United States securities and exchange commission logo

                               April 14, 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
                                                            Draft Registration
Statement on Form S-1
                                                            Submitted March 19,
                                                            CIK No. 0001845337

       Dear Dr. Bender:

              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

             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

       Draft Registration Statement on Form S-1 submitted March 19, 2021

       Prospectus Summary
       Overview, page 1

   1.                                                   We note your statement
on page 1 that your lead product candidate    has the potential to be
                                                        first-in-class    and
several other references to    first-in-class    on pages 2, 3, 104, 121, 126,
                                                        and elsewhere in the
prospectus. This term suggest that the product candidate is effective
                                                        and likely to be
approved by the FDA. Please delete these references throughout your
                                                        registration statement.
To the extent your use of this term was intended to convey your
                                                        belief that the product
is based on a novel technology or approach and/or is further along
                                                        in the development
process, you may discuss how your technology differs from
                                                        technology used by
competitors and, as applicable, that you are not aware of competing
 Jeremy Bender, Ph.D., M.B.A.
FirstName   LastNameJeremy Bender,
                           HoldingPh.D., M.B.A.
Day One Biopharmaceuticals         Co LLC
April       NameDay One Biopharmaceuticals Holding Co LLC
       14, 2021
April 214, 2021 Page 2
FirstName LastName
         products that are further along in the development process. Statements
such as these
         should be accompanied by cautionary language that the statements are
not intended to
         give any indication that the product candidate has been proven
effective or that it will
         receive regulatory approval.
2.       We note on page 2 that you plan to    further [y]our leadership
position.    Please
         substantiate this and other claims of leadership throughout the
prospectus or revise them
         to state them as your beliefs.
3.       Please balance your statements that your portfolio is wholly-owned and
that you hold
         worldwide exclusive rights to DAY101 for all oncology indications and
to pimasertib for
         all therapeutic areas with reference to your milestone and royalty
obligations under the
         Viracta license agreement, as discussed on page 148.
Our Product Candidates, page 3

4.       We note your statement on page 20 that you expect that the Phase 2
trial of DAY101 in
         pLGG will provide a sufficient dataset to support marketing approval
with only 60
         patients. Please revise to state the basis for your claim and your
references to the
         FIREFLY-1 trial as    pivotal.
5.       Please shorten the arrows in the pipeline chart to correspond to the
current stage of
         development. For example, neither you nor the investigator has
completed any Phase 2
         clinical trial of DAY 101 for pediatric relapsed pLGG or as a
frontline therapy in pLGG.
         We will not object to footnote or explanatory disclosure to describe
your anticipated
         development plans.
6.       The third product candidate disclosed in your pipeline table,
MSC201503B, has not been
         associated with a target disease. Please tell us why you believe it is
material to your
         business or remove it from your pipeline table. In this regard, we
note that there does not
         appear to be any discussion of this product candidate or your
development efforts other
         than in the disclosure of your license and patent agreements.
Our Strategy, page 5

7.       We note your references to    rapidly    advancing your lead product
candidate through
         clinical development, the potential for    expedited    clinical
execution and the ability to
            rapidly advance    clinical development of oncology product
candidates in pediatric
         patients. Please revise these statements here and throughout your
registration statement to
         remove any implication that you will be successful in commercializing
your product
         candidates in a rapid or accelerated manner.
Summary Consolidated Financial Data, page 11

8.       Please revise to provide pro forma EPS in your consolidated statements
of operations and
         comprehensive loss data which reflects the conversion of your
preferred shares. Refer to
         Rule 11 of Regulation S-X.
 Jeremy Bender, Ph.D., M.B.A.
Day One Biopharmaceuticals Holding Co LLC
April 14, 2021
Page 3
Risk Factors
Risks related to our common stock and this offering, page 78

9.       Please revise the exclusive forum risk factor on page 85 to disclose
that investors cannot
         waive compliance with the federal securities laws and the rules and
regulations thereunder
         and that there is also a risk that your forum selection provisions may
result in increased
         costs for investors to bring a claim.
Business, page 121

10.      Revise your graphics throughout this section so that the fonts are
large enough to be
Clinical trial results for pLGG, page 135

11.      We note you have disclosed some treatment emergent adverse events here
and some
         adverse events related to the pimasertib study on page 144. Please
revise to identify all
         treatment-related serious adverse events and the number of patients
that experienced them
         for each clinical trial you discuss.
12.      Please tell us whether the MRI images on page 137 are representative
of results observed
         in your Phase 1 trial of DAY101. If this graphic is not
representative, please remove it.
Intellectual Property, page 150

13.      In the third paragraph, you state you own or co-own a patent portfolio
consisting of seven
         patent families, however, in the following paragraph it appears you
only discuss five
         patent families. Please revise to clarify. Also, for each patent
family, please disclose the
         applicable jurisdictions for your granted patents, rather than
generalizing    foreign
Consolidated Financial Statements
Note 17. Subsequent Events, page F-31

14.      Please revise to disclose the key terms for the Series B redeemable
convertible preferred

15.    Please 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
FirstName LastNameJeremy Bender, Ph.D., M.B.A.
       potential investors in reliance on Section 5(d) of the Securities Act,
whether or not they
Comapany    NameDay
       retain copies ofOne  Biopharmaceuticals Holding Co LLC
                        the communications.
April 14, 2021 Page 3
FirstName LastName
 Jeremy Bender, Ph.D., M.B.A.
FirstName   LastNameJeremy Bender,
                           HoldingPh.D., M.B.A.
Day One Biopharmaceuticals         Co LLC
April       NameDay One Biopharmaceuticals Holding Co LLC
       14, 2021
April 414, 2021 Page 4
FirstName LastName
       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
other questions.


                                                         Division of
Corporation Finance
                                                         Office of Life
cc:      Julia Forbess, Esq.