January 23, 2025

Mark McDonnell
Managing Director
ARCH Venture Management, LLC
8755 W. Higgins Road Suite 1025
Chicago, IL 60631

       Re: ARCH Venture Management, LLC
           Boundless Bio, Inc.
           Schedule 13D Filed by ARCH Venture Fund IX, L.P. et al.
           Filed October 29, 2024
           File No. 005-94492
Dear Mark McDonnell:

       We have conducted a limited review of the above-captioned filing and
have the
following comment.

        Please respond to this letter by amending the filing or by providing
the requested
information. If you do not believe our comment applies to your facts and
circumstances or
that an amendment is appropriate, please advise us why in a response letter.

       After reviewing any amendment to the filing and any information provided
in
response to this comment, we may have additional comments.

Schedule 13D Filed October 29, 2024
General

1.     We note that the event reported as requiring the filing of the Schedule
13D was April
       2, 2024. Rule 13d-1(a) of Regulation 13D-G requires the filing of a
Schedule 13D
       within five business days after the date beneficial ownership of more
than five percent
       of a class of equity securities specified in Rule 13d-1(i)(1) was
acquired. Based on the
       April 2, 2024 event date, the Schedule 13D submitted on October 29, 2024
was not
       timely filed. Please advise us why the Schedule 13D was not filed within
the required
       five business days after the date of the acquisition.
        We remind you that the filing persons are responsible for the accuracy
and adequacy
of their disclosures, notwithstanding any review, comments, action or absence
of action by
the staff.
 January 23, 2025
Page 2

      Please direct any questions to Brian Soares at 202-551-3690 or Nicholas
Panos at
202-551-3266.



                                                         Sincerely,

                                                         Division of
Corporation Finance
                                                         Office of Mergers &
Acquisitions