August 14, 2019

Paula Ragan
Chief Executive Officer
X4 Pharmaceuticals, Inc.
955 Massachusetts Avenue, 4th Floor
Cambridge, MA 02139

       Re: X4 Pharmaceuticals, Inc.
           Registration Statement on Form S-3
           Filed August 9, 2019
           File No. 333-233161

Dear Dr. Ragan:

       We have limited our review of your registration statement to those
issues we have
addressed in our 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
response.

       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-3

Description of Capital Stock
Anti-Takeover Provisions
Exclusive Forum Selection, page 15

1.     We note that your forum selection provision identifies the Court of
Chancery of the State
       of Delaware as the exclusive forum for certain litigation, including any
"derivative
       action." Please disclose whether this provision applies to actions
arising under the
       Securities Act or Exchange Act. In that regard, we note that Section 27
of the Exchange
       Act creates exclusive federal jurisdiction over all suits brought to
enforce any duty or
       liability created by the Exchange Act or the rules and regulations
thereunder, and Section
       22 of the Securities Act creates concurrent jurisdiction for federal and
state courts over all
       suits brought to enforce any duty or liability created by the Securities
Act or the rules and
 Paula Ragan
X4 Pharmaceuticals, Inc.
August 14, 2019
Page 2
         regulations thereunder. If the provision applies to Securities Act
claims, please also revise
         your prospectus to state that there is uncertainty as to whether a
court would enforce such
         provision and that investors cannot waive compliance with the federal
securities laws and
         the rules and regulations thereunder. If this provision does not apply
to actions arising
         under the Securities Act or Exchange Act, please also tell us how you
will inform
         investors in future filings that the provision does not apply to any
actions arising under the
         Securities Act or Exchange Act.
        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
statement.

        Please contact Sonia Bednarowski at 202-551-3666 or Justin Dobbie,
Legal Branch
Chief, at 202-551-3469 with any questions.



FirstName LastNamePaula Ragan                                   Sincerely,
Comapany NameX4 Pharmaceuticals, Inc.
                                                                Division of
Corporation Finance
August 14, 2019 Page 2                                          Office of
Healthcare & Insurance
FirstName LastName