POWER OF ATTORNEY

Know all by these presents, that the undersigned hereby constitutes and appoints
each of Laura Shawver, Tighe Reardon and Christian V. Kuhlen of Synthorx, Inc.
(the "Company"), signing individually, the undersigned's true and lawful
attorney-in-fact and agent to:

(1) execute for and on behalf of the undersigned, an officer, director or holder
of 10% of more of a registered class of securities of the Company, Forms 3, 4
and 5 in accordance with Section 16(a) of the Securities Exchange Act of 1934,
as amended (the "Exchange Act") and the rules thereunder;

(2) do and perform any and all acts for and on behalf of the undersigned that
may be necessary or desirable to complete and execute such Form 3, 4 or 5,
complete and execute any amendment or amendments thereto, and timely file such
forms or amendments with the United States Securities and Exchange Commission
and any stock exchange or similar authority; and

(3) take any other action of any nature whatsoever in connection with the
foregoing which, in the opinion of such attorney-in-fact, may be of benefit, in
the best interest of, or legally required by, the undersigned, it being
understood that the documents executed by such attorney-in-fact on behalf of the
undersigned pursuant to this Power of Attorney shall be in such form and shall
contain such terms and conditions as such attorney-in-fact may approve in such
attorney-in-fact's discretion.

The undersigned hereby grants to each such attorney-in-fact full power and
authority to do and perform any and every act and thing whatsoever requisite,
necessary, or proper to be done in the exercise of any of the rights and powers
herein granted, as fully to all intents and purposes as the undersigned might or
could do if personally present, with full power of substitution or revocation,
hereby ratifying and confirming all that such attorney-in-fact, or such
attorney-in-fact's substitute or substitutes, shall lawfully do or cause to be
done by virtue of this power of attorney and the rights and powers herein
granted.  The undersigned acknowledges that each of the foregoing
attorneys-in-fact, in serving in such capacity at the request of the
undersigned, is not assuming, nor is the Company assuming, any of the
undersigned's responsibilities to comply with Section 16 of the Exchange Act.

This Power of Attorney shall remain in full force and effect until the earliest
to occur of (a) the undersigned is no longer required to file Forms 3, 4 and 5
with respect to the undersigned's holdings of and transactions in securities
issued by the Company, (b) revocation by the undersigned in a signed writing
delivered to the foregoing attorney-in-fact or (c) until such attorney-in-fact
shall no longer be employed by the Company.

IN WITNESS WHEREOF, the undersigned has caused this Power of Attorney to be
executed as of this 19th day of December, 2018.

/s/ Andrew Powell