UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

SCHEDULE 14A INFORMATION

Proxy Statement Pursuant to Section 14(a) of the

Securities Exchange Act of 1934

(Amendment No.)

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Check the appropriate box:

X4 Pharmaceuticals, Inc.

(Name of Registrant as Specified In Its Charter)

(Name of Person(s) Filing Proxy Statement if other thanOther Than the Registrant)

Payment of Filing Fee (Check the appropriate box):

X4 Pharmaceuticals, Inc.PHARMACEUTICALS, INC.

955 Massachusetts Avenue, 4th4^th Floor

Cambridge, MAMassachusetts 02139

May 22, 2019NOTICE OF ANNUAL MEETING OF STOCKHOLDERS

To Our Stockholders:Be Held On June 10, 2020

Dear Stockholder:

You are cordially invited to attend the 2019 annual meetingAnnual Meeting of stockholdersStockholders of X4 Pharmaceuticals, Inc. to, a Delaware corporation (the “Company”). The Annual Meeting will be held on Wednesday, June 10, 2020 at 10:11:00 a.m. EDT on Monday, June 17, 2019local time at the offices of Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C., One Financial Center,Cooley LLP, 500 Boylston Street, 14^th Floor, Boston, Massachusetts 02111.02116 for the following purposes:

Details regarding1.     To elect the meeting,Board’s nominees, Gary J. Bridger, Ph.D. and Murray Stewart, M.D., to the Board of Directors to hold office until the 2023 Annual Meeting of Stockholders.

2.     To approve the Company’s Amended and Restated 2017 Equity Incentive Plan.

3.     To approve an amendment to the Company’s Certificate of Incorporation to increase the authorized number of shares of common stock from 33,333,333 shares to 125,000,000 shares.

4.     To ratify the selection by the Audit Committee of the Board of Directors of PricewaterhouseCoopers LLP as the independent registered public accounting firm of the Company for its fiscal year ending December 31, 2020.

5.     To conduct any other business to be conductedproperly brought before the Annual Meeting.

These items of business are more fully described in the Proxy Statement accompanying this Notice.

The record date for the Annual Meeting is April 15, 2020. Only stockholders of record at the meeting,close of business on that date may vote at the Annual Meeting or any adjournment or postponement thereof.

The Company is closely monitoring developments related to the coronavirus pandemic(COVID-19). It could become necessary to change the date, time, location and/or means of holding the Annual Meeting (including by means of remote communication). If such a change is made, the Company will announce the change in advance, and information about X4 Pharmaceuticals, Inc. thatdetails on how to participate will be issued by press release, posted on the Company’s website and filed as additional proxy materials with the Securities and Exchange Commission.

Cambridge, Massachusetts

April     , 2020

TABLE OF CONTENTS

i

X4 PHARMACEUTICALS, INC.

955 Massachusetts Avenue, 4^th Floor

Cambridge, Massachusetts 02139

PROXY STATEMENT

FOR THE 2020 ANNUAL MEETING OF SHAREHOLDERS

June 10, 2020

QUESTIONS AND ANSWERS ABOUT THESE PROXY MATERIALS AND VOTING

Why did I receivea notice regarding the annual meeting, one personavailability of proxy materials on the Internet?

Pursuant to rules adopted by the Securities and Exchange Commission (the “SEC”), we have elected to provide access to our proxy materials over the Internet. Accordingly, we have sent you a Notice of Internet Availability of Proxy Materials (the “Notice”) because the Board of Directors (the “Board”) of X4 Pharmaceuticals, Inc. (sometimes referred to as the “Company” or “X4”) is soliciting your proxy to vote at the 2020 Annual Meeting of Stockholders (the “Annual Meeting”), including at any adjournments or postponements of the Annual Meeting. All stockholders will have the ability to access the proxy materials on the website referred to in the Notice or request to receive a printed set of the proxy materials. Instructions on how to access the proxy materials over the internet or to request a printed copy may be found in the Notice.

We intend to mail the Notice on or about April    , 2020 to all stockholders of record entitled to vote at the Annual Meeting.

Will I receive any other proxy materials by mail?

We may send you a proxy card, along with a second Notice, on or after May    , 2020.

How do I attend the Annual Meeting?

The Annual Meeting will be electedheld on Wednesday, June 10, 2020 at 11:00 a.m. local time at the offices Cooley LLP, 500 Boylston Street, 14^th Floor, Boston, Massachusetts 02116. Directions to the Annual Meeting may be found at http://www.proxypush.com/XFOR.Information on how to vote in person at the Annual Meeting is discussed below.

Please note that while we intend to hold the Annual Meeting in person at the location set forth above, we are sensitive to the public health and travel concerns that our management, directors and stockholders may have regarding the evolvingCOVID-19 pandemic and the related protocols that federal, state and local governments may impose. As a result, we may decide to hold the Annual Meeting in a different location or solely by means of remote communication (i.e., a virtual-only meeting). Please retain the control numbers from your proxy card so that you can access the Annual Meeting if it is converted to a virtual-only meeting. Any such change will be announced via a press release that will be filed on our website and as additional soliciting material with the SEC as soon as reasonably practicable before the Annual Meeting.

Who can vote at the Annual Meeting?

Only stockholders of record at the close of business on April 15, 2020 will be entitled to vote at the Annual Meeting. On this record date, there were 16,142,275 shares of common stock outstanding and entitled to vote. A list of stockholders entitled to vote at the Annual Meeting will be available for examination during normal business hours by any stockholder for any purpose germane to the meeting for ten days prior to the Annual Meeting, and during the Annual Meeting such list will be available for examination.

Stockholder of Record: Shares Registered in Your Name

If on April 15, 2020, your shares were registered directly in your name with our transfer agent, Computershare Trust Company, N.A., then you are a stockholder of record. As a stockholder of record, you may vote in person at the Annual Meeting or vote by proxy. Whether or not you plan to attend the Annual Meeting, we urge you to vote by proxy electronically through the Internet, over the telephone or by completing and returning a printed proxy card that you may request or that we may elect to deliver at a later time to ensure your vote is counted.

Beneficial Owner: Shares Registered in the Name of a Broker or Bank

If on April 15, 2020,your shares were held not in your name but rather in an account at a brokerage firm, bank or other similar organization, then you are the beneficial owner of shares held in “street name” and the Notice isbeing forwarded to you by that organization. The organization holding your account is considered to be the stockholder of record for purposes of voting at the Annual Meeting. As a beneficial owner, you have the right to direct your broker, bank or other agent regarding how to vote the shares in your account. You are also invited to attend the Annual Meeting. However, since you are not the stockholder of record, you may not vote your shares in person at the Annual Meeting unless you request and obtain a valid proxy from your broker, bank or other agent.

What am I voting on?

There are four matters scheduled for a vote:

What if another matter is properly brought before the Annual Meeting?

The Board knows of directors recommends the approval of each of these proposals. Suchno other businessmatters that will be transacted as maypresented for consideration at the Annual Meeting. If any other matters are properly comebrought before the annual meeting.

We hope you will be able to attend the annual meeting. Whether you plan to attend the annual meeting, or not, it is important that you cast yourthe intention of the persons named in the accompanying proxy to vote either in person or by proxy. You may vote over the Internet as well as by telephone or by mail. When you have finished reading the proxy statement, you are urged to voteon those matters in accordance with their best judgment.

How do I vote?

You may either vote “FOR” all the instructions set forthnominees to the Board or you may “WITHHOLD” your vote for any nominee you specify. For each of the other matters to be voted on, you may vote “FOR” or “AGAINST” or abstain from voting.

The procedures for voting are fairly simple:

Stockholder of Record: Shares Registered in Your Name

If you are a stockholder of record, you may vote in person at the proxy statement. We encourage you toAnnual Meeting, vote by proxy sothrough the Internet, vote by proxy over the telephone, or vote by proxy using a proxy card that your shares will be represented and voted at the meeting, whetheryou may request or not you can attend.

Thank you for your continued support of X4 Pharmaceuticals, Inc. We look forward to seeing you at the annual meeting.

Sincerely,

Paula Ragan, Ph.D.

President, Chief Executive Officer and Secretarythat we

X4 Pharmaceuticals, Inc.

955 Massachusetts Avenue, 4th Floor

Cambridge, MA 02139

May 22, 2019

NOTICE OF 2019 ANNUAL MEETING OF STOCKHOLDERS

PURPOSES:

WHO MAY VOTE:

You may vote if you were the record owner of X4 Pharmaceuticals, Inc. common stockelect to deliver at the close of business on April 29, 2019. A list of stockholders of record will be available at the annual meeting and, during the 10 days prior to the annual meeting, at our principal executive offices located at 955 Massachusetts Avenue, 4th Floor, Cambridge, Massachusetts 02139.

All stockholders are cordially invited to attend the annual meeting.a later time. Whether or not you plan to attend the annual meeting,Annual Meeting, we urge you to vote by proxy to ensure your vote is counted. You may still attend the Annual Meeting and submitvote in person even if you have already voted by proxy.

Beneficial Owner: Shares Registered in the Name of Broker or Bank

If you are a beneficial owner of shares registered in the name of your broker, bank or other agent, you should have received a Notice containing voting instructions from that organization rather than from us. Simply follow the voting instructions in the Notice to ensure that your vote is counted. To vote in person at the Annual Meeting, you must obtain a valid proxy from your broker, bank or other agent. Follow the instructions from your broker, bank or other agent included with these proxy materials, or contact that organization to request a proxy form.

How many votes do I have?

On each matter to be voted upon, you have one vote for each share of common stock you own as of April 15, 2020.

If I am a stockholder of record and I do not vote, or if I return a proxy card or otherwise vote without giving specific voting instructions, what happens?

If you are a stockholder of record and do not vote by completing a proxy card, by telephone, through the Internetor in person at the Annual Meeting, your shares will not be voted.

If you return a signed and dated proxy card or mailotherwise vote without marking voting selections, your shares will be voted, as applicable, “FOR” the election of both nominees for director, “FOR” the approval of the Amended and Restated 2017 Equity Incentive Plan, “FOR” the amendment to our Restated Certificate of Incorporation and “FOR” the ratification of the selection of our independent registered public accounting firm. If any other matter is properly presented at the Annual Meeting, your proxyholder (one of the individuals named on your proxy card) will vote your shares using his or her best judgment.

If I am a beneficial owner of shares held in street name and I do not provide my broker or bank with voting instructions, what happens?

If you are a beneficial owner of shares held in street name and you do not instruct your broker, bank or other agent how to vote your shares, your broker, bank or other agent may still be able to vote your shares in its

discretion. In this regard, under the rules of the New York Stock Exchange (“NYSE”), brokers, banks and other securities intermediaries that are subject to NYSE rules may use their discretion to vote your “uninstructed” shares with respect to matters considered to be “routine” under NYSE rules, but not with respect to“non-routine” matters. In this regard, the NYSE has advised us that Proposals 1 and 2 are considered to be“non-routine” under NYSE rules meaning that your broker may not vote your shares on those proposals in the absence of your voting instructions. However, Proposals 3 and 4 are considered to be “routine” matters under NYSE rules, meaning that if you do not return voting instructions to your broker by its deadline, your shares may be voted by your broker in its discretion on Proposals 3 and 4.

If you a beneficial owner of shares held in street name, in order to ensure your shares are voted in the presenceway you would prefer, youmust provide voting instructions to your broker, bank or other agent by the deadline provided in the materials you receive from your broker, bank or other agent.

Who is paying for this proxy solicitation?

We will pay for the entire cost of a quorum.soliciting proxies. In addition to these proxy materials, our directors and employees may also solicit proxies in person, by telephone, or by other means of communication. Directors and employees will not be paid any additional compensation for soliciting proxies.We may also reimburse brokerage firms, banks and other agents for the cost of forwarding proxy materials to beneficial owners.

What does it mean if I receive more than one Notice?

If you receive more than one Notice, your shares may be registered in more than one name or in different accounts. Please follow the voting instructions on each of the Notices to ensure that all of your shares are voted.

Can I change my vote after submitting my proxy?

Stockholder of Record: Shares Registered in Your Name

Yes. You may change orcan revoke your proxy at any time before it is voted at the meeting.

BY ORDER OF OUR BOARD OF DIRECTORS

Paula Ragan, Ph.D.

President, Chief Executive Officer and Secretary

Our bylaws also establish an advance notice procedure if you wish to present a proposal (including a director nomination) before an annual meeting of stockholders but you are not requesting that your proposal or nomination be included in next year’s proxy materials. To be timely for our 2021 Annual Meeting of Stockholders, our Corporate Secretary must receive the written notice at our principal executive offices not later than the close of business on March 12, 2021 nor earlier than the close of business on February 10, 2021. However, if we hold our 2021 Annual Meeting of Stockholders more than 30 days before or after June 10, 2021 (theone-year anniversary date of the 2020 Annual Meeting of Stockholders), then timely notice of a stockholder proposal that is not intended to be included in our proxy statement must be received not earlier than the close of business on the 120th day prior to the 2021 Annual Meeting of Stockholders and not later than the close of business on the later of (a) the 90th day prior to the 2021 Annual Meeting of Stockholders and (b) the 10th day following the day on which the one of the following first occurs: (i) mailing of notice of the date of the 2021 Annual Meeting of Stockholders and (ii) the first public announcement of the date of the 2021 Annual Meeting of Stockholders. You are also advised to review our bylaws, which contain additional requirements about advance notice of stockholder proposals and director nominations.

How are votes counted?

Votes will be counted by the inspector of election appointed for the Annual Meeting, who will separately count, for the proposal to elect directors, votes “For,” “Withhold” and brokernon-vote, and, with respect to the other proposals, votes “For” and “Against,” abstentions and, if applicable, brokernon-votes. Abstentions will be counted towards the vote total for each of Proposals 2, 3 and 4, and will have stock certificates, theythe same effect as “Against” votes. Brokernon-votes on Proposals 1 and 2 will have no effect and will not be counted if you do nottowards the vote as describedtotal for any of those proposals.

What are “brokernon-votes”?

As discussed above, under “How Do I Vote?” If yourwhen a beneficial owner of shares are held in street name and you dodoes not providegive voting instructions to thehis or her broker, bank broker or other nomineesecurities intermediary holding his or her shares as to how to vote on matters deemed to be“non-routine” under NYSE rules, the broker, bank or other such agent cannot vote the shares. Theseun-voted shares are counted as “brokernon-votes.” The NYSE has advised us that holdsProposals 1 and 2 are considered to be“non-routine” under NYSE rules and we therefore expect brokernon-votes to exist in connection with those proposals.

As a reminder, if you are a beneficial owner of shares held in street name, in order to ensure your shares as described above,are voted in the bank, broker or other nominee that holds your shares has the authority to vote your unvoted shares only on the

How many votes are needed to approve each proposal?

The following table summarizes the minimum vote your shares on a particular matter because it has not received instructions from youneeded to approve each proposal and does not have discretionary voting authority on that matter or because yourthe effect of abstentions and broker chooses not to vote on a matter for which it does have discretionary voting authority.

What Vote Is Required To Approve Each Proposal and How Are Votes Counted?non-votes.

Is Voting Confidential?

What is the quorum requirement?

We will keep all the proxies, ballots and voting tabulations private. We only let our InspectorA quorum of Election, Computershare Trust Company, N.A.,examine these documents. Management will not know how you voted on a specific proposal unless itstockholders is necessary to meet legal requirements. Wehold a valid meeting. A quorum will however, forwardbe present if stockholdersholdingat least a majority in voting power of the outstanding shares entitled to management any written commentsvote are present at the Annual Meeting in person or represented by proxy. On the record date, there were 16,142,275 sharesoutstanding and entitled to vote.Thus, the holders of8,071,138shares must be present in person or represented by proxy at the Annual Meeting to have a quorum.

Your shares will be counted towards the quorum only if you makesubmit a valid proxy (or one is submitted on your behalf by your broker, bank or other nominee) or if you vote in person at the Annual Meeting. Abstentions and brokernon-votes will be counted towards the quorum requirement. If there is no quorum, the holders of a majority of shares present at the Annual Meeting in person or represented by proxy card or that you otherwise provide.may adjourn the Annual Meeting to another date.

Where CanHow can I Findfind out the Voting Resultsresults of the voting at the Annual Meeting?

The preliminaryPreliminary voting results will be announced at the annual meeting, and weAnnual Meeting. In addition, final voting results will publish preliminary results, or final results if available,be published in a Current Reportcurrent report on Form8-K that we expect to file within four business days after the Annual

Meeting. If final voting results are not available to us in time to file aForm 8-K within four business days ofafter the annual meeting. If final results are unavailable at the timeAnnual Meeting, we intend to file the aForm8-K, then we will file an amended Current Report on Form8-K to disclose the final votingpublish preliminary results and, within four business days after the final voting results are known.known to us, file an additionalForm 8-K to publish the final results.

What AreHow does the Costs of Soliciting These Proxies?

We will pay all of the costs of soliciting these proxies. Our directors and employees may solicit proxies in person or by telephone, fax or email. We will pay these employees and directors no additional compensation for

PROPOSAL 1

ELECTIONOF DIRECTORS

Our Board of Directors is divided into three classes. Each class consists, as nearly as possible, ofone-third of the total number of directors, and priceseach class has a three-year term. Vacancies on the Board may be filled only by persons elected by a majority of the remaining directors. A director elected by the Board to fill a vacancy in a class, including vacancies created by an increase in the number of directors, shall serve for the remainder of the full term of that class and until the director’s successor is duly elected and qualified.

The Board of Directors presently has seven members. There are two directors in the class whose term of office expires at our Annual Meeting: Gary J. Bridger, Ph.D. was appointed to our Board in connection with the Merger in March 2019, and Murray Stewart, M.D. was appointed to our Board in March 2019 to fill a vacancy on the Board. If elected at the Annual Meeting, each of these nominees would serve until the 2023 annual meeting and until his successor has been duly elected and qualified, or, if sooner, until the director’s death, resignation or removal. It is our policy that all directors and nominees for director make every effort to attend the Annual Meeting. All of the then-serving directorsattended the 2019 annual meeting of stockholders.

Vote Required

Directors are elected by a plurality of the votes of the holders of shares present in person or represented by proxy and entitled to vote on the election of directors. Accordingly, the two nominees receiving the highest number of affirmative votes will be elected.Shares represented by executed proxies will be voted, if authority to do so is not withheld, for the election of the twonominees named below. If any nominee becomes unavailable for election as a result of an unexpected occurrence, shares that would have been voted for that nominee will instead will be voted for the election of a substitute nominee proposed by us. Each person nominated for election has agreed to serve if elected. We have no reason to believe that any nominee will be unable to serve.

Director Nominees and Continuing Directors

The following is a brief biography and the ages of each nominee and each director whose term will continue after the Annual Meeting, and a discussion of the specific experience, qualifications, attributes or skills of each nominee that led the Nominating and Corporate Governance Committee to recommend that person as a nominee for director, as of the date of this Proxy Statement.

There are no family relationships between or among any of our directors or nominees. There is no arrangement or understanding between any of our directors or nominees and any other person or persons pursuant to which he or she is to be selected as a director or nominee.

There are no legal proceedings to which any of our directors is a party adverse to us or any of our subsidiaries or in which any such person has a material interest adverse to us or any of our subsidiaries.

Nominees for Election as a Class III Director for a Three-Year Term Expiring at the 2023 Annual Meeting of Stockholders

Gary J. Bridger, Ph.D., age 57, became a member of the board of directors of X4 Pharmaceuticals, Inc., a private company, in October 2018 and continues to serve as a member of our Board following the completion of the Merger in March 2019. From February 2015 to December 2017, Dr. Bridger served as a consultant to Xenon Pharmaceuticals Inc., a biopharmaceutical company, where he previously served as the Executive Vice President of Research and Development from January 2013 to February 2015. From October 2013 to October 2015, Dr. Bridger served as a Managing Director at Five Corners Capital Inc. From June 2010 to June 2012, Dr. Bridger served as a venture partner at Venture West Capital Management, a venture capital firm. From November 2006 to December 2007, Dr. Bridger served as Senior Vice President of Research and Development of Genzyme Corporation, a biotechnology company, which was acquired by Sanofi, S.A. In June 1996, Dr. Bridger

co-founded AnorMED Inc., a biopharmaceutical company, and was its Vice President of Research and Development and Chief Scientific Officer from 2000 until its acquisition by Genzyme in November 2006. Dr. Bridger has served on the board of directors of Aquinox Pharmaceuticals, Inc., a pharmaceutical company, since 2013, has served on the board of directors of Liminal BioSciences Inc., a biotechnology company, since May 2019, and previously served on the board of directors of Alder BioPharmaceuticals, Inc., a biopharmaceutical company, from 2013 to 2016. Dr. Bridger serves on the scientific advisory board of Alectos Therapeutics Inc., a biopharmaceutical company. Dr. Bridger holds a Ph.D. in Organic Chemistry from the University of Manchester Institute of Science and Technology. We believe that Dr. Bridger’s experience as an executive officer or director of several public and private life sciences companies provides him with the qualifications and skills to serve on our Board.

Murray W. Stewart, M.D., age 59, has served as a member of our Board since March 2019. Dr. Stewart has served as Chief Medical Officer of Rhythm Pharmaceuticals, Inc., a biopharmaceutical company focused on developing and commercializing therapies for the treatment of rare genetic disorders of obesity, since October 2018. He previously served as Head of Research and Development for Novelion Therapeutics Inc., a biopharmaceutical company focused on developing new standards of care for individuals living with rare disease, from November 2017 to October 2018. Prior to that, Dr. Stewart held various positions of increasing responsibility at GlaxoSmithKline, including serving as Chief Medical Officer from April 2014 to November 2017, with global responsibility for patient well-being across the vaccine, pharmaceutical, and consumer business units. He also held multiple research and development leadership roles, including Chief Medical Officer for the pharmaceutical business, Clinical Head of the Biopharma Unit, and Therapy Area Head for metabolic and cardiovascular diseases. Prior to his tenure to joining GlaxoSmithKline, he worked as a consultant physician and honorary senior lecturer at the Diabetes Center in Newcastle upon Tyne in the United Kingdom. Dr. Stewart holds an M.D. from Southampton Medical School and is a Fellow of the Royal College of Physicians. We believe that Dr. Stewart’s extensive biopharmaceutical leadership experience, including experience in research, clinical development and regulatory strategy, provide him with the qualifications and skills to serve on our Board.

THE BOARD OF DIRECTORS RECOMMENDS

A VOTE IN FAVOR OF EACH NAMED NOMINEE.

Class II Directors Continuing in Office Until the 2022 Annual Meeting of Stockholders

William E. Aliski, MPA, age 72, has served as a member of our Board since September 2019. Since 2011, Mr. Aliski has served as an independent consultant to life sciences companies focused on orphan diseases. Prior to 2011, he served as Chief Commercial Officer for FoldRX Pharmaceuticals, an early stage company purchased by Pfizer in 2011, served as General Manager for Biomarin Europe, a biopharmaceutical company focused on rare diseases with genetic causes, and held executive level positions at Transkaryotic Therapies (TKT)/Shire and Genzyme Corp., both biopharmaceutical companies. Currently, Mr. Aliski serves as a member of the boards of directors of Ultragenyx Pharmaceutical Inc. and Applied Genetic Therapies Corporation. Mr. Aliski holds a M.P.A. from Harvard University and a Master of Social Planning from Boston College. We believe that Mr. Aliski’s experience, in both consulting and operational roles, with life sciences companies focused on orphan diseases, as well as his public company board experience, qualify him to serve on our Board.

René Russo, Pharm.D.,age 45, became a member of the board of directors of Arsanis, Inc. in April 2016 and continues to serve as a member of our Board following the completion of the Merger in March 2019. Since May 2019, Dr. Russo has served as the Chief Executive Officer of Xilio Therapeutics (formerly Akrevia Therapeutics), a privately held biopharmaceutical company focused on developing tumor-selective, immuno-oncology therapeutics. Prior to then, she served as President and Chief Executive Officer of Arsanis, Inc. from April 2016 to November 2018, and as its Chief Development Officer from July 2015 until April 2016. Prior to joining Arsanis, Dr. Russo served in various roles of increasing responsibility overan 11-year period at Cubist

Pharmaceuticals, Inc., a public pharmaceutical development company, focused on the development and commercialization of infectious disease therapeutics, from 2003 until its acquisition by Merck in May 2015, most recently as its Vice President, Global Medical Affairs. From 1999 to 2004, she held roles of increasing responsibility at Bristol-Myers Squibb following a Postdoctoral Fellow in Industrial Pharmacy Infectious Diseases. Prior to joining the biotechnology industry, Dr. Russo held clinical positions at Robert Wood Johnson University Hospital and Princeton Hospital. Dr. Russo received her Pharm.D. and B.S. from Rutgers University. We believe that Dr. Russo’s expertise and experience as an executive at public and private pharmaceutical companies and her expertise in clinical development and commercialization of therapeutics targeting infectious diseases, provide her with the qualifications and skills to serve on our Board.

Class I Directors Continuing in Office Until the 2021 Annual Meeting of Stockholders

David McGirr, M.B.A.,age 65, became a member of the board of directors of Arsanis, Inc. in September 2017 and continues to serve as a member of our Board following the completion of the Merger in March 2019. From March 2013 until June 2014, Mr. McGirr was Senior Advisor to the chief executive officer of Cubist Pharmaceuticals, Inc., a biopharmaceutical company where he also served as Senior Vice President and Chief Financial Officer from November 2002 to March 2013. Prior to joining Cubist in 2002, Mr. McGirr was the President and Chief Operating Officer of hippo inc., a venture-financed internet technology company. From 1996 to 1999, he was the President of GAB Robins North America, Inc., a risk management company, serving also as Chief Executive Officer from 1997 to 1999. Mr. McGirr was a private equity investor from 1995 to 1996. From 1978 to 1995, Mr. McGirr served in various positions within the S.G. Warburg Group, ultimately as Chief Financial Officer, Chief Administrative Officer and Managing Director of S.G. Warburg & Co., Inc., a position held from 1992 to 1995. Mr. McGirr is currently a member of the board of directors of Insmed Incorporated, a publicly traded biopharmaceutical company, where he has served since October 2013, and Rhythm Pharmaceuticals, Inc., a publicly traded biopharmaceutical company, where he has served since November 2015. Mr. McGirr previously served on the board of directors of Roka Bioscience, Inc., a molecular diagnostics company, from December 2013 to January 2018 and Menlo Therapeutics, Inc., a publicly traded biopharmaceutical company, from November 2017 until its merger with Foamix Pharmaceuticals, Ltd., another publicly traded biopharmaceutical company, in March 2019. Mr. McGirr received a B.Sc. in Civil Engineering from the University of Glasgow and received an M.B.A. from The Wharton School at the University of Pennsylvania. We believe that Mr. McGirr’s extensive experience with public and private pharmaceutical companies provide him with the qualifications and skills to serve on our Board.

Paula Ragan, Ph.D., age 50, has served as our President, Chief Executive Officer, Secretary and a member of our board of directors since July 2014. Prior to joining us, from August 2012 to September 2014, Dr. Ragan consulted as Chief Business Officer at Lysosomal Therapeutics Inc., a private biopharmaceutical company, where she led the company’s business development activities. Prior to then, from January 2007 to August 2012, Dr. Ragan held leadership roles in corporate development and operations at Genzyme Corporation, a Sanofi company, where she led strategic partnering efforts for Genzyme’s Rare Disease business and headed the supply chain planning for Genzyme’s flagship commercial products. Other professional roles include business roles at Hydra Biosciences, Oscient Corporation and Celera Corporation. Dr. Ragan received her B.S. from Tufts University and her Ph.D. from Massachusetts Institute of Technology and completed post-doctoral studies at Harvard Medical School. We believe that Dr. Ragan’s perspective and experience as our President and Chief Executive Officer, which provides the Board with historic knowledge, operational expertise and continuity, provides her with the qualifications and skills to serve on our Board.

Michael S. Wyzga, age 65, became a member of the board of directors of X4 Pharmaceuticals, Inc., a private company, in July 2018 and continues to serve as a member of our Board and our Chairman following the completion of the Merger in March 2019. Mr. Wyzga is currently the President of MSW Consulting Inc., a strategic consulting group focused in the life sciences area. From December 2011 until November 2013, Mr. Wyzga served as President and Chief Executive Officer and a member of the board of directors of Radius Health, Inc., a publicly traded biopharmaceutical company. Prior to that, Mr. Wyzga served in various senior

management positions at Genzyme Corporation, a publicly traded global biotechnology company. Mr. Wyzga joined Genzyme in February 1998 and most recently served as Executive Vice President, Finance from May 2003 until November 2011 and as chief financial officer from July 1999 until November 2011. Mr. Wyzga is also a member of the boards of directors of Exact Sciences Corporation, a publicly traded medical technology company, where he has served since February 2015; LogicBio Therapeutics, Inc., a publicly traded genetic medicines company, where he has served since September 2018; GenSight Biologics S.A., a Euronext traded biopharmaceutical company, where he has served since February 2016 and currently holds the position as chairman of the board of directors; and Mereo BioPharma Group plc (formerly Oncomed Pharmaceuticals, Inc.), where has served since October 2013 through the business combination in April 2019. Mr. Wyzga previously served as a member of the boards of directors of Akebia Therapeutics, Inc., a publicly traded biopharmaceutical company, from February 2014 to December 2018, and Idenix Pharmaceuticals, Inc., a publicly traded biotechnology company that was acquired by Merck in August 2014, and as a member of the supervisory board of Prosensa Holding B.V., a publicly traded biopharmaceutical company, from June 2014 until the Prosensa acquisition by BioMarin Falcon B.V. in December 2014. He received an M.B.A. from Providence College and a B.S. from Suffolk University. We believe that Mr. Wyzga’s senior management experience at biopharmaceutical and biotechnology companies, his current and past experience on boards of directors of public companies, and his financial expertise qualify him to serve as a member of our Board.

INFORMATION REGARDING THE BOARD OF DIRECTORS AND CORPORATE GOVERNANCE

INDEPENDENCEOF THE BOARDOF DIRECTORS

As required under the Nasdaq Stock Market (“Nasdaq”)listing standards, a majority of the members of a listed company’s Board of Directors must qualify as “independent,” as affirmatively determined by the Board of Directors. The Board consults with its counsel to ensure that the Board’s determinations are consistent with relevant securities and other laws and regulations regarding the definition of “independent,” including those set forth in pertinent listing standards of Nasdaq, as in effect from time to time.

Consistent with these considerations, after review of all relevant identified transactions or relationships between each director, or any of his or her family members, and the Company, its senior management and its independent auditors, the Board has affirmatively determined that the following five directors are independent directors within the meaning of the applicable Nasdaq listing standards: Mr. Aliski, Dr. Bridger, Mr. McGirr, Dr. Stewart and Mr. Wyzga. In making this determination, the Board found that none of these directors or nominees for director had a material or other disqualifying relationship with the Company. Dr. Ragan is employed by us and is therefore not independent under Nasdaq listing standards, and Dr. Russo was employed by us during the past three years and is therefore not independent under Nasdaq listing standards.

BOARD LEADERSHIP STRUCTURE

The Board has an independent Chairman, Mr. Wyzga, who has authority, among other things, to call and preside over Board meetings, including meetings of the independent directors, to set meeting agendas and to determine materials to be distributed to the Board. Accordingly, the Chairman has substantial ability to shape the work of the Board. We believe that separation of the positions of Chairman and Chief Executive Officer reinforces the independence of the Board in its oversight of our business and affairs. Our Board also recognizes the time, effort and energy that our Chief Executive Officer must devote to her position in the current business environment, as well as the commitment required to serve as our Chairman, particularly as the board of directors’ oversight responsibilities continue to grow. In addition, we believe that having an independent Chairman creates an environment that is more conducive to objective evaluation and oversight of management’s performance, increasing management accountability and improving the ability of the Board to monitor whether management’s actions are in the best interests of our Company and our stockholders. As a result, we believe that having an independent Chairman can enhance the effectiveness of the Board as a whole.

ROLEOFTHE BOARDIN RISK OVERSIGHT

One of the Board’s key functions is informed oversight of our risk management process. The Board does not have a standing risk management committee, but rather administers this oversight function directly through the Board as a whole, as well as through various Board standing committees that address risks inherent in their respective areas of oversight. For example, in determining whether and under what circumstances we will engage in financing transactions or enter into licensing, collaboration or similar arrangements, the Board is involved in our management of risks related to our financial condition or of the risks inherent in drug development and commercialization. As part of its oversight, our Board receives reports by each committee chair regarding the committee’s considerations and actions. In particular, the Audit Committee is responsible for discussing the adequacy of our risk management activities with management and our independent registered public accounting firm. The Audit Committee’s primary emphasis is managing our financial risk exposures, including our internal control over financial reporting. The Audit Committee also monitors compliance with legal and regulatory requirements and considers and approves or disapproves any related person transactions. In addition, the Compensation Committee is responsible for considering whether our compensation programs and practices are reasonably likely to have a material adverse effect on us.

At each of its meetings, the Board receives business updates from various members of management. These updates may identify matters that have emerged within that member of management’s scope of responsibility that

involve operational, financial, legal or regulatory risks and, in these cases, the Board provides guidance to management. Our Board believes that full and open communication between management and the Board is essential for effective risk management and oversight.

In carrying out their risk oversight functions, the Board of Directors and its committees routinely request and review management updates, reports from the independent auditors and legal and regulatory advice from outside experts, as appropriate, to assist in discerning and managing important risks that may be faced by us. The Board of Directors is committed to continuing to ensure and evolve its risk oversight practices as appropriate given the stage of our evolution as a late-stage clinical biopharmaceutical company and the fast-paced changes of the life sciences industry. Regarding the COVID-19 pandemic, our management is meeting regularly to address concerns of our employees and business, as well as updating and communicating with the full Board regularly. The full Board has oversight and has been engaged concerning the monitoring and identification of risks to our Company, and actions we are taking to mitigate risks related to this pandemic.

MEETINGSOF THE BOARDOF DIRECTORS

The Board of Directors met seventimes during the last fiscal year. Each of our incumbent Board members, other than Dr. Russo, attended 75% or more of the aggregate number of meetings of the Board and of the committees on which he or sheserved, held during the portion of the last fiscal year for which he or shewas a director or committee member.

INFORMATION REGARDING COMMITTEESOFTHE BOARDOF DIRECTORS

The Board has three standing committees:an Audit Committee, a Compensation Committee and a Nominating and Corporate Governance Committee. Each of the committees has authority to engage legal counsel or other experts or consultants, as it deems appropriate to carry out its responsibilities. The composition and function of each of these committees are described below.

Audit Committee

The Audit Committee is currently composed of threemembers: Mr. McGirr (chair), Dr. Stewart and Mr. Wyzga. The Audit Committee met sixtimes during 2019. The Board has adopted a written Audit Committee charter that is available to stockholders on our website at http://investors.x4pharma.com/.

The Audit Committee established by the Board in accordance with Section 3(a)(58)(A) of the Securities Exchange Act of 1934, as amended (the “Exchange Act”), to oversee our corporate accounting and financial reporting processes and audits of our financial statements. For this purpose, the Audit Committee performs several functions. The Audit Committee:

The Board of Directors reviews the Nasdaq listing standards definition of independence for Audit Committee members on an annual basis and has determined that all members of the Company’s Audit Committee are independent (as independence is currently defined in Rule 5605(c)(2)(A)(i) and (ii) of the Nasdaq listing standards).

The Board of Directors has also determined that each of Mr. McGirr and Mr. Wyzga qualifies as an “audit committee financial expert,” as defined in applicable SEC rules.

Report of the Audit Committee of the Board of Directors

The material in this report is not “soliciting material,” is not deemed “filed” with the SEC and is not to be incorporated by reference in any filing of the Company under the Securities Act of 1933, as amended (the “Securities Act”) or the Exchange Act, whether made before or after the date hereof and irrespective of any general incorporation language in any such filing.

The Audit Committee has reviewed and discussed the audited financial statements for the fiscal year ended December 31, 2019 with management of the Company. The Audit Committee has discussed with the independent registered public accounting firm the matters required to be discussed by the applicable requirements of the Public Company Accounting Oversight Board (“PCAOB”) and the SEC. The Audit Committee has also received the written disclosures and the letter from the independent registered public accounting firm required by applicable requirements of the PCAOB regarding the independent accountants’ communications with the Audit Committee concerning independence, and has discussed with the independent registered public accounting firm the accounting firm’s independence. Based on the foregoing, the Audit Committee has recommended to the Board of Directors that the audited financial statements be included in the Company’s Annual Report onForm 10-K for the fiscal year ended December 31, 2019.

Respectfully submitted,

David McGirr, M.B.A (Chairman)

Murray W. Stewart, M.D.

Michael S. Wyzga

Compensation Committee

The Compensation Committee is currently composed of threedirectors: Dr. Stewart (chair), Mr. Aliski and Dr. Bridger. All members of our Compensation Committee are independent (as independence is currently defined in Rule 5605(d)(2) of the Nasdaq Listing Rules.) The Compensation Committee met four times during 2019. The Board has adopted a written Compensation Committee charter that is available to stockholders on our website at http://investors.x4pharma.com/.

The Compensation Committee acts on behalf of the Board to review, approve or recommend for approval by the Board and oversee our compensation strategy, policies, plans and programs, including:

Compensation Committee Processes and Procedure

In setting base salaries and bonuses and granting equity incentive awards for our executive officers, our Compensation Committee considers compensation for comparable positions in the market, the historical compensation levels of our executives, individual and corporate performance as compared to our expectations and objectives, our desire to motivate our employees to achieve short- and long-term results that are in the best interests of our stockholders, and a long-term commitment to our company. As part of this process, Dr. Ragan, as our president and chief executive officer, prepares performance evaluations for the other executive officers and recommends annual salary increases, annual stock option awards and cash bonuses to the Compensation Committee. The Compensation Committee conducts a performance evaluation of Dr. Ragan. As necessary, the Compensation Committee consults with the Board as to the achievement of corporate objectives that drive contingent compensation awards.

We have utilized a compensation consultant, Arnosti Consulting, with expertise in the local biotechnology market to help assess executive and employee compensation and to help inform our compensation strategy, and we expect to engage a compensation consultant in the future.

Nominating and Corporate Governance Committee

The Nominating and Corporate Governance Committeeis current composed of three directors: Dr. Bridger (chair), Mr. McGirr, and Mr. Wyzga. All members of the Nominating and Corporate Governance Committee are independent (as independence is currently defined in Rule 5605(a)(2) of the Nasdaq Listing Rules). The Nominating and Corporate Governance Committee met two times during 2019. The Board has adopted a written Nominating and Corporate Governance Committee charter that is available to our stockholders on our website at http://investors.x4pharma.com/.

The Nominating and Corporate Governance Committee of the Board is responsible for identifying, reviewing and evaluating candidates to serve as directors of the Company (consistent with criteria approved by the Board), reviewing and evaluating incumbent directors, recommending to the Boardcandidates for election to the Board, making recommendations to the Board regarding the membership of the committees of the Board (including with respect to making changes or rotation of members, the creation of additional Board committees, or changes in Committee charters), overseeing an annual self-assessment of the Board, overseeing succession planning for senior executives of the Company, and developing and recommending to the Board a set of corporate governance principles for the Company.

The Nominating and Corporate Governance Committee believes that candidates for director should have certain minimum qualifications, including a reputation for integrity, honesty and adherence to high ethical standards; demonstrated business acumen, experience and the ability to exercise sound judgements in matters that relate to the current and long-term objectives of the Company and a willingness and ability to contribute positively to the decision-making process of the Company; a commitment to understand the Company and its industry and to regularly attend and participate in meetings of the Board and its committees; the interest and ability to understand the sometimes conflicting interests of the various constituencies of the Company, which include stockholders, employees, customers, governmental units, creditors and the general public, and to act in the interests of all stockholders; and the ability to serve for at least three years before reaching the age of 75. The Nominating and Corporate Governance Committee also believes that candidates for director should not have, nor appear to have,

a conflict of interest that would impair the candidate’s ability to represent the interests of all the Company’s stockholders and to fulfill the responsibilities of a director. However, the Nominating and Corporate Governance Committee retains the right to modify these qualifications from time to time. Candidates for director nominees are reviewed in the context of the current composition of the Board, the operating requirements of the Company and the long-term interests of stockholders, and the Nominating and Corporate Governance Committee has direct input from the Chairman of the Board and the Chief Executive Officer.

Our Nominating and Corporate Governance Committee has not adopted a formal diversity policy in connection with the consideration of director nominations or the selection of nominees but believes that our Board, taken as a whole, should embody a diverse set of skills, experiences and backgrounds. In this regard, the Nominating and Corporate Governance Committee will consider issues of diversity among its members in identifying and considering nominees for director, and strive where appropriate to achieve a diverse balance of backgrounds, perspectives, experience, age, gender, ethnicity and country of citizenship on our board of directors and its committees. The Nominating and Corporate Governance Committee does not make any particular weighting of diversity or any other characteristic in evaluating nominees and directors.

The Nominating and Corporate Governance Committee appreciates the value of thoughtful Board refreshment, and regularly identifies and considers qualities, skills and other director attributes that would enhance the composition of the Board. In the case of incumbent directors whose terms of office are set to expire, the Committee reviews these directors’ overall service to the Company during their terms, including the number of meetings attended, level of participation, quality of performance and any other relationships and transactions that might impair the directors’ independence. The Nominating and Corporate Governance Committee also takes into account the results of the Board’s self-evaluation, conducted annually on a group and individual basis. In the case of new director candidates, the Nominating and Corporate Governance Committee also determines whether the nominee is independent for Nasdaq purposes, which determination is based upon applicable Nasdaq listing standards, applicable SEC rules and regulations and the advice of counsel, if necessary. The Nominating and Corporate Governance Committee then uses its network of contacts to compile a list of potential candidates, but may also engage, if it deems appropriate, a professional search firm. The Nominating and Corporate Governance Committee conducts any appropriate and necessary inquiries into the backgrounds and qualifications of possible candidates after considering the function and needs of the Board. The Nominating and Corporate Governance Committee meets to discuss and consider the candidates’ qualifications and then selects a nominee for recommendation to the Board by majority vote.

The Nominating and Corporate Governance Committee will consider director candidates recommended by stockholders.⁷ The Nominating and Corporate Governance Committee does not intend to alter the manner in which it evaluates candidates, including the minimum criteria set forth above, based on whether or not the candidate was recommended by a stockholder. Stockholders who wish to recommend individuals for consideration by the Nominating and Corporate Governance Committee to become nominees for election to the Board may do so by delivering a written recommendation to the Nominating and Corporate Governance Committee at the following address: c/o X4 Pharmaceuticals, Inc., 955 Massachusetts Avenue, 4^th Floor, Cambridge, Massachusetts 02139 at least 120 days prior to the anniversary date of the mailing of our proxy statement for the last Annual Meeting of Stockholders. Submissions must include the full name of the proposed nominee, a description of the proposed nominee’s business experience for at least the previous five years, complete biographical information, a description of the proposed nominee’s qualifications as a director and a representation that the nominating stockholder is a beneficial or record holder of our common stock and has been a holder for at least one year. Any such submission must be accompanied by the written consent of the proposed nominee to be named as a nominee and to serve as a director if elected.

STOCKHOLDER COMMUNICATIONS WITH THE BOARD OF DIRECTORS

These communications will be reviewed by one or more employees of the Company designated by the Board, who will determine whether the communication should be presented to the Board. Stockholders who wish to communicate with our Board of Directors, or any individual director, may do so by sending written

communications addressed to our Corporate Secretary at 955 Massachusetts Avenue,4th Floor, Cambridge, Massachusetts 02139.

Communications will be distributed to Board of Directors, or to any individual director or directors as appropriate, depending on the facts and circumstances outlined in the communications. Items that are unrelated to the duties and responsibilities of our Board of Directors may be excluded, such as:

In addition, any material that is unduly hostile, threatening, or illegal in nature may be excluded, in which case it will be made available to any outside director upon request.

CODEOF ETHICS

We have adopted aCode of Business Conduct and Ethics that applies to all of our officers, directors and employees. The Code of Business Conduct and Ethics is available on our website at http://investors.x4pharma.com.If the Company makes any substantive amendments to the Code of Business Conduct and Ethics or grants any waiver from a provision of the Code to any executive officer or director, the Company will promptly disclose the nature of the amendment or waiver on its website.

PROPOSAL 2

APPROVALOF THE AMENDEDAND RESTATED 2017 EQUITY INCENTIVE PLAN

The X4 Pharmaceuticals, Inc. 2017 Equity Incentive Plan (the “2017 Plan”) was originally approved by Arsanis’ board of directors and its stockholders prior to the effectiveness of Arsanis’ initial public offering in 2017. On March 25, 2020, our Board approved the amendment and restatement of the 2017 Plan, subject to stockholder approval, including following three amendments:

Our Board did not approve any other changes to the 2017 Plan. We are seeking stockholder approval of the 2017 Plan, as amended and restated (the “Amended and Restated 2017 Plan”).

Our Board approved the Amended and Restated 2017 Plan, subject to stockholder approval, primarily to replenish the shares available to X4 for the grant of equity awards. Our Board believes that the effective use of long-term equity compensation is vital to our ability to achieve strong performance in the future and that our continued growth and success depends, in large part, on our ability to attract, retain and motivate key employees with relevant experience and critical skills, particularly as we grow and in light of the highly-competitive market for employee talent in which we operate.

Our Board has determined that it is in the best interests of X4 and its stockholders to approve this proposal. Our Board has approved the Amended and Restated 2017 Plan, subject to stockholder approval, and recommends that stockholders vote in favor of this proposal at the Annual Meeting. Stockholder approval of this proposal requires the affirmative vote of a majority of the votes cast by the holders of shares present in person or by proxy and entitled to vote at the Annual Meeting.

If our stockholders approve this proposal, the Amended and Restated 2017 Plan will become effective as of the date of stockholder approval. If our stockholders do not approve this proposal, the Amended and Restated 2017 Plan will not become effective and the 2017 Plan will continue to be administered in its current form.

Background

The 2017 Plan was initially adopted by Arsanis’ board of directors and approved by its stockholders prior to the effectiveness of Arsanis’ initial public offering. The 2017 Plan had an initial share reserve, as of the IPO, equal to the sum of (1) 585,994 shares (not reflecting the Reverse Stock Split that occurred in connection with the Merger in 2019, as described below) and (2) such additional number of shares up to 1,368,918 shares (not reflecting the Reverse Stock Split) as is equal to the sum of (a) the number of shares reserved for issuance under our 2010 Special Stock Incentive Plan, as amended (the “2010 Plan”) and our 2011 Stock Incentive Plan (the “2011 Plan” and, together with the 2010 Plan, the “Prior Plans”), and (b) the number of shares subject to awards granted under the Prior Plans which expire, terminate or are otherwise surrendered, canceled, forfeited or repurchased. As mentioned above, the 2017 Plan’s share reserve also includes an evergreen provision that automatically increases

the share reserve on the first day of each year beginning with the year ending December 31, 2018 and continuing until, and including, the year ending December 31, 2027, in an amount equal to the least of (a) 4% of outstanding shares on such date, (b) 1,025,490 (not reflecting the Reverse Stock Split) or (c) the number of shares determined by our Board of Directors. In addition, any shares subject outstanding awards under the 2017 Plan that expire or are otherwise forfeited to, or repurchased by, the Company become available again for future awards under the 2017 Plan. For information on how shares are counted for purposes of the 2017 Plan, please refer to “Authorized Shares” below.

In connection with the6-for-1 Reverse Stock Split that occurred in connection with the Merger in March 2019, the total share reserve was reduced to the sum of (1) 97,665 shares and (2) such additional number of shares up to 228,153 shares as is equal to the sum of (a) the number of shares reserved for issuance the Prior Plans and (b) the number of shares subject to awards granted under the Prior Plans which expire, terminate or are otherwise surrendered, canceled, forfeited or repurchased. The fixed cap in our evergreen provision was reduced from 1,025,490 to 170,915, which represented approximately 1.1 % of our shares outstanding as of December 31, 2019.

As of April 15, 2020, 114,022 shares remain available for grant under the 2017 Plan, excluding the April 2020 grants of awards discussed below, which are subject to stockholder approval of this Proposal 2. The table below includes aggregate information regarding number of shares subject to outstanding equity awards granted under the 2017 Plan, as of April 15, 2020; the number of shares available for future awards under the 2017 Plan, as of April 15, 2020; and the proposed number of shares that would be available for issuance of future awards under the Amended and Restated 2017 Plan if this proposal is approved by our stockholders. The table below does not include either the future effect, in 2021 and beyond, of the evergreen provisions of the 2017 Plan, which would also be modified if stockholders approve this proposal, or the April 2020 grant of awards discussed below, which will be granted if stockholders approve this proposal.

Reasons for Voting for the Proposal

Long-Term Equity is a Key Component of our Compensation Programs

Additional Shares are Required for Future Grants and Retention of Key Employees

Consider Unique Factors to X4

Our Company is Committed to the Effective Utilization of Shares

The 2017 Plan Requires Additional Shares to Meet our Forecasted Needs

We have granted equity awards in the form of RSUs representing 592,248 shares in April 2020 (roughly 3.7% of our outstanding shares as of April 15, 2020), all of which are subject to stockholder approval of this Proposal 2. We anticipate granting additional awards in 2020 to incentivize and retain employees and as our Company grows.

As a result, our Board of Directors, based on the recommendation of the Compensation Committee, concluded that increasing the number of shares available for issuance under our 2017 Plan would provide the Company with the ability to provide market competitive compensation while preserving our cash reserves to help us retain and motivate employees who hold underwater stock options, which are not permitted to be repriced without stockholder approval.

The Board of Directors, based on the recommendation of the Compensation Committee, concluded that fixing the maximum number of shares that may be subject to awards granted in the form of ISOs at 3,146,241 was appropriate in order to allow us to continue granting ISOs following the approval of the Amended and Restated 2017 Plan.

In addition, our Board of Directors, based on the recommendation of the Compensation Committee, concluded that amending the evergreen provision was appropriate due to the constraints imposed by the fixed share limitation in the evergreen provision. As a result, our Board of Directors, based on the recommendation of the Compensation Committee, approved an amendment to the evergreen provision of the 2017 Plan to eliminate its fixed share limitation. As proposed to be amended, the annual evergreen amount will be the lesser of (a) 4% of the shares outstanding on the first day of the year and (b) a number of shares determined by the Board of Directors.

The other material features of the 2017 Plan remain the same as under the terms of the 2017 Plan previously approved by our stockholders. A summary of the Amended and Restated 2017 Plan are described below. The following summary is qualified in its entirety by reference to the Amended and Restated 2017 Plan, a copy of which is set forth as Appendix A.

Material Features of the Amended and Restated 2017 Plan

The material terms of the Amended and Restated 2017 Plan are summarized below. The following description of the Amended and Restated 2017 Plan is a summary only and is qualified in its entirety by reference to the complete text of the Amended and Restated 2017 Plan. Shareholders are urged to read the actual text of the Amended and Restated 2017 Plan in its entirety, a copy of which is attached to this proxy statement asAnnex A.

Administration

The Amended and Restated 2017 Plan will continue to be administered by our Board of Directors. Our Board of Directors has the authority to, among other things, interpret the Amended and Restated 2017 Plan, determine eligibility for and grant awards, determine, modify or waive the terms and conditions of any awards, determine the form of settlement of awards (whether in cash, shares of our common stock or other property), and do all things necessary or appropriate to carry out the purposes of the Amended and Restated 2017 Plan. Our Board of Director’s determinations under the Amended and Restated 2017 Plan are conclusive and binding. Our Board of Directors may delegate its authority under the Amended and Restated 2017 Plan to one or more committees of the Board of Directors or certain officers of the Company and has, in fact, delegated such authority to the Compensation Committee. References in this summary to our Board of Directors refer to our Board of Directors and its applicable delegates, including the Compensation Committee.

Eligibility

Our employees, officers, directors, consultants and advisors are eligible to participate in the Amended and Restated 2017 Plan. As of April 15, 2020, we estimate that approximately 60 employees (excluding officers), two officers, seven directors, and approximately 20 consultants and advisors were eligible to participate. Incentive stock options may be granted under the Amended and Restated 2017 Plan only to our employees, including our executive officers.

Authorized Shares

Subject to adjustment, as described below, and excluding the future effect of the evergreen provision, the maximum number of shares of our common stock that may be delivered in satisfaction of awards under the 2017 Plan as of April 15, 2020 was equal to the sum of (1) 97,665 shares and (2) such additional number of shares up to 228,153 shares as is equal to the sum of (a) the number of shares reserved for issuance under the Prior Plans and (b) the number of shares subject to awards granted under the Prior Plans which expire, terminate or are otherwise surrendered, canceled, forfeited or repurchased. A total of 114,022 shares remained available for grant as of April 15, 2020, not including the April 2020 grants that are subject to stockholder approval of the Amended and Restated 2017 Plan. If this proposal is approved by stockholders, an additional 474,465 shares will become available under the Amended and Restated 2017 Plan, subject to adjustment, as described below, and excluding the future effect of the evergreen provision.

In addition, if this proposal is approved by our stockholders, the number of shares of our common stock available for issuance under the Amended and Restated 2017 Plan will be automatically increased pursuant to the evergreen provision on the first day of each fiscal year until, and including the fiscal year ending December 31, 2027, in an amount equal to the lesser of (1) 4% of outstanding shares as of such date and (2) the number of shares determined by our Board of Directors.

In addition to the 474,465 shares that will be available for future grants if this proposal is approved and the shares that will become available for future grants as a result of the amended evergreen provision, any shares subject to outstanding awards under the 2017 Plan that (1) expire, terminate, or are surrendered or are canceled without having been fully exercised, or are forfeited in whole or in part (including as the result of shares of common stock subject to such award being repurchased by us) or (2) are not issued will again be available for issuance under the Amended and Restated 2017 Plan. Further, if any shares are delivered (by actual delivery, attestation, or net exercise) to us by a participant to (i) purchase shares of common stock upon the exercise of an award or (ii) satisfy tax withholding obligations with respect to awards, then such shares will be added back to the number of shares available for issuance under the Amended and Restated 2017 Plan.

If this proposal is approved by our stockholders, subject to adjustment, as described below, no more than 3,146,241 shares of our common stock may be delivered in satisfaction of ISOs awarded under the Amended and Restated 2017 Plan.

The shares of our common stock to be issued under the Amended and Restated 2017 Plan may be authorized but unissued shares of our common stock or treasury shares. The closing price of our common stock as reported on Nasdaq on April 15, 2020 was $9.49 per share.

Types of Awards

The Amended and Restated 2017 Plan provides for awards of stock options, stock appreciation rights (“SARs”), restricted stock, restricted stock units, and other stock-based awards convertible into or otherwise based on shares of our common stock.

Stock options and SARs.The exercise price of a stock option, and the base value against which a SAR, is to be measured, may not be less than the fair market value (or, in the case of an ISO granted to a ten percent shareholder, 110% of the fair market value) of shares of our common stock on the date of grant. Our Board of Directors will determine the time or times at which stock options or SARs become exercisable and the terms on which such awards remain exercisable, except no stock option or SAR may have a term in excess of ten years.

Restricted stock. A restricted stock award is an award of shares of our common stock subject to forfeiture restrictions. Restricted stock awards are subject to our right to repurchase all or part of such shares at their issue price or other stated or formula price (or to require forfeiture of such shares if issued at no cost) from the

recipient in the event that conditions specified by the Board of Directors in the applicable award are not satisfied prior to the end of the applicable restriction period or periods established by the Board of Directors for such award. Dividend rights may be provided in connection with a restricted stock award. Our Board of Directors will determine the terms and conditions of a restricted stock award, including the conditions for vesting and repurchase (or forfeiture) and the issue price, if any.

Restricted stock units. A restricted stock unit award is an award denominated in shares of our common stock that entitles the participant to receive shares of our common stock or cash measured by the value of the shares of our common stock in the future. The delivery of shares of our common stock or cash under a restricted stock unit may be subject to the satisfaction of performance conditions or other vesting conditions. Dividend equivalent rights may be provided in connection with a restricted stock unit award. Our Board of Directors will determine the terms and conditions of a restricted stock unit award, including the conditions for vesting and repurchase (or forfeiture) and the issue price, if any.

Other stock-based awards. Other stock-based awards are awards that are valued in whole or in part by reference to, or are otherwise based on, shares of our common stock or other property. Our Board of Directors will determine the terms and conditions of other stock-based awards, including any purchase price applicable thereto.

Limitation on Repricing of Stock Options and SARs

Unless such action is approved by the Company’s stockholders, the Company may not: (1) amend any outstanding stock option or SAR granted under the Amended and Restated 2017 Plan to provide an exercise price per share that is lower than the then-current exercise price per share of such outstanding stock option or SAR, (2) cancel any outstanding option or SAR (whether or not granted under the Amended and Restated 2017 Plan) and grant in substitution therefor new awards under the Amended and Restated 2017 Plan (other than substitute awards granted in connection with a merger, consolidation or other acquisition) covering the same or a different number of shares and having an exercise price per share lower than the then-current exercise price per share of the cancelled option or SAR, (3) cancel in exchange for a cash payment any outstanding option or SAR with an exercise price per share above the then-current fair market value of our common stock or (4) take any other action under the Plan that constitutes a “repricing” within the meaning of the rules of Nasdaq.

Vesting; Change in Status

Our Board of Directors has the authority to determine the vesting schedule applicable to each award, and to accelerate the vesting or exercisability of any award. Our Board of Directors also has the authority to determine the effect of a change in employment or other status on an award.

Non-Transferability of Awards

Awards under the Amended and Restated 2017 Equity Plan may not be transferred, other than by will or the laws of descent and distribution, or other than in the case of ISOs, pursuant to a qualified domestic relations order, unless otherwise provided by our Board of Directors.

Certain Transactions; Certain Adjustments

In the event of (1) a merger or consolidation as a result of which all of our common stock is converted into or exchanged for the right to receive cash, securities or other property or is cancelled, (2) any transfer or disposition of all of our common stock, or (3) our dissolution or liquidation (any of the foregoing, a “covered transaction”), our Board of Directors may, with respect to awards (other than restricted stock awards), (a) provide that such awards will be assumed, or substantially equivalent awards will be substituted, by the acquiring or succeeding corporation (or an affiliate thereof), (b) upon written notice to a participant, provide that all of the participant’s unvested awards will be forfeited immediately prior to the consummation of such covered transaction and/or

unexercised awards will terminate immediately prior to the consummation of such covered transaction unless exercised by the participant (to the extent then exercisable) within a specified period following the date of such notice, (c) provide that outstanding awards will become exercisable, realizable or deliverable, or restrictions applicable to an award will lapse, in whole or in part prior to or upon such covered transaction, (d) in the event of a covered transaction under the terms of which holders of our common stock will receive upon consummation thereof a cash payment for each share surrendered in the covered transaction (the “acquisition price”), make or provide for a cash payment to participants with respect to each award held by a participant equal to (i) the number of shares of our common stock subject to the vested portion of the award (after giving effect to any acceleration of vesting that occurs upon or immediately prior to such covered transaction) multiplied by (ii) the excess, if any, of (A) the acquisition price over (B) the exercise, measurement or purchase price of such award and any applicable tax withholdings, in exchange for the termination of such award, (e) provide that, in connection with a liquidation or dissolution of the Company, awards will convert into the right to receive liquidation proceeds (if applicable, net of the exercise, measurement or purchase price thereof and any applicable tax withholdings) and (f) any combination of the foregoing.

Upon the occurrence of a covered transaction other than a liquidation or dissolution of the Company, the repurchase and other rights of the Company with respect to outstanding restricted stock awards will inure to the benefit of the Company’s successor and will, unless the Board of Directors determines otherwise, apply to the cash, securities or other property which our common stock was converted into or exchanged for pursuant to such covered transaction in the same manner and to the same extent as they applied to such restricted stock award. The Board of Directors may provide for termination or deemed satisfaction of such repurchase or other rights under the instrument evidencing any restricted stock award or any other agreement between a participant and the Company, either initially or by amendment. Upon the occurrence of a covered transaction involving the liquidation or dissolution of the Company, except to the extent specifically provided to the contrary in the instrument evidencing any restricted stock award or any other agreement between a participant and the Company, all restrictions and conditions on all restricted stock awards then outstanding will automatically be deemed terminated or satisfied.

In the event of any stock split, reverse stock split, stock dividend, recapitalization, combination of shares, reclassification of shares,spin-off or other similar change in capitalization or event, or any dividend or distribution to holders of our common stock other than an ordinary cash dividend, (1) the number and class of securities available under the Amended and Restated 2017 Plan, (2) the share counting rules of the Amended and Restated 2017 Plan, (3) the number and class of securities and exercise price per share of each outstanding stock option, (4) the share andper-share provisions and the measurement price of each outstanding SAR, (5) the number of shares subject to and the repurchase price per share subject to each outstanding restricted stock and restricted stock unit awards and (6) the share andper-share-related provisions and the purchase price, if any, of each outstanding other stock-based award, will be equitably adjusted by the Company (or substituted awards may be made, if applicable) in the manner determined by the Board of Directors.

Amendment; Termination

Our Board of Directors may amend the Amended and Restated 2017 Plan or outstanding awards, or terminate the Amended and Restated 2017 Plan as to future grants of awards, except that our Board of Directors will not be able to alter the terms of an award if it would affect materially and adversely a participant’s rights under the award without the participant’s consent. Stockholder approval will be required for any amendment to the Amended and Restated 2017 Plan to the extent such approval is required by law, including applicable stock exchange requirements.

Certain U.S. Federal Income Tax Consequences

The following is a summary of the principal U.S. federal income tax consequences to participants and us with respect to awards granted under the Amended and Restated 2017 Plan. This summary is not intended to be

exhaustive and does not discuss the income tax laws of any local, state or foreign jurisdiction in which a participant may reside. The information is based upon current U.S. federal income tax rules and therefore is subject to change when those rules change. Because the tax consequences to any participant may depend on his or her personal circumstances, each participant should consult the participant’s tax adviser regarding the federal, state, local and other tax consequences of the grant or exercise of an award or the disposition of shares acquired under the Amended and Restated 2017 Plan. The Amended and Restated 2017 Plan is not qualified under the provisions of Section 401(a) of the Code and is not subject to any of the provisions of the Employee Retirement Income Security Act of 1974, as amended. Our ability to realize the benefit of any tax deductions described below depends on our generation of taxable income as well as the requirement of reasonableness and the satisfaction of our tax reporting obligations.

Stock Options (other than ISOs)

In general, a participant has no taxable income upon the grant of a stock option that is not intended to be an ISO (an “NSO”) but realizes ordinary income in connection with the exercise of the NSO in an amount equal to the excess (at the time of exercise) of the fair market value of the shares acquired upon exercise over the exercise price. Upon a subsequent sale or exchange of the shares, any recognized gain or loss is treated as a capital gain or loss for which the Company is not entitled to a deduction.

Subject to the requirement of reasonableness, the provisions of Section 162(m) of the Code, and the satisfaction of a tax reporting obligation, we will generally be entitled to a tax deduction equal to the taxable ordinary income realized by the participant.

ISOs

A participant generally is not subject to ordinary income tax upon the grant or exercise of an ISO. If the participant holds a share received upon exercise of an ISO for more than two years from the date the stock option was granted and more than one year from the date the stock option was exercised, which is referred to as the required holding period, the difference, if any, between the amount realized on a sale or other taxable disposition of that share and the participant’s tax basis in that share will be long-term capital gain or loss.

If, however, a participant disposes of a share acquired upon exercise of an ISO before the end of the required holding period, which is referred to as a disqualifying disposition, the participant generally will recognize ordinary income in the year of the disqualifying disposition equal to the excess, if any, of the fair market value of the share on the date of exercise of the stock option over the exercise price. However, if the sales proceeds are less than the fair market value of the share on the date of exercise of the stock option, the amount of ordinary income recognized by the participant will not exceed the gain, if any, realized on the sale. If the amount realized on a disqualifying disposition exceeds the fair market value of the share on the date of exercise of the stock option, that excess will be short-term or long-term capital gain, depending on whether the holding period for the share exceeds one year.

For purposes of the alternative minimum tax, the amount by which the fair market value of a share of stock acquired upon exercise of an ISO exceeds the exercise price of the stock option generally will be an adjustment included in the participant’s alternative minimum taxable income for the year in which the stock option is exercised. If, however, there is a disqualifying disposition of the share in the year in which the stock option is exercised, there will be no adjustment for alternative minimum tax purposes with respect to that share. In computing alternative minimum taxable income, the tax basis of a share acquired upon exercise of an ISO is increased by the amount of the adjustment taken into account with respect to that share for alternative minimum tax purposes in the year the stock option is exercised.

We are not allowed a tax deduction with respect to the grant or exercise of an ISO or the disposition of a share acquired upon exercise of an ISO after the required holding period. If there is a disqualifying disposition of a

share, however, we will generally be entitled to a tax deduction equal to the taxable ordinary income realized by the participant, subject to the requirement of reasonableness, the provisions of Section 162(m) of the Code, and provided that either the employee includes that amount in income or we timely satisfy our reporting requirements with respect to that amount.

SARs

The grant of a SAR does not itself result in taxable income, nor does taxable income result merely because a SAR becomes exercisable. In general, a participant who exercises a SAR for shares of stock or receives payment in cancellation of a SAR will have ordinary income equal to the amount of any cash and the fair market value of any stock received. A corresponding deduction is generally available to the Company, subject to the requirement of reasonableness, the provisions of Section 162(m) of the Code, and the satisfaction of a tax reporting obligation.

Restricted Stock Awards

A participant who is awarded or purchases shares subject to a substantial risk of forfeiture generally does not have income until the risk of forfeiture lapses. When the risk of forfeiture lapses, the participant has ordinary income equal to the excess of the fair market value of the shares at that time over the purchase price, if any, and a corresponding deduction is generally available to the Company, subject to the requirement of reasonableness, the provisions of Section 162(m) of the Code, and the satisfaction of a tax reporting obligation. However, a participant may make an election under Section 83(b) of the Code to be taxed on restricted stock when it is acquired rather than later, when the substantial risk of forfeiture lapses. A participant who makes an effective 83(b) election will realize ordinary income equal to the fair market value of the shares as of the time of acquisition less any price paid for the shares. A corresponding deduction will generally be available to the Company, subject to the requirement of reasonableness, the provisions of Section 162(m) of the Code, and the satisfaction of a tax reporting obligation. If a participant makes an effective 83(b) election, no additional income results by reason of the lapsing of the restrictions.

For purposes of determining capital gain or loss on a sale of shares awarded under the Amended and Restated 2017 Plan, the holding period in the shares begins when the participant recognizes taxable income with respect to the transfer. The participant’s tax basis in the shares equals the amount paid for the shares plus any income realized with respect to the transfer. However, if a participant makes an effective 83(b) election and later forfeits the shares, the tax loss realized as a result of the forfeiture is limited to the excess of what the participant paid for the shares (if anything) over the amount (if any) realized in connection with the forfeiture.

Restricted Stock Units

Generally, the recipient of a restricted stock unit award structured to comply with the requirements of Section 409A of the Code or an exception to Section 409A of the Code will recognize ordinary income at the time the stock is delivered equal to the excess, if any, of the fair market value of the stock received over any amount paid by the recipient in exchange for the stock. To comply with the requirements of Section 409A of the Code, the stock subject to a restricted stock unit award may generally only be delivered upon one of the following events: a fixed calendar date (or dates), separation from service, death, disability or a change in control. If delivery occurs on another date, unless the restricted stock unit award otherwise complies with or qualifies for an exception to the requirements of Section 409A of the Code (including delivery upon achievement of a performance goal), in addition to the tax treatment described above, the recipient will owe an additional 20% federal tax and interest on any taxes owed.

The recipient’s basis for the determination of gain or loss upon the subsequent disposition of shares acquired from a restricted stock unit award will be the amount paid for such shares plus any ordinary income recognized when the stock is delivered.

Subject to the requirement of reasonableness, the provisions of Section 162(m) of the Code, and the satisfaction of a tax reporting obligation, we will generally be entitled to a tax deduction equal to the taxable ordinary income realized by the recipient of the restricted stock unit award.

Section 162(m) Limitations

Section 162(m) of the Code generally disallows a tax deduction to public companies for compensation in excess of $1 million paid to each of the company’s chief executive officer and the three most highly compensated executive officers (other than the chief executive officer and chief financial officer). Pursuant to tax legislation signed into law on December 22, 2017 (the “Tax Act”), for taxable years beginning after December 31, 2017, the Section 162(m) deduction limitation is expanded so that it also applies to compensation in excess of $1 million paid to a public company’s chief financial officer. Historically, compensation that qualified under Section 162(m) as performance-based compensation was exempt from the deduction limitation. However, subject to certain transition rules, the Tax Act eliminated the qualified performance-based compensation exception. As a result, for taxable years beginning after December 31, 2017, all compensation in excess of $1 million paid to each of the executives described above (other than certain grandfathered compensation or compensation paid pursuant to certain equity awards granted before or during a transition period following Arsanis’ IPO) will not be deductible by us.

New Plan Benefits

The following table sets forth certain information regarding future benefits under the Amended and Restated 2017 Plan:

2017 Plan Benefits

The following table presents certain information with respect to awards that have been granted (even if not currently outstanding) under the 2017 Plan as of April 15, 2020:

Equity Compensation Plan Information

The following table provides certain information with respect to all of our equity compensation plans in effect as of December 31, 2019.

Vote Required

The affirmative vote of a majority of the votes cast by the holders of shares present in person or by proxy and entitled to vote at the Annual Meeting will be required to approve Proposal 2.

THE BOARDOF DIRECTORS RECOMMENDS

A VOTEIN FAVOROF PROPOSAL 2.

PROPOSAL 3

APPROVALOF INCREASEIN NUMBEROF AUTHORIZED SHARESOF COMMON STOCK

The Board is requesting stockholder approval of an amendment to our Restated Certificate of Incorporation to increase our authorized number of shares of common stock from 33,333,333 shares to 125,000,000 shares.

The additional common stock to be authorized by adoption of the amendment would have rights identical to our currently outstanding common stock. Adoption of the proposed amendment and issuance of the common stock would not affect the rights of the holders of our currently outstanding common stock, except for effects incidental to increasing the number of shares of our common stock outstanding, such as dilution of the earnings per share and voting rights of current holders of common stock. If the amendment is adopted, it will become effective upon filing of a Certificate of Amendment of our Restated Certificate of Incorporation with the Secretary of State of the State of Delaware.

In addition to the 16,134,495 shares of common stock outstanding on December 31, 2019, the Board has reserved 1,928,948 shares for grants of restricted stock, issuance upon exercise of options and rights and other equity awards granted under our incentive stock and stock purchase plans (not including the any additional shares to be added if the stockholders approval Proposal 2), and up to approximately 13,162,821 shares of common stock which may be issued upon exercise of warrants and prefunded warrants issued in public offerings in April and November 2019.

Although, at present, the Board has no other plans to issue the additional shares of common stock, beyond those to be added to the 2017 Plan if the stockholders approve Proposal 2, the Board desires to have the shares available to provide additional flexibility to use its capital stock for business and financial purposes in the future. The additional shares may be used for various purposes without further stockholder approval. These purposes may include raising capital; providing equity incentives to employees, officers or directors; establishing strategic relationships or collaborations with other companies; issuing shares in connection with a joint venture or other strategic commercial transaction; and expanding our business or pipeline through the acquisition of other businesses, property or other assets.

The additional shares of common stock that would become available for issuance if the proposal were adopted could also be used by us to oppose a hostile takeover attempt or to delay or prevent changes in control or management of our Company. For example, without further stockholder approval, the Board could strategically sell shares of common stock in a private transaction to purchasers who would oppose a takeover or favor the current Board. Although this proxyproposal to increase the authorized common stock has been prompted by business and financial considerations and not by the threat of any hostile takeover attempt (nor is the Board currently aware of any such attempts directed at us), nevertheless, stockholders should be aware that approval of proposal could facilitate future efforts by us to deter or prevent changes in control of our Company, including transactions in which the stockholders might otherwise receive a premium for their shares over then current market prices.

Vote Required

The affirmative vote of the holders of a majority of the outstanding shares of the common stock will be required to approve this amendment to our Restated Certificate of Incorporation.

THE BOARDOF DIRECTORS RECOMMENDS

A VOTEIN FAVOROF PROPOSAL 3.

PROPOSAL 4

RATIFICATIONOF SELECTIONOF INDEPENDENT REGISTERED PUBLIC ACCOUNTING FIRM

The Audit Committee of the Board has selected PricewaterhouseCoopers LLP as our independent registered public accounting firm for the fiscal year ending December 31, 2020 and has further directed that management submit the selection of its independent registered public accounting firm for ratification by the stockholders at the Annual Meeting. PricewaterhouseCoopers LLP has audited our financial statements since 2016 and also served as our auditor when we were a private company since 2016. Representatives ofPricewaterhouseCoopers LLP are expected to be present at the Annual Meeting. They will have an opportunity to make a statement reflectif they so desire and will be available to respond to appropriate questions.

Neither our Bylaws nor other governing documents or law require stockholder ratification of the Reverse Stock Split. As used herein,selection of PricewaterhouseCoopers LLP as our independent registered public accounting firm. However, the word “Arsanis” refersAudit Committee of the Board is submitting the selection of PricewaterhouseCoopers LLP to the Company priorstockholders for ratification as a matter of good corporate practice. If the stockholders fail to ratify the selection, the Audit Committee of the Board will reconsider whether or not to retain that firm. Even if the selection is ratified, the Audit Committee of the Board in its discretion may direct the appointment of different independent auditors at any time during the year if they determine that such a change would be in our best interests and our stockholders.

PRINCIPAL ACCOUNTANT FEESAND SERVICES

The following table represents aggregate fees billed to us for the fiscal years ended December 31, 2018 and December 31, 2019 by PricewaterhouseCoopers LLP, our principal accountant.

engagement of an independent registered public accounting firm for the next year’s audit, management will submit an aggregate of services expected to be rendered during that year for each of four categories of services (audit services, audit-related services, tax services and other fees) to the Audit Committee for approval. Prior to engagement, the Audit Committeepre-approves these services by category of service. The fees are budgeted, and the Audit Committee requires our independent registered public accounting firm and management to report actual fees versus the budget periodically throughout the year by category of service. During the year, circumstances may arise when it may become necessary to engage our independent registered public accounting firm for additional services not contemplated in the originalpre-approval. In those instances, the Audit Committee requires specificpre-approval before engaging our independent registered public accounting firm. Thepre-approval of services may be delegated to one or more of the Audit Committee’s members, but the decision must be reported to the full Audit Committee at its next scheduled meeting.

The Audit Committee has determined that the rendering of services other than audit services by PricewaterhouseCoopers LLP is compatible with maintaining the principal accountant’s independence.

Vote Required

The affirmative vote of a majority of the votes cast by the holders of shares present in person or by proxy and entitled to vote at the Annual Meeting will be required to ratify the selection of PricewaterhouseCoopers LLP.

THE BOARD OF DIRECTORS RECOMMENDS

A VOTE IN FAVOR OF PROPOSAL 4

.

EXECUTIVE OFFICERS

The following table sets forth information with respect to our executive officers and their ages as of the date of this Proxy Statement. There are no family relationships between any of our executive officers, and there is no arrangement or understanding between any executive officer and any other person pursuant to which the executive officer was selected.

Paula Ragan, Ph.D.Biographical information for Dr. Ragan is presented above under the caption “Class II Directors Continuing in Office Until the 2021 Annual Meeting of Stockholders.”

Adam S. Mostafa. Mr. Mostafa has served as our Chief Financial Officer and Treasurer since the Merger in March 2019. Prior to the Merger, Mr. Mostafa served as Private X4’s Chief Financial Officer since September 2018. Prior to joining Private X4, Mr. Mostafa served as chief financial officer of Abpro Corporation, a biotechnology company focused on antibody therapeutics, from June 2016 to August 2018. Prior to that, Mr. Mostafa was a managing director in the healthcare investment banking group at Cantor Fitzgerald from January 2015 to May 2016, and from June 2011 to January 2015, Mr. Mostafa was a senior banker in the healthcare investment banking group at Needham & Company. Prior to that, Mr. Mostafa was a vice president in the investment banking group at CRT Capital Group from March 2007 to May 2011, and from September 2003 to March 2007, Mr. Mostafa was a portfolio management associate in the global stock selection group at AQR Capital. Mr. Mostafa began his career as an analyst in the healthcare investment banking group at Salomon Smith Barney. Mr. Mostafa earned an A.B. in Economics from Brown University.

Derek Meisner,J.D. Mr. Meisner has served as our General Counsel since November 2019. From September 2018 until October 2019, Mr. Meisner held the role of General Counsel at Genocea Biosciences, a publicly traded, clinical-stage biopharmaceutical company. Prior to Genocea, he served as General Counsel of multiple Boston-based financial services firms, including Quantopian, Inc., a venture-backedstart-up, from April 2016 until September 2018, and life science investor RA Capital Management from January 2015 to April 2016. Mr. Meisner was also previously a partner at the international law firm K&L Gates and Branch Chief in the Division of Enforcement of the U.S. Securities and Exchange Commission. Mr. Meisner holds a B.A. from the University of Michigan and a J.D. from the Washington College of Law at American University.

SECURITY OWNERSHIP OF

CERTAIN BENEFICIAL OWNERS AND MANAGEMENT

The following table sets forth certain information with respect toregarding the beneficial ownership of ourthe Company’s common stock as of April 30, 201915, 2020 by: (i) each director and nominee for (a)director; (ii) each of the executive officers named in the Summary Compensation Table on page 18 of this proxy statement, (b) each of our directors and director nominees, (c)Table; (iii) all of our current directors and executive officers and directors of the Company as a groupgroup; and (d) each stockholder(iv) all those known by usthe Company to own beneficiallybe beneficial owners of more than 5%five percent of ourits common stock. Beneficial ownership is determined in accordance with the rules of the SEC and includes voting or investment power with respect to the securities. We deem shares of common stock that may be acquired by an individual or group within 60 days of April 30, 2019 pursuant to the exercise of options or warrants to be outstanding for the purpose of computing the percentage ownership of such individual or group, but those sharesApplicable percentages are not deemed to be outstanding for the purpose of computing the percentage ownership of any other person shown in the table. Except as indicated in footnotes to this table, we believe that the stockholders named in this table have sole voting and investment power with respect to all shares of common stock shown to be beneficially owned by them based on information provided to us by these stockholders. Percentage of ownership is based on 12,420,77816,142,275 shares of common stock outstanding on April 30, 2019.15, 2020, adjusted as required by rules promulgated by the SEC. Unless otherwise noted below, the address of each stockholder below is c/o X4 Pharmaceuticals, Inc., 955 Massachusetts Avenue, 4th Floor, Cambridge, Massachusetts 02139.

MANAGEMENT AND CORPORATE GOVERNANCE

Our Board of Directors

Our restated certificate of incorporation, as amended, and our amended and restatedby-laws provide that our business is to be managed by or under the direction of our board of directors. Our board of directors is divided into three classes for purposes of election. One class is elected at each annual meeting of stockholders to serve for a three-year term. Our board of directors currently consists of seven members, classified into three classes as follows: (1) René Russo and Isaac Blech constitute a class with a term expiring at the 2019 annual meeting of stockholders; (2) Gary J. Bridger, Ph.D. and Murray Stewart, M.D. constitute a class with a term ending at the 2020 annual meeting of stockholders; and (3) David McGirr, Paula Ragan, Ph.D. and Michael S. Wyzga constitute a class with a term ending at the 2021 annual meeting of stockholders.

On May 13, 2019, our board of directors accepted the recommendation of our nominating and corporate governance committee and voted to nominate René Russo for election at the annual meeting for a term of three years to serve until the 2022 annual meeting of stockholders, and until her successor has been elected and qualified. Mr. Blech will continue to serve as a director through the end of his term, which concludes at the 2019 annual meeting of stockholders, upon which the number of directors constituting our board of directors will be reduced to six (6).

Set forth below are the names of our directors, including the person nominated for election as a director and those directors whose terms do not expire this year, their ages, their offices in the Company, if any, their principal occupations or employment for at least the past five years, the length of their tenure as directors and the names of other public companies in which such persons hold or have held directorships during the past five years as of April 30, 2019. Additionally, information about the specific experience, qualifications, attributes or skills that led to our board of directors’ conclusion at the time of filing of this proxy statement that each person listed below should serve as a director is set forth below:

Paula Ragan, Ph.D. Dr. Ragan has been our President, Chief Executive Officer, Secretary and a member of our board of directors since the closing of the Merger in March 2019. Dr. Ragan has been the President, Chief Executive Officer and a member of the board of directors of X4 since July 2014. She has more than 18 years of experience building companies in the biotechnology industry. From August 2012 to September 2014, Dr. Ragan consulted as Chief Business Officer at Lysosomal Therapeutics Inc., a private biopharmaceutical company, where she led the company’s business development activities. Prior to that, from January 2007 to August 2012, Dr. Ragan held leadership roles in corporate development and operations at Genzyme Corporation, a Sanofi company, where she led strategic partnering efforts for Genzyme’s Rare Disease business and headed the supply chain planning for Genzyme’s flagship commercial products. Other professional roles include business roles at Hydra Biosciences, Oscient Corporation and Celera Corporation. Dr. Ragan received her B.S. from Tufts University and her Ph.D. from Massachusetts Institute of Technology and completed post-doctoral studies at Harvard Medical School. We believe that Dr. Ragan’s perspective and experience as our President and Chief

Outstanding Equity Awards at Fiscal Year End 2018OUTSTANDING EQUITY AWARDSAT FISCAL YEAR END

The following table sets forthshows for the fiscal year ended December 31, 2019, certain information regarding outstanding equity awards which consist of X4 stock options and X4 restricted stock, held by Dr. Ragan and Mr. Mostafa as of Decemberat fiscal year end for the Named Executive Officers.

OUTSTANDING EQUITY AWARDS AT DECEMBER 31, 2018. The share and per share amounts included in this table have not been adjusted to reflect the conversion of the shares in the Merger based on the exchange ratio of 0.5702 and do not reflect the Reverse Stock Split.2019