United States securities and exchange commission logo April 28, 2022 Paula Ragan, Ph.D. Chief Executive Officer X4 Pharmaceuticals, Inc 61 North Beacon Street, 4th Floor Boston, Massachusetts 02134 Re: X4 Pharmaceuticals, Inc Registration Statement on Form S-3 Filed April 22, 2022 File No. 333-264459 Dear Dr. Ragan: This is to advise you that we have not reviewed and will not review your registration statement. Please refer to Rules 460 and 461 regarding requests for acceleration. 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. Please contact Tyler Howes at 202-551-3370 with any questions. Sincerely, Division of Corporation Finance Office of Life Sciences cc: Courtney Thorne, Esq.