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.)

Filed by the Registrant  ☒                            Filed by a Party other than the Registrant  ☐

Check the appropriate box:

ARSANIS, INC.X4 Pharmaceuticals, Inc.

(Name of Registrant as Specified inIn Its Charter)

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

Payment of Filing Fee (Check the appropriate box):

ARSANIS,X4 PHARMACEUTICALS, INC.

890 Winter Street, Suite 230955 Massachusetts Avenue, 4^th Floor

Waltham,Cambridge, Massachusetts 02451

(781)819-570402139

NOTICE OF 2018 ANNUAL MEETING OF STOCKHOLDERS

To Be Held onOn June 5, 201810, 2020

Dear Stockholders:Stockholder:

You are cordially invited to attend the 2018 annual meetingAnnual Meeting of stockholdersStockholders of Arsanis,X4 Pharmaceuticals, Inc. to, a Delaware corporation (the “Company”). The Annual Meeting will be held on Tuesday,Wednesday, June 5, 201810, 2020 at 2:11:00 p.m., Eastern Daylight Time,a.m. local time at the offices of WilmerHale located at 60 StateCooley LLP, 500 Boylston Street, 14^th Floor, Boston, Massachusetts 02109. At the annual meeting, stockholders will consider and vote on02116 for the following matters:purposes:

1.     To elect the 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.

Stockholders of record at the close of business on April 16, 2018 will be entitled to notice of and tothat date may vote at the annual meetingAnnual Meeting or any adjournment or postponement thereof. This

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 details 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

René Russo, Pharm.D., BCPS

President and Chief Executive Officer

Waltham, Massachusetts

April 23, 2018

TABLE OF CONTENTS

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ARSANIS,X4 PHARMACEUTICALS, INC.

890 Winter Street, Suite 230955 Massachusetts Avenue, 4^th Floor

Waltham,Cambridge, Massachusetts 0245102139

(781)819-5704

PROXY STATEMENT

2018FOR THE 2020 ANNUAL MEETING OF STOCKHOLDERSSHAREHOLDERS

To Be Held on June 5, 201810, 2020

INFORMATION CONCERNING SOLICITATIONQUESTIONS AND ANSWERS ABOUT THESE PROXY MATERIALS AND VOTING

ThisWhy did I receivea notice regarding the availability of proxy statement andmaterials on the enclosed proxy card are being furnished in connection with the solicitation of proxiesInternet?

Pursuant to rules adopted by the board of directors of Arsanis, Inc. for use at the annual meeting of stockholders to be held on Tuesday, June 5, 2018 at 2:00 p.m., Eastern Daylight Time, at the offices of WilmerHale located at 60 State Street, Boston, Massachusetts 02109, and at any adjournment thereof. Except where the context otherwise requires, references to “Arsanis,” “the Company,” “we,” “us,” “our” and similar terms refer to Arsanis, Inc.

This proxy statement summarizes information about the proposals to be considered at the meeting and other information you may find useful in determining how to vote. The proxy card is the means by which you actually authorize another person to vote your shares in accordance with your instructions. We are making this proxy statement, the related proxy card and our annual report to stockholders for the fiscal year ended December 31, 2017 available to stockholders for the first time on or about April 23, 2018.

Important Notice Regarding the Availability of Proxy Materials for

the Annual Meeting of Stockholders to be Held on June 5, 2018:

This proxy statement and our 2017 Annual Report to Stockholders are

available for viewing, printing and downloading at http://www.proxyvote.com.

A copy of our Annual Report on Form10-K for the fiscal year ended December 31, 2017, as filed with 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 SEC, except for exhibits,“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 furnished without chargeheld 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 upon written or oral requestfor any purpose germane to Arsanis, Inc., 890 Winter Street, Suite 230, Waltham, Massachusetts 02451, Attention: Investor Relations. This proxy statementthe meeting for ten days prior to the Annual Meeting, and ourduring the Annual Report on Form10-KMeeting such list will be available for the fiscal year ended December 31, 2017 are also available on the SEC’s website at www.sec.gov.

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IMPORTANT INFORMATION ABOUT THE ANNUAL MEETING AND VOTINGStockholder of Record: Shares Registered in Your Name

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may elect to deliver at a later time. Whether or not you plan to attend the Annual Meeting, we urge you to vote by proxy to ensure your vote is counted. You may still attend the Annual Meeting and “brokervote 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 respectthese 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 countedvoted.

If you return a signed and dated proxy card or otherwise vote without marking voting selections, your shares will be voted, as votes in favor of such matter, and will also not be counted as shares voting on such matter. Accordingly, abstentions and “brokernon-votes” will have no effect on the voting on Proposal 1 or Proposal 2 referenced above.

FORapplicable, “FOR” the election of both nominees for director, “FOR” the three nomineesapproval of the Amended and Restated 2017 Equity Incentive Plan, “FOR” the amendment to serve as Class I directors, each for a three year term;our Restated Certificate of Incorporation and

FOR “FOR” the ratification of the appointmentselection of PricewaterhouseCoopers LLP as our independent registered public accounting firmfirm. 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 way 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 fiscal year ending December 31, 2018.entire cost of 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 can revoke your proxy at any time before the final vote at the Annual Meeting. If you are the record holder of your shares, you may revoke your proxy in any one of the following ways:

Your most current proxy card or telephone or Internet proxy is the one that is counted.

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

If your shares are held by your broker, bank or other agent, you should follow the instructions provided by your broker, bank or other agent.

When are stockholder proposals and director nominations due for next year’s annual meeting?

To be considered for inclusion in next year’s proxy materials, your proposal must be submitted in writing by December 29, 2020, to our Company Secretary, c/o X4 Pharmaceuticals, Inc., 955 Massachusetts Avenue, 4 -^th Floor, Cambridge, Massachusetts 02139.

BOARD OF DIRECTORS AND CORPORATE GOVERNANCE

Election of Directors

Our board of directors is divided into three classes, with members of each class holding office for staggered three-year terms. There are currently three Class I directors (William Clark, David McGirr and Claudio Nessi), whose terms expire at this annual meeting of stockholders; three Class II directors (René Russo, Michael Ross and Amy Schulman), whose terms expire at the 2019 annual meeting of stockholders; and three Class III directors (Tillman U. Gerngross, Carl Gordon and Terrance McGuire), whose terms expire at the 2020bylaws also establish an advance notice procedure if you wish to present a proposal (including a director nomination) before an annual meeting of stockholders (in all cases subjectbut 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 qualification of their successors orbrokernon-vote, and, with respect to their earlier death, resignation or removal).

Set forth below are the names ofother proposals, votes “For” and certain information“Against,” abstentions and, if applicable, brokernon-votes. Abstentions will be counted towards the vote total for each member of our board, includingProposals 2, 3 and 4, and will have the nomineessame effect as “Against” votes. Brokernon-votes on Proposals 1 and 2 will have no effect and will not be counted towards the vote total for electionany of those proposals.

What are “brokernon-votes”?

As discussed above, when a beneficial owner of shares held in street name does not give voting instructions to his or her broker, bank or other securities intermediary holding his or her shares as Class I directors.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 information presented includesNYSE has advised us that Proposals 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 are voted in the way 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.

How many votes are needed to approve each director’s and nominee’s principal occupation and business experience forproposal?

The following table summarizes the past five years,minimum vote needed to approve each proposal and the nameseffect of other public companies of which he or she has served as a director during the past five years. The information presented below regarding the specific experience, qualifications, attributesabstentions and skills of each director and nominee led our nominating and corporate governance committee and our board of directors to conclude that he or she should serve as a director. In addition, we believe that all of our directors and nominees possess the attributes or characteristics described in “Corporate Governance Matters—Director Nomination Process” that the nominating and corporate governance committee expects of each director. There are no family relationships among any of our directors, nominees for director, or executive officers.brokernon-votes.

What is the quorum requirement?

A quorum of stockholders is necessary to hold a valid meeting. A quorum will be present if stockholdersholdingat least a majority in voting power of the outstanding shares entitled to vote 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.ClassThus, 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 submit 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 may adjourn the Annual Meeting to another date.

How can I Director Nomineesfind out the results of the voting at the Annual Meeting?

Preliminary voting results will be announced at the Annual Meeting. In addition, final voting results will be published in a current 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 after the Annual Meeting, we intend to file aForm 8-K to publish preliminary results and, within four business days after the final results are known to us, file an additionalForm 8-K to publish the final results.

How does the March 2019 merger affect the disclosures in this proxy statement?

On March 13, 2019, X4 (formerly Arsanis, Inc.) completed its business combination in accordance with the terms of the Agreement and Plan of Merger, dated as of November 26, 2018, as amended on December 20, 2018 and March 8, 2019, or the Merger Agreement, by and among X4, X4 Therapeutics, Inc. (formerly X4 Pharmaceuticals, Inc., or “Private X4”) and Artemis AC Corp., a Delaware corporation and a wholly-owned subsidiary of the Company (the “Merger Sub”), pursuant to which, among other matters, Merger Sub merged with and into Private X4, with Private X4 continuing as a wholly-owned subsidiary of X4 and the surviving corporation of the merger (the “Merger”). Following the Merger, on March 13, 2019, X4 effected a1-for-6 reverse stock split of its common stock (the “Reverse Stock Split”), and changed its name to “X4 Pharmaceuticals, Inc.” Following the completion of the Merger, the business conducted by X4 became primarily the business conducted by Private X4, a clinical-stage biopharmaceutical company focused on the research, development and commercialization of novel therapeutics for the treatment of rare diseases. All references to common stock shares and per share amounts in this Proxy Statement have been retroactively adjusted to reflect, where applicable, the Reverse Stock Split, as indicated.

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 each 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

William Clark, M.B.A.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.Mr. Clark, age 59, has served as a member of our board of directorsBoard since September 2017. Mr. Clark is currently the President and Chief Executive Officer of Genocea Biosciences, Inc., or Genocea, a publicly traded biopharmaceutical company, a position heMarch 2019. Dr. Stewart has held since February 2011. He also served as Genocea’s Chief Business Officer from August 2010 to February 2011. Prior to joining Genocea, Mr. Clark served as Chief BusinessMedical Officer at Vandaof Rhythm Pharmaceuticals, Inc., or Vanda, a biopharmaceutical company heco-founded in 2004.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 Vanda, Mr. Clark wasthat, 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 principal at Care Capital, LLC, a venture capital firm investing in biopharmaceutical companies, after serving in a variety of commercialconsultant physician and strategic roles at SmithKline Beecham (now GlaxoSmithKline). Mr. Clark currently serves on the board of directors of Genocea,

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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 2017.2017 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., or Cubist, 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, aninc., a venture-financed internet technology venture-financed 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;2013, and Rhythm Pharmaceuticals, Inc., a publicly traded biopharmaceutical company, where he has served since November 2015; and Menlo Therapeutics, Inc., a publicly traded biopharmaceutical company where he has served since November 2017.2015. Mr. McGirr alsopreviously served on the board of directors of Roka Bioscience, Inc., a molecular diagnostics company, from December 2013 to January 2018.2018 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. Our board of directors believesWe believe that Mr. McGirr’s extensive experience as an executive officer or director of a number ofwith public and private pharmaceutical companies provide him with the qualifications and skills to serve on our board of directors.Board.

Claudio Nessi,Paula Ragan, Ph.D., M.B.A.    Dr. Nessiage 50, has served as our President, Chief Executive Officer, Secretary and a member of our board of directors since August 2013. Dr. Nessi has served as Managing Partner at NeoMed Management, a venture capital firm, since 2016, where he has served as a Partner since 2005 and served as an Investment Director from 2001 until 2005. Also, Dr. Nessi has served as Managing Director of Omega Funds, an investment advisory firm, since November 2016. Dr. Nessi held other board positions at Endosense SA from October 2005 to August 2013, Kuros BioSurgery AG from October 2002 to February 2013, PregLem SA from June 2007 to October 2010 and Creabilis Ltd. from February 2008 to December 2016. Dr. Nessi also currently serves on the Board of Directors of the private biotechnology companies Avitide, Inc. and Anaconda Biomed and is the Chairman of the Board of Directors of the public biotechnology company GenKyoTex SA. Dr. Nessi received his M.B.A. from Erasmus University in the Netherlands, and received his Ph.D. in Genetics from the University of Pavia, Italy. Our board of directors believes Dr. Nessi’s expertise and experience in the biotechnology industry through his roles of increasing responsibility at NeoMed Management spanning a period of over 15 years, in which he has been involved in the evaluation, investment and oversight of several biotechnology companies; his scientific and business-focused educational background, as well as his experience as a director of other companies provide him with the qualifications and skills to serve on our board of directors.

Class II Directors

René Russo, Pharm.D., BCPS.Dr. Russo has served as a member of our board of directors and as our President and Chief Executive Officer since April 2016. Dr. Russo served as our Chief Development Officer from July 2015 until April 2016. Previously, Dr. Russo served in various roles over an11-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 where she started her industry career as a Postdoctoral Fellow in Industrial Pharmacy Infectious Diseases.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 biotechnology industry,company’s business development activities. Prior to then, from January 2007 to August 2012, Dr. RussoRagan held clinical positionsleadership roles in corporate development and operations at Robert Wood Johnson University HospitalGenzyme Corporation, a Sanofi company, where she led strategic partnering efforts for Genzyme’s Rare Disease business and Princeton Hospital.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. RussoRagan received her Pharm.D. and B.S. from Rutgers University. Our boardTufts University and her Ph.D. from Massachusetts Institute of directors believesTechnology and completed post-doctoral studies at Harvard Medical School. We believe that Dr. Russo’s expertiseRagan’s perspective and experience as our President and Chief Executive Officer, her perspectivewhich provides the Board with historic knowledge, operational expertise and experience as an executive at public and private

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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. Please see “BoardThe 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 Directors“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 Corporate Governance – Electionthe 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 Directors” abovethe 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 biographical information regardingdirector had a material or other disqualifying relationship with the Company. Dr. Russo.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.

Eszter Nagy, M.D., Ph.D.BOARD LEADERSHIP STRUCTURE    Dr. Nagyco-founded Arsanis

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 2010its oversight of our business and built a multi-disciplinary researchaffairs. Our Board also recognizes the time, effort and preclinical teamenergy that our Chief Executive Officer must devote to her position in Vienna. Dr. Nagy has servedthe current business environment, as well as the commitment required to serve as our Chief Scientific OfficerChairman, 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 Managing Directoroversight 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 wholly owned subsidiary, Arsanis Biosciences GmbH, since October 2011. Dr. Nagy also served onCompany 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 Directorsour 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 January 2011 until November 2017. From August 2013various 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 December 2015, Dr. Nagy served as our President. From January 1999 to September 2010, Dr. Nagy served in various roles during her 12 years at Intercell AG (now Valneva SE), most recently as its Senior Vice President of Global Research. Dr. Nagyco-founded EveliQure Biotechnologies (Vienna) in 2012,management. Our Board believes that full and was onopen 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 of WittyCell, S.A.S (now part of Abivax, SE), both of which are vaccine development companies. Prior to joining the biotechnology industry, Dr. Nagy spent 10 years in academic research, including as an Associate Professor at the University Medical School of Pécs and a Visiting Scientist in the Department of Cancer Genetics at the Roswell Park Cancer Institute. Dr. Nagy received her M.D.its committees routinely request and Ph.D.review management updates, reports from the University Medical Schoolindependent 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 Pécs, Hungary,Directors is committed to continuing to ensure and was a Postdoctoral Fellow at Dartmouth Medical School.

Michael Gray, M.B.A., C.P.A.Mr. Gray has servedevolve its risk oversight practices as appropriate given the stage of our Chief Financial Officer since March 2016 and as our Chief Operating Officer since September 2017. Mr. Gray also served as our Chief Business Officer from March 2016 to September 2017. Prior to joining us, Mr. Gray served in various leadership positions from August 2000 through February 2016 at Curis, Inc., a publicly held oncology drug development company. He served as Curis’ Chief Financial Officer and Chief Business Officer from February 2014 to February 2016 and as its Chief Financial Officer and Chief Operating Officer from December 2006 to February 2014. From December 2003 until December 2006, Mr. Gray served as Curis’ Vice President of Finance and Chief Financial Officer and from August 2000 until December 2003, served as its Senior Director of Finance and Controller. Previously, Mr. Gray held positions including Controller and de facto Chief Financial Officer at Reprogenesis, a biotechnology company focused on the development of cell therapy drug candidates, and as an audit professional for the accounting and consulting firm of Ernst & Young, LLP. Mr. Gray received his M.B.A. in corporate finance and entrepreneurial management from the F.W. Olin Graduate School of Business at Babson College and a B.S. in accounting from Bryant College.

Chris Stevens, M.D.    Dr. Stevens has served as our Chief Medical Officer since June 2016. Prior to joining us, Dr. Stevens servedevolution as a consultant for over 30 companies, from 2004late-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 2016, where he assisted in all stagesaddress concerns of drug development across the United Statesour employees and in Europe. Dr. Stevens served key clients during this time, including Cubist Pharmaceuticals, Inc., Dyax, Inc. and Millennium/Takeda, all biotechnology companies. Previously, he served as Senior Vice President of Clinical Development at Alnara Pharmaceuticals from 2009 to 2011 through its acquisition by Eli Lilly and also previously held medical director roles at Circe Biomedical and Altus Pharmaceuticals. Additionally, Dr. Stevens spent 10 years as a clinical and research gastroenterologist at Beth Israel Deaconess Medical Center in Boston and as an Assistant Professor of Medicine at Harvard Medical

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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 an analysisbe incorporated by reference in any filing of these policies. For 2017,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 “namedCompensation 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:

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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 committeeis 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.

Our Board believes that additional shares are necessary to meet our anticipated equity compensation needs. Our Board believes we can meet these anticipated needs for approximately the next seven years after the Annual Meeting if this Proposal 2 is approved by stockholders. This estimate is based on a forecast that takes into account our anticipated rate of growth in hiring, an estimated range of our stock price over time and our historical forfeiture rates.

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, retain compensationamong 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 other outside advisors are eligible to assistparticipate in the evaluationAmended 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 compensation. Duringand 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 proposal 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 2017, our compensation committee retained Willis Towers Watson asyear ending December 31, 2020 and has further directed that management submit the selection of its independent compensation consultantregistered 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 provide comparative data on executivebe present at the Annual Meeting. They will have an opportunity to make a statement if they so desire and director compensation practiceswill be available to respond to appropriate questions.

Neither our Bylaws nor other governing documents or law require stockholder ratification of the selection of PricewaterhouseCoopers LLP as our independent registered public accounting firm. However, the Audit Committee of the Board is submitting the selection of PricewaterhouseCoopers LLP to the stockholders 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 industrybest interests and assess our executives’stockholders.

PRINCIPAL ACCOUNTANT FEESAND SERVICES

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

All fees described above werepre-approved by the Audit Committee. There were no services that were approved by the Audit Committee pursuant to Rule2-01(c)(7)(i)(C) (relating to the approval of ade minimis amount ofnon-audit services after the fact but before completion of the audit).

The Audit Committee has adopted a policy and procedures for thepre-approval of audit andnon-audit services rendered by our independent registered public accounting firm, PricewaterhouseCoopers LLP. Prior to

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 regarding the ownership of the Company’s common stock as of April 15, 2020 by: (i) each director and nominee for director; (ii) each of the executive officers named in the Summary Compensation Table; (iii) all current executive officers and directors of the Company as a group; and (iv) all those known by the Company to be beneficial owners of more than five percent of its common stock. Applicable percentages are based on 16,142,275 shares outstanding on April 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.

EXECUTIVE COMPENSATION

SUMMARY COMPENSATION TABLE

The following table shows the total compensation relativepaid or accrued during the fiscal years ended December 31, 2018 and December 31, 2019 to comparable companies.(1) the individuals who served as our Chief Executive Officer during the fiscal year ended December 31, 2019 and (2) our two next most highly compensated executive officers who earned more than $100,000 during the fiscal year ended December 31, 2019 and were serving as executive officers as of such date. We refer to these individuals in this Proxy Statement as our Named Executive Officers.

Our Named Executive Officers for 2019 who appear in the Summary Compensation Table are:

SUMMARY COMPENSATION TABLE

OUTSTANDING EQUITY AWARDSAT FISCAL YEAR END

The following table shows for the fiscal year ended December 31, 2019, certain information regarding outstanding equity awards at fiscal year end for the Named Executive Officers.

OUTSTANDING EQUITY AWARDS AT DECEMBER 31, 2019

NARRATIVETO SUMMARY COMPENSATION TABLE

The material terms of the elements of our executive compensation program are described below.

Base Salary.    In 2016, we paid Dr. Russo an annualized

Each Named Executive Officer’s base salary is a fixed component of $380,000. In 2016, we paidannual compensation for performing specific duties and functions, and has been established by our Board taking into account each individual’s role, responsibilities, skills, and experience.

Non-Equity Incentive Plan Compensation

Our annual bonus program is intended to reward our Named Executive Officers for meeting objective or subjective individual and/or company-wide performance goals for a fiscal year. For 2019 and 2018, our Named Executive Officers, other than Mr. Gray, received incentive compensation based upon the Board or Compensation Committee’s determination of their respective achievement of and performance with respect to corporate objectives.

Long-Term Equity Incentives

Our equity grant program is intended to align the interests of our Named Executive Officers with those of our stockholders and to motivate them to make important contributions to our performance.

Employment and Change in Control Arrangements

Paula Ragan, Ph.D.

On June 23, 2015, Private X4 entered into an annualizedexecutive employment agreement with Dr. Ragan. Pursuant to the executive employment agreement, Dr. Ragan’s compensation consisted of base salary, of $350,000, which was prorated to reflectannual bonus as determined by the number of days he served with our company following his hire in March 2016. In 2016, we paid Dr. Stevens an annualized base salary of $380,000, which was prorated to reflect the number of days he served with our company following his hire in June 2016. In

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In connection with the execution of her amended and restated employment agreement, Dr. Ragan also agreed to continue to abide by the terms of hernon-competition,non-solicitation,non-disclosure and intellectual property agreement.

Michael P. Gray

In connection with the appointment of Michael P. Gray as Arsanis’ President and Chief Executive Officer on November 26, 2018, Arsanis entered into an amended and restated employment agreement with Mr. Gray, which agreement became effective as of November 27, 2018 and replaced his October 10, 2017 amended and restated letter agreement (the “Gray 2018 Agreement”). Under the Gray 2018 Agreement, Mr. Gray’s base salary was $450,000 per annum and he was entitled to participate in Arsanis’ medical and other benefits programs, and he was entitled to receive an annual bonus based on his individual performance and the company’s performance during the applicable year, all as determined by Arsanis’ Board of Directors in its sole discretion, at a target bonus rate of 55% of his annualized base salaries of Dr. Russo and Mr. Gray to $450,000 and $400,000, respectively, in each case subjectsalary.

On March 13, 2019, immediately prior to and effective upon the closing of our initial public offeringthe Merger, Mr. Gray resigned for good reason for purposes of the Gray 2018 Agreement. In accordance with the terms of the Gray 2018 Agreement, in connection with such resignation, Mr. Gray became entitled to the following severance benefits: (i) an amount equal to the sum of (x) 1.5 times his then-current base salary and in(y) 1.5 times his target bonus for 2019, and (ii) COBRA premium benefits for up to 12 months. In addition, pursuant to the caseterms of the Gray 2018 Agreement, Mr. Gray’s increase,outstanding equity awards vested in full, and the option award granted to Mr. Gray in June 2017 may be retroactiveexercised for up to two years following his separation date.

Adam S. Mostafa

On July 23, 2018, Private X4 entered into an executive employment agreement with Mr. Mostafa, pursuant to which Mr. Mostafa agreed to serve as Private X4’s Chief Financial Officer effective as of September 27, 2017,5, 2018. Pursuant to the executive employment agreement, Mr. Mostafa’s compensation consisted of base salary, potential annual bonus as determined by the Private X4 board of directors, a potential equity grant subject to approval by the Private X4 board of directors, fringe benefits, vacation and reimbursement of ordinary and reasonableout-of-pocket expenses. Mr. Mostafa’s initial base salary, which was subject to review and adjustment, was $340,000. The agreement provided for Private X4 to pay annual bonuses at Private X4’s sole discretion, as determined by the effective datePrivate X4 board of directors, with an annual target bonus set at 35% of annual base salary. Private X4 also agreed to grant Mr. Gray’s promotionMostafa an option to purchase 659,067 shares of Private X4 common stock, subject to approval by the roleX4 board of Chief Operating Officerdirectors and Chief Financial Officer.subject to certain vesting conditions, which option was granted on October 19, 2018. In connection with the Merger, this option became an option to purchase 62,633 shares of our common stock.