey) Limited
2.	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (a)  ☐        (b)  ☒
3.	SEC USE ONLY
4.	CITIZENSHIP OR PLACE OF ORGANIZATION Jersey
NUMBER OF SHARES BENEFICIALLY   OWNED BY EACH REPORTING PERSON WITH		5.	SOLE VOTING POWER 2,281,095
		6.	SHARED VOTING POWER 0
		7.	SOLE DISPOSITIVE POWER 2,281,095
		8.	SHARED DISPOSITIVE POWER 0
9.	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 2,281,095
10.	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES  ☐
11.	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9 7.54% (1)
12.	TYPE OF REPORTING PERSON OO

    CUSIP No. 87167A 10 3       SCHEDULE 13G       Page 6 of 10 Pages  

Item 1.		Issuer
	(a)	Name of Issuer:
		Synthorx, Inc. (the “Issuer”)
	(b)	Address of Issuer’s Principal Executive Offices:
		11099 N. Torrey Pines Road Suite 190 La Jolla, California 92037
Item 2.		Filing Person
	(a) – (c)	Name of Persons Filing; Address; Citizenship:
		(i)  Medicxi Ventures I LP (“Ventures I”); (ii)  Medicxi Co-Invest I LP (“Co-Invest I”); (iii)   Medicxi Ventures I GP Limited (“Ventures I GP”), which is the sole general partner of Ventures I and Co-Invest I; (iv) Medicxi Ventures Management (Jersey) Limited (“Manager”), which is appointed by Ventures I GP as the manager of Ventures I and Co-Invest I. The address of the principal business office of each of the Reporting Persons is Nick McHardy c/o Intertrust Fund Services (Jersey) Limited, 44 Esplanade, St. Helier, Jersey JE4 9WG. Each of Ventures I and Co-Invest I is a Jersey limited partnership. Each of Ventures I GP and Manager is a Jersey limited liability company.
	(d)	Title of Class of Securities:
		Common stock, par value $0.001 per share
	(e)	CUSIP Number: 87167A 10 3
Item 3.	If this statement is filed pursuant to Rules 13d-1(b), or 13d-2(b) or (c), check whether the person filing is a:
	(a)	☐     Broker or dealer registered under Section 15 of the Act;
	(b)	☐     Bank as defined in Section 3(a)(6) of the Act;
	(c)	☐     Insurance company as defined in Section 3(a)(19) of the Act;
	(d)	☐     Investment company registered under Section 8 of the Investment Company Act of 1940;
	(e)	☐     An investment adviser in accordance with Rule 13d-1(b)(1)(ii)(E);
	(f)	☐     An employee benefit plan or endowment fund in accordance with Rule 13d-1(b)(1)(ii)(F);
	(g)	☐     A parent holding company or control person in accordance with Rule 13d-1(b)(1)(ii)(G);
	(h)	☐     A savings associations as defined in Section 3(b) of the Federal Deposit Insurance Act (12 U.S.C. 1813);
	(i)	☐     A church plan that is excluded from the definition of an investment company under section 3(c)(14) of the Investment Company Act of 1940;
	(j)	☐     A non-U.S. institution in accordance with Rule 240.13d-1(b)(1)(ii)(J);

    CUSIP No. 87167A 10 3       SCHEDULE 13G       Page 7 of 10 Pages  

	(k)	☐       Group, in accordance with Rule 240.13d-1(b)(1)(ii)(K). If filing as a non-U.S. institution in accordance with Rule 240.13d-1(b)(1)(ii)(J), please specify the type of institution:
		If filing as a non-U.S. institution in accordance with §240.13d-1(b)(1)(ii)(J), please specify the type of institution:
Item 4.	Ownership.
	(a) and (b)	Amount beneficially owned:
		(i) Ventures I directly owns 2,252,582 shares of Common Stock (the “Ventures I Shares”), which represents approximately 7.45% of the outstanding shares of Common Stock.
		(ii)  Co-Invest I directly owns 28,513 shares of Common Stock (the “Co-Invest I Shares”), which represents approximately 0.09% of the outstanding shares of Common Stock.
		(iii)  Ventures I GP is the general partner of Ventures I and Co-Invest I and may be deemed to beneficially own the Ventures I Shares and the Co-Invest I Shares (the “Shares”), which in the aggregate is 2,281,095 shares of Common Stock and represents approximately 7.54% of the outstanding shares of Common Stock.
		(iv) As Manager has been appointed by Ventures I GP as manager of Ventures I and Co-Invest I, Manager may be deemed to beneficially own the Shares.
	(c)	Number of shares as to which such person has:

	Number of Shares of Common Stock
Reporting Person	(i)		(ii)		(iii)		(iv)
Ventures I		2,252,582		0		2,252,582		0
Co-Invest I		28,513		0		28,513		0
Ventures I GP		2,281,095		0		2,281,095		0
Manager		2,281,095		0		2,281,095		0

	(i)   Sole power to vote or direct the vote (ii)    Shared power to vote or to direct the vote (iii)    Sole power to dispose or to direct the disposition of (iv)   Shared power to dispose or to direct the disposition of
	The percentage of class was calculated based on 30,230,319 shares of Common Stock, as set forth the Issuer’s final prospectus, dated December 6, 2018, and filed with the Commission on December 7, 2018.
Item 5.	Ownership of Five Percent or Less of a Class. Not applicable.
Item 6.	Ownership of More than Five Percent on Behalf of Another Person. Not applicable.
Item 7.	Identification and Classification of the Subsidiary Which Acquired the Security Being Reported on by the Parent Holding Company or Control Person. Not applicable.

    CUSIP No. 87167A 10 3       SCHEDULE 13G       Page 8 of 10 Pages  

Item 8.	Identification and Classification of Members of the Group. Not applicable.
Item 9.	Notice of Dissolution of Group. Not applicable.
Item 10.	Certification. By signing below I certify that, to the best of my knowledge and belief, the securities referred to above were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect.

    CUSIP No. 87167A 10 3       SCHEDULE 13G       Page 9 of 10 Pages  

SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

Date: December 19, 2018

Medicxi Ventures I LP
By:	/s/ Alex Di Santo
Name:	Alex Di Santo
Title:	Director
For and on behalf of Medicxi Ventures Management (Jersey) Limited as manager of Medicxi Ventures I LP
Medicxi Co-Invest I LP
By:	/s/ Alex Di Santo
Name:	Alex Di Santo
Title:	Director
For and on behalf of Medicxi Ventures Management (Jersey) Limited as manager of Medicxi Co-Invest I LP
Medicxi Ventures I GP Limited
By:	/s/ Alex Di Santo
Name:	Alex Di Santo
Title:	Director
Medicxi Ventures Management (Jersey) Limited
By:	/s/ Alex Di Santo
Name:	Alex Di Santo
Title:	Director

    CUSIP No. 87167A 10 3       SCHEDULE 13G       Page 10 of 10 Pages  

EXHIBIT INDEX

Exhibit
99.1	Joint Filing Statement

EX-99.1

EXHIBIT 99.1

Joint Filing Agreement

The undersigned acknowledge and agree that the foregoing statement on Schedule 13G is filed on behalf of each of the undersigned and that all subsequent amendments to this statement on Schedule 13G shall be filed on behalf of each of the undersigned without the necessity of filing additional joint filing statements. The undersigned acknowledge that each shall be responsible for the timely filing of such amendments, and for the completeness and accuracy of the information concerning him, her or it contained herein, but shall not be responsible for the completeness and accuracy of the information concerning the other entities or persons, except to the extent that he, she or it knows or has reason to believe that such information is inaccurate.

Date: December 19, 2018

Medicxi Ventures I LP
By:	/s/ Alex Di Santo
Name:	Alex Di Santo
Title:	Director
For and on behalf of Medicxi Ventures Management (Jersey) Limited as manager of Medicxi Ventures I LP
Medicxi Co-Invest I LP
By:	/s/ Alex Di Santo
Name:	Alex Di Santo
Title:	Director
For and on behalf of Medicxi Ventures Management (Jersey) Limited as manager of Medicxi Co-Invest I LP
Medicxi Ventures I GP Limited
By:	/s/ Alex Di Santo
Name:	Alex Di Santo
Title:	Director
Medicxi Ventures Management (Jersey) Limited
By:	/s/ Alex Di Santo
Name:	Alex Di Santo
Title:	Director