POWER OF ATTORNEY

      The undersigned, as a Section 16 reporting person of Theseus
Pharmaceuticals, Inc., a Delaware corporation (the "Company"), hereby
constitutes and appoints each of Bradford Dahms and Kristine Callahan as the
undersigned's true and lawful attorney-in-fact to:

      (1)    complete and execute Form ID, "Update Passphrase Confirmation" form
and Forms 3, 4 and 5 and other forms and all amendments thereto as such
attorney-in-fact shall in his or her discretion determined to be required or
advisable pursuant to Section 16 of the Securities Exchange Act of 1934 (as
amended) and the rules and regulations promulgated thereunder, or any successor
laws and regulations, as a consequence of the undersigned's ownership,
acquisition or disposition of securities of the Company; and

      (2)    do all acts necessary in order to file such forms with the
Securities and Exchange Commission, any securities exchange or national
association, the Company and such other person or agency as the attorney-in-fact
shall deem appropriate.

      The undersigned hereby ratifies and confirms all that said attorneys-in-
fact and agents shall do or cause to be done by virtue hereof.  The undersigned
acknowledges that the foregoing attorneys-in-fact, in serving in such capacity
at the request of the undersigned, are not assuming, nor is the Company
assuming, any of the undersigned's responsibilities to comply with the
Securities Exchange Act of 1934 (as amended).

      This Power of Attorney shall remain in full force and effect until 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,
unless earlier revoked by the undersigned in a signed writing delivered to the
foregoing attorneys-in-fact.

      IN WITNESS WHEREOF, the undersigned has caused this Power of Attorney to
be executed as of this 30th day of July, 2021.


                                        /s/ Iain Dukes
                                        ----------------------------------------
                                        Iain Dukes