United States securities and exchange commission logo

                             May 18, 2021

       Jeremy Bender, Ph.D., M.B.A.
       Chief Executive Officer
       Day One Biopharmaceuticals Holding Co LLC
       395 Oyster Point Blvd., Suite 217
       South San Francisco, CA 94080

                                                        Re: Day One
Biopharmaceuticals Holding Co LLC
Statement on Form S-1
                                                            Filed May 4, 2021
                                                            File No. 333-255754

       Dear Dr. Bender:

              We have reviewed your 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 amending your registration
statement and providing the
       requested information. 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

              After reviewing any amendment to your registration statement and
the information you
       provide in response to these comments, we may have additional comments.

       Registration Statement on Form S-1

       Prospectus Summary
       Our Product Candidates, page 3

   1.                                                   We note your disclosure
in the pipeline table indicating that you anticipate filing a new
                                                        drug application (NDA)
for DAY101 for the treatment of pediatric patients with relapsed
                                                        or progressive
low-grade glioma in 2023. Please revise to remove any implication that
                                                        your FIREFLY-1 or other
trials will be successful. Please make similar revisions
                                                        throughout your
prospectus, e.g., on pages 106, 127, 129, 130 and 134.
 Jeremy Bender, Ph.D., M.B.A.
Day One Biopharmaceuticals Holding Co LLC
May 18, 2021
Page 2
Risk Factors
Risks related to our common stock and this offering
The exclusive forum provision in our organizational documents may limit a
ability..., page 86

2.    We note your response to comment 9 and your disclosure stating that your
forum selection
      provision designating the Delaware Court of Chancery as the exclusive
forum for certain
      actions, including any derivative action, does not apply to suits brought
to enforce any
      duty or liability created under the Exchange Act. Please ensure that the
exclusive forum
      provision in your governing documents states this clearly.
Business, page 127

3.    We reissue comment 10 in part. Please revise the graphics on pages 139
and 140, which
      still contain fonts that are too small or illegible.
        We remind you that the company and its management are responsible for
the accuracy
and adequacy of their disclosures, notwithstanding any review, comments, action
or absence of
action by the staff.

       Refer to Rules 460 and 461 regarding requests for acceleration. Please
allow adequate
time for us to review any amendment prior to the requested effective date of
the registration

       You may contact Li Xiao at 202-551-4391 or Kevin Kuhar at 202-551-3662
if you have
questions regarding comments on the financial statements and related matters.
Please contact
Abby Adams at 202-551-6902 or Christine Westbrook at 202-551-5019 with any

FirstName LastNameJeremy Bender, Ph.D., M.B.A.
                                                   Division of Corporation
Comapany NameDay One Biopharmaceuticals Holding Co LLC
                                                   Office of Life Sciences
May 18, 2021 Page 2
cc:       Julia Forbess, Esq.
FirstName LastName