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                             August 11, 2021

       Timothy P. Clackson, Ph.D.
       President and Chief Executive Officer
       Theseus Pharmaceuticals, Inc.
       210 Broadway Street
       Cambridge, Massachusetts 02139

                                                        Re: Theseus
Pharmaceuticals, Inc.
                                                            Draft Registration
Statement on Form S-1
                                                            Submitted July 20,
2021
                                                            CIK 0001745020

       Dear Dr. Clackson:

              We have reviewed your draft registration statement and have the
following comments. In
       some of our comments, we may ask you to provide us with information so
we may better
       understand your disclosure.

              Please respond to this letter by providing the requested
information and either submitting
       an amended draft registration statement or publicly filing your
registration statement on
       EDGAR. If you do not believe our comments apply to your facts and
circumstances or do not
       believe an amendment is appropriate, please tell us why in your
response.

             After reviewing the information you provide in response to these
comments and your
       amended draft registration statement or filed registration statement, we
may have additional
       comments.

       Draft Registration Statement on Form S-1

       Prospectus Summary
       Overview, page 1

   1.                                                   We note your disclosure
here and on pages 85 and 99 stating that the global market for
                                                        TKIs was approximately
$23 billion in 2020 and is estimated to grow to more than $40
                                                        billion by 2026. Please
revise your disclosure to only present the market sizes for the
                                                        indications for which
you are currently developing product candidates.
   2.                                                   Please revise the
Overview subsection of the Prospectus Summary to disclose that THE-
                                                        630 is subject to the
ARIAD License Agreement, as indicated elsewhere in the prospectus.
 Timothy P. Clackson, Ph.D.
FirstName LastNameTimothy
Theseus Pharmaceuticals, Inc. P. Clackson, Ph.D.
Comapany
August 11, NameTheseus
           2021          Pharmaceuticals, Inc.
August
Page 2 11, 2021 Page 2
FirstName LastName
Our Pipeline, page 2

3.       Please remove or revise the "Product Rights" column in your pipeline
table to reflect your
         disclosure elsewhere in the prospectus that THE-630 is subject to your
license agreement
         with ARIAD Pharmaceuticals.
Pan-KIT Program -- THE-630, page 2

4.       Please remove your statements here and on pages 99 and 103 indicating
that THE-630 has
         the potential to be a    best-in-class    next generation KIT
inhibitor as this language
         suggests that THE-630 is effective and likely to be approved.
Risk Factors
We may not be able to utilize a significant portion..., page 44

5.       Please quantify the NOLs that are subject to limitation and clarify
the factors that will
         determine the extent of the limitation.
Industry and Market Data, page 76

6.       We note your statement that the content of the third-party sources,
except to the extent set
         forth in this prospectus, does not constitute a portion of the
prospectus. We further note
         your statements that (i) you have not separately verified third party
data and (ii) no third
         party has verified your internal research. These statements may imply
an inappropriate
         disclaimer of responsibility with respect to such information that
appears in your
         prospectus. Please either delete these statements or specifically
state that you are liable for
         the disclosure regarding to the market and industry data and your
internal company
         research that appears in the prospectus.
Business
Our Research and Discovery Approach, page 102

7.       We note your disclosure throughout this section stating that (i) you
believe your approach
         substantially reduces the biological and clinical risk of your
programs, (ii) your predictive
         screening allows you to apply your drug discovery expertise to select
inhibitors with
         optimal properties and (iii) you design clinical development programs
that provide you
         with an early and accurate assessment of activity against individual
mutant variants
         and will enable you to executive efficient clinical development.

         Please revise your disclosure throughout this section to clarify, if
true, that (x) the drug
         development progress is inherently uncertain and cannot be fully
de-risked, (y) your
         predictive screening approach has not been clinically validated and
that your TKIs may
         not function as anticipated in future clinical trials and (z) there is
no guarantee that the
         clinical trials you may conduct in the future will provide you with
positive or actionable
         data that will facilitate efficient clinical development.
 Timothy P. Clackson, Ph.D.
Theseus Pharmaceuticals, Inc.
August 11, 2021
Page 3
Our Pan-KIT Inhibitor Program: THE-630, page 103

8.     We note your statement that you intend to pursue a streamlined
development pathway.
       Please revise your disclosure to clarify what is meant by "streamlined"
and describe how
       you will streamline the development of THE-630 as compared to competing
product
       candidates that are being developed for the same indication. We further
note your
       statement that you intend to "quickly" advance THE-630 into earlier
lines of treatment if
       you receive positive data from your planned Phase 1/2 trials. Please
revise this disclosure
       to remove any implication that you will be successful in commercializing
your product
       candidates in an accelerated manner as such statements are speculative.
Intellectual Property, page 122

9.     Please revise your disclosure regarding your patent portfolio to specify
the number of
       patents and patent applications you own and license separately. Please
also specify
       whether you own or license the issued U.S. patent covering the KIT
inhibitor program and
       the type of patent protection it has (e.g., composition of matter,
method of use, etc.).
General

10.    Please supplementally provide us with copies of all written
communications, as defined in
       Rule 405 under the Securities Act, that you, or anyone authorized to do
so on your behalf,
       present to potential investors in reliance on Section 5(d) of the
Securities Act, whether or
       not they retain copies of the communications.
       You may contact Christie Wong at 202-551-3684 or Terence O'Brien at
202-551-3355 if
you have questions regarding comments on the financial statements and related
matters. Please
contact Alan Campbell at 202-551-4224 or Irene Paik at 202-551-6553 with any
other questions.



                                                             Sincerely,
FirstName LastNameTimothy P. Clackson, Ph.D.
                                                             Division of
Corporation Finance
Comapany NameTheseus Pharmaceuticals, Inc.
                                                             Office of Life
Sciences
August 11, 2021 Page 3
cc:       Colin G. Conklin
FirstName LastName