Document:

EX-4.3

 Exhibit 4.3 

Form of Maxim Warrant 
 THIS WARRANT
AND THE UNDERLYING SECURITIES ISSUABLE UPON THE EXERCISE THEREOF HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED (THE “ACT”). NO SALE OR DISPOSITION MAY BE EFFECTED EXCEPT IN COMPLIANCE WITH RULE 144 UNDER SAID ACT OR
AN EFFECTIVE REGISTRATION STATEMENT RELATED THERETO OR AN OPINION OF COUNSEL FOR THE HOLDER SATISFACTORY TO THE COMPANY THAT SUCH REGISTRATION IS NOT REQUIRED UNDER THE ACT. 

X4 PHARMACEUTICALS, INC. 

WARRANT TO PURCHASE PREFERRED STOCK 
  

			
	No. 2015-    	  	                    , 2015

 VOID AFTER
                    , 2020 

THIS CERTIFIES THAT, for value received, Maxim Partners, LLC, with its principal
office at 405 Lexington Ave., New York, NY 10174, or assigns (the “Holder”), is entitled to subscribe for and purchase from X4 PHARMACEUTICALS, INC, a Delaware corporation, with its principal office at 784 Memorial
Drive, Suite #140, Cambridge, MA 02139 (the “Company”), the Exercise Shares at the Exercise Price (each subject to adjustment as provided herein). Unless indicated otherwise, the aggregate number of Exercise Shares
that Holder may purchase by exercising this Warrant is equal to [        ] Exercise Shares, subject to adjustment pursuant to the terms hereof, including but not limited to adjustments pursuant to
Sections 5 and 7 below. 
 1. DEFINITIONS. Capitalized terms used but not defined herein shall have the
meanings set forth in the Agreement. As used herein, the following terms shall have the following respective meanings: 
 (a)
“Equity Securities” shall mean shares of the Company’s shares of Series A Preferred Stock, par value U.S. $0.001 per share. 

(b) “Exercise Period” shall mean the period commencing on the date hereof and ending five (5) years later,
unless sooner terminated as provided below 
 (c) “Exercise Price” shall mean $2.07; provided, that, the
Exercise Price shall be subject to adjustment pursuant to Sections 5 and 7 below. 
 (d) “Exercise Shares”
shall mean the Equity Securities issuable upon exercise of this Warrant and any Conversion Securities (as defined below). 

2. EXERCISE OF WARRANT. The rights represented by this Warrant may be exercised in
whole or in part at any time during the Exercise Period, by delivery of the following to the Company at its address set forth above (or at such other address as it may designate by notice in writing to the Holder): 

  
 1. 

 (a) an executed Notice of Exercise in the form attached hereto; 

(b) payment of the Exercise Price either (i) in cash or by check, (ii) by wire transfer or (iii) pursuant to
Section 2.1 below; and 
 (c) this Warrant. 

Upon the exercise of the rights represented by this Warrant, a certificate or certificates for the Exercise Shares so purchased, registered in the name of the
Holder shall be issued and delivered to the Holder within a reasonable time after the rights represented by this Warrant shall have been so exercised. In the event that this Warrant is being exercised for less than all of the then-current number of
Exercise Shares purchasable hereunder, the Company shall, concurrently with the issuance by the Company of the number of Exercise Shares for which this Warrant is then being exercised, issue a new Warrant exercisable for the remaining number of
Exercise Shares purchasable hereunder. 
 The person in whose name any certificate or certificates for Exercise Shares are to be issued upon exercise of
this Warrant shall be deemed to have become the holder of record of such shares on the date on which this Warrant was surrendered and payment of the Exercise Price was made, irrespective of the date of delivery of such certificate or certificates,
except that, if the date of such surrender and payment is a date when the stock transfer books of the Company are closed, such person shall be deemed to have become the holder of such shares at the close of business on the next succeeding date on
which the stock transfer books are open. 
 2.1 Net Exercise. Notwithstanding any provisions herein to the contrary, if the
fair market value of one Exercise Share is greater than the Exercise Price (at the date of calculation as set forth below), in lieu of exercising this Warrant by payment of cash, the Holder may elect to receive shares equal to the value (as
determined below) of this Warrant (or the portion thereof being canceled) by surrender of this Warrant at the principal office of the Company together with the properly endorsed Notice of Exercise in which event the Company shall issue to the Holder
a number of Exercise Shares computed using the following formula: 
 X = Y (A-B) 

    A 
 Where
X = the number of Exercise Shares to be issued to the Holder 
  

	 	Y =	 the number of Exercise Shares purchasable under the Warrant or, if only a portion of the Warrant is being
exercised, that portion of the Warrant being canceled (at the date of such calculation) 

  

	 	A =	 the fair market value of one Exercise Share (at the date of such calculation) 

 

	 	B =	 Exercise Price (as adjusted to the date of such calculation) 

For purposes of the above calculation, the fair market value of one Exercise Share shall be determined by the Company’s Board of Directors in good faith;
provided, however, that in the event that this Warrant is exercised pursuant to this Section 2.1 in connection with the Company’s initial public offering of its Common Stock, the fair market value per share shall be the product of
(i) the per share offering price to the public of the Company’s initial public offering, and (ii) the number of shares of Common Stock into which each Exercise Share is convertible at the time of such exercise. 

  
 2. 

 3. COVENANTS OF THE COMPANY.

 3.1 Covenants as to Exercise Shares. Except as provided in the last sentence of this Section 3.1, the Company
covenants and agrees that all Exercise Shares that may be issued upon the exercise of the rights represented by this Warrant will, upon issuance, be validly issued and outstanding, fully paid and nonassessable, and free from all taxes, liens and
charges with respect to the issuance thereof. The Company further covenants and agrees that the Company will at all times during the Exercise Period, have authorized and reserved, free from preemptive rights, a sufficient number of shares of the
series of equity securities comprising the Exercise Shares to provide for the exercise of the rights represented by this Warrant. If at any time during the Exercise Period the number of authorized but unissued shares of such series of the
Company’s equity securities shall not be sufficient to permit exercise of this Warrant, the Company will take such corporate action as may, in the opinion of its counsel, be necessary to increase its authorized but unissued shares of such
series of the Company’s equity securities to such number of shares as shall be sufficient for such purposes. 
 3.2 Notices
of Record Date. In the event of any taking by the Company of a record of the holders of any class of securities for the purpose of determining the holders thereof who are entitled to receive any dividend or other distribution, the Company shall
mail to the Holder, at least ten (10) days prior to the date specified herein, a notice specifying the date on which any such record is to be taken for the purpose of such dividend or distribution. 

4. REPRESENTATIONS OF HOLDER. 

4.1 Acquisition of Warrant for Personal Account. The Holder represents and warrants that it is acquiring the Warrant and the
Exercise Shares solely for its account for investment and not with a view to or for sale or distribution of said Warrant or Exercise Shares or any part thereof. The Holder also represents that the entire legal and beneficial interests of the Warrant
and Exercise Shares the Holder is acquiring is being acquired for, and will be held for, its account only. 
 4.2 Securities Are Not
Registered. 
 (a) The Holder understands that the Warrant and the Exercise Shares have not been registered under the Securities
Act of 1933, as amended (the “Act”) on the basis that no distribution or public offering of the stock of the Company is to be effected. The Holder realizes that the basis for the exemption may not be present if,
notwithstanding its representations, the Holder has a present intention of acquiring the securities for a fixed or determinable period in the future, selling (in connection with a distribution or otherwise), granting any participation in, or
otherwise distributing the securities. The Holder has no such present intention. 

  
 3. 

 (b) The Holder recognizes that the Warrant and the Exercise Shares must be held
indefinitely unless they are subsequently registered under the Act or an exemption from such registration is available. The Holder recognizes that the Company has no obligation to register the Warrant or the Exercise Shares of the Company, or to
comply with any exemption from such registration. 
 (c) The Holder is aware that neither the Warrant nor the Exercise Shares may be
sold pursuant to Rule 144 adopted under the Act unless certain conditions are met, including, among other things, the existence of a public market for the shares, the availability of certain current public information about the Company, the resale
following the required holding period under Rule 144 and the number of shares being sold during any three month period not exceeding specified limitations. Holder is aware that the conditions for resale set forth in Rule 144 have not been satisfied
and that the Company presently has no plans to satisfy these conditions in the foreseeable future. 
 4.3 Disposition of Warrant and
Exercise Shares. 
 (a) The Holder further agrees not to make any disposition of all or any part of the Warrant or Exercise
Shares in any event unless and until: 
 (i) the Company shall have received a letter secured by the Holder from the Securities and
Exchange Commission stating that no action will be recommended to the Commission with respect to the proposed disposition; 
 (ii)
there is then in effect a registration statement under the Act covering such proposed disposition and such disposition is made in accordance with said registration statement; or 

(iii) the Holder shall have notified the Company of the proposed disposition and shall have furnished the Company with a detailed
statement of the circumstances surrounding the proposed disposition, and if reasonably requested by the Company, the Holder shall have furnished the Company with an opinion of counsel, reasonably satisfactory to the Company, for the Holder to the
effect that such disposition will not require registration of such Warrant or Exercise Shares under the Act or any applicable state securities laws. 

(b) The Holder understands and agrees that all certificates evidencing the shares to be issued to the Holder may bear the following
legend: 
 THESE SECURITIES HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED (THE “ACT”). THEY
MAY NOT BE SOLD, OFFERED FOR SALE, PLEDGED OR HYPOTHECATED IN THE ABSENCE OF AN EFFECTIVE REGISTRATION STATEMENT AS TO THE SECURITIES UNDER THE ACT OR AN OPINION OF COUNSEL SATISFACTORY TO THE COMPANY THAT SUCH REGISTRATION IS NOT REQUIRED. 

4.4 Accredited Investor Status. The Holder is an “accredited investor” as defined in Regulation D promulgated under
the Act. 

  
 4. 

 5. ADJUSTMENT OF EXERCISE
PRICE AND NUMBER OF EXERCISE SHARES. 

5.1 Changes in Securities. In the event of changes in the series of equity securities of the Company comprising the Exercise
Shares by reason of stock dividends, splits, recapitalizations, reclassifications, combinations or exchanges of shares, separations, reorganizations, liquidations, or the like, the number and class of Exercise Shares available under the Warrant in
the aggregate and the Exercise Price shall be correspondingly adjusted to give the Holder of the Warrant, on exercise for the same aggregate Exercise Price, the total number, class, and kind of shares as the Holder would have owned had the Warrant
been exercised prior to the event and had the Holder continued to hold such shares until after the event requiring adjustment. For purposes of this Section 5 and Section 7, the “Aggregate Exercise Price” shall mean
the aggregate Exercise Price payable in connection with the exercise in full of this Warrant. The form of this Warrant need not be changed because of any adjustment in the number of Exercise Shares subject to this Warrant. 

5.2 Conversion. Upon the conversion of all outstanding shares of the series of equity securities comprising the Exercise Shares,
this Warrant shall become exercisable for that class, series, kind, and number of shares of equity securities (the “Conversion Securities”) into which the Exercise Shares would then be convertible and the Exercise Price shall
be correspondingly adjusted to give the Holder of the Warrant, on exercise for the same aggregate Exercise Price, the total number, class, series, and kind of shares as the Holder would have owned if this Warrant had been exercised prior to such
event and such Exercise Shares had been acquired. In such case, all references to “Exercise Shares” shall mean the Conversion Securities. Conversion Securities shall be subject to further and successive adjustments pursuant to this
Section 5. 
 6. FRACTIONAL SHARES. No fractional shares shall be issued upon the exercise
of this Warrant as a consequence of any adjustment pursuant hereto. All Exercise Shares (including fractions) to be issued upon exercise of this Warrant shall be aggregated for purposes of determining whether the exercise would result in the
issuance of any fractional share. If, after aggregation, the exercise would result in the issuance of a fractional share, the Company shall, in lieu of issuance of any fractional share, pay the Holder otherwise entitled to such fraction a sum in
cash equal to the product resulting from multiplying the then current fair market value of one Exercise Share by such fraction. 
 7.
EARLY TERMINATION. In the event of, at any time during the Exercise Period, an initial public offering of securities of the Company registered under the Act, or a [Deemed Liquidation Event] (as defined in
the Company’s Certificate of Incorporation), the Company shall provide to the Holder ten (10) days advance written notice of the estimated closing date of such public offering or Deemed Liquidation Event, and, unless the Holder exercises
this Warrant pursuant to the terms of Section 2 within five (5) days of the receipt of such advance written notice, this Warrant shall be deemed exercised pursuant to Section 2.1 immediately prior to the date such public offering is
closed or the closing of such Deemed Liquidation Event. 

  
 5. 

 8. MARKET
STAND-OFF AGREEMENT. The Holder hereby agrees that it will not, without the prior written consent of the managing underwriter,
during the period beginning on and including the date that the Company’s Common Stock is first quoted on a national exchange and continuing through the close of trading on the date that is 180 days thereafter , or such other period as may be
requested by the Company or an underwriter to comply with regulatory restrictions on (1) the publication or other distribution of research reports and (2) analyst recommendations and options, including, but not limited to, the restrictions
contained in FINRA Rule 2711(f)(4) or NYSE Rule 472(f)(4), or any successor provisions or amendments thereto), (i) lend; offer; pledge; sell; contract to sell; sell any option or contract to purchase; purchase any option or contract to sell; grant
any option, right, or warrant to purchase; or otherwise transfer or dispose of, directly or indirectly, any shares of Common Stock or any securities convertible into or exercisable or exchangeable (directly or indirectly) for Common Stock (whether
such shares or any such securities are then owned by the Holder or are thereafter acquired) or (ii) enter into any swap or other arrangement that transfers to another, in whole or in part, any of the economic consequences of ownership of such
securities, whether any such transaction described in clause (i) or (ii) above is to be settled by delivery of Common Stock or other securities, in cash, or otherwise. The foregoing provisions of this Section shall apply only to an initial
public offering, shall not apply to the sale of any shares to an underwriter pursuant to an underwriting agreement, or the transfer of any shares to any trust for the direct or indirect benefit of the Holder or the immediate family of the Holder,
provided that the trustee of the trust agrees to be bound in writing by the restrictions set forth herein, and provided further that any such transfer shall not involve a disposition for value (an “Estate Planning Transfer”),
and shall be applicable to the Holder only if all officers and directors are subject to the same restrictions and the Company uses commercially reasonable efforts to obtain a similar agreement from all stockholders individually owning more than one
percent (1%) of the Company’s outstanding Common Stock (after giving effect to conversion into Common Stock of all outstanding Preferred Stock). The underwriters in connection with such registration are intended
third-party beneficiaries of this Section and shall have the right, power, and authority to enforce the provisions hereof as though they were a party hereto. Each Holder further agrees to execute such
agreements as may be reasonably requested by the underwriters in connection with such registration that are consistent with this Section or that are necessary to give further effect thereto. Any discretionary waiver or termination of the
restrictions of any or all of such agreements by the Company or the underwriters shall apply pro rata to all Holders subject to such agreements, based on the number of shares subject to such agreements. Holder agrees that any transferee of the
Warrant (or other securities) of the Company held by Holder shall be bound by this Section 8. 
 9. Rights and Restrictions
of the Holder. As contemplated by that certain Series A Preferred Stock Purchase Agreement by and among the Company and the purchasers named therein dated as of August 14, 2015 (the “Purchase Agreement”), the
Equity Securities shall be deemed to have been issued under the Purchase Agreement and accordingly the Equity Securities and the Exercise Shares issuable upon the conversion of such Equity Securities shall be entitled to the rights, benefits and
restrictions accorded to the Equity Securities, and the Equity Shares issuable upon conversion thereof, issued pursuant to the Purchase Agreement, including the rights, benefits and restrictions set forth in the Purchase Agreement and the related
Investor Rights Agreement, in each case as amended, and the applicable provisions of the Company’s Amended and Restated Certificate of Incorporation, the Purchase Agreement and the related Investor Rights Agreement and Right of First Refusal
and Co-Sale Agreement are hereby incorporated herein by reference and made a part hereof as if set forth herein in their entirety. 

  
 6. 

 10. NO STOCKHOLDER RIGHTS. This
Warrant in and of itself shall not entitle the Holder to any voting rights or other rights as a stockholder of the Company. 
 11.
TRANSFER OF WARRANT. Without the prior written consent of the Company, this Warrant shall not be assigned, pledged or hypothecated in any way (whether by operation of law or otherwise) and shall
not be subject to execution, attachment or similar process, provided, however, that the Holder may assign this Warrant to its affiliate(s) without such prior written consent of the Company. Any attempted transfer, assignment, pledge, hypothecation
or other disposition of this Warrant or of any rights granted hereunder contrary to the provisions of this Section 11, or the levy of any attachment or similar process upon this Warrant or such rights, shall be null and void. 

12. LOST, STOLEN, MUTILATED OR DESTROYED
WARRANT. If this Warrant is lost, stolen, mutilated or destroyed, the Company may, on such terms as to indemnity or otherwise as it may reasonably impose (which shall, in the case of a mutilated Warrant, include the surrender
thereof), issue a new Warrant of like denomination and tenor as the Warrant so lost, stolen, mutilated or destroyed. Any such new Warrant shall constitute an original contractual obligation of the Company, whether or not the allegedly lost, stolen,
mutilated or destroyed Warrant shall be at any time enforceable by anyone. 
 13.
AMENDMENT. Any term of this Warrant may be amended or waived with the written consent of the Company and Holders of at least a majority in interest of the outstanding Warrants provided that all
Warrants are similarly affected. Upon the effectuation of such amendment or waiver in conformance with this Section 13, the Company shall promptly give written notice thereof to the record holders of the Warrants who have not previously
consented thereto in writing. 
 14. NOTICES, ETC. All notices required or permitted hereunder
shall be in writing and shall be deemed effectively given: (a) upon personal delivery to the party to be notified, (b) when sent by confirmed telex or facsimile if sent during normal business hours of the recipient, if not, then on the
next business day, (c) five (5) days after having been sent by registered or certified mail, return receipt requested, postage prepaid, or (d) one (1) day after deposit with a nationally recognized overnight courier, specifying
next day delivery, with written verification of receipt. All communications shall be sent to the Company at the address listed on the signature page and to the Holder at 405 Lexington Ave., New York, NY 10174, or at such other address as the Company
or Holder may designate by ten (10) days advance written notice to the other parties hereto. 
 15.
ACCEPTANCE. Receipt of this Warrant by the Holder shall constitute acceptance of and agreement to all of the terms and conditions contained herein. 

16. GOVERNING LAW. This Warrant and all rights, obligations and liabilities hereunder shall be
governed by and construed under the laws of the Commonwealth of Massachusetts as applied to agreements among Massachusetts residents, made and to be performed entirely within the Commonwealth of Massachusetts without giving effect to conflicts of
laws principles. 

  
 7. 

 IN WITNESS WHEREOF, the Company has
caused this Warrant to be executed by its duly authorized officer as of [●], 2015. 
  

			
	X4 PHARMACEUTICALS, INC 

 
			
		
	By: 	 	 

 
			
	Name:	 	Paula Ragan
	Title:	 	Chief Executive Officer
	Address:	 	784 Memorial Drive, Suite #140,
Cambridge, MA 02139

 SIGNATURE PAGE TO WARRANT
TO PURCHASE PREFERRED STOCK 

 NOTICE OF EXERCISE 

TO: X4 PHARMACEUTICALS, INC 
 (1) ☐ The
undersigned hereby elects to purchase ________ shares of ___________ (the “Exercise Shares”) of X4 PHARMACEUTICALS, INC. (the “Company”) pursuant to the terms of the attached Warrant, and
tenders herewith payment of the exercise price in full, together with all applicable transfer taxes, if any. 
 ☐ The undersigned
hereby elects to purchase ________ shares of __________ (the “Exercise Shares”) of X4 PHARMACEUTICALS, INC (the “Company”) pursuant to the terms of the net exercise provisions set forth in
Section 2.1 of the attached Warrant, and shall tender payment of all applicable transfer taxes, if any. 
 (2) Please issue a certificate or
certificates representing said Exercise Shares in the name of the undersigned or in such other name as is specified below: 
  

 
 (Name) 

 
  

 
  

(Address) 
 (3) The undersigned represents
that (i) the aforesaid Exercise Shares are being acquired for the account of the undersigned for investment and not with a view to, or for resale in connection with, the distribution thereof and that the undersigned has no present intention of
distributing or reselling such shares; (ii) the undersigned is aware of the Company’s business affairs and financial condition and has acquired sufficient information about the Company to reach an informed and knowledgeable decision
regarding its investment in the Company; (iii) the undersigned is experienced in making investments of this type and has such knowledge and background in financial and business matters that the undersigned is capable of evaluating the merits
and risks of this investment and protecting the undersigned’s own interests; (iv) the undersigned understands that Exercise Shares issuable upon exercise of this Warrant have not been registered under the Securities Act of 1933, as amended
(the “Securities Act”), by reason of a specific exemption from the registration provisions of the Securities Act, which exemption depends upon, among other things, the bona fide nature of the investment intent as expressed
herein, and, because such securities have not been registered under the Securities Act, they must be held indefinitely unless subsequently registered under the Securities Act or an exemption from such registration is available; (v) the
undersigned is aware that the aforesaid Exercise Shares may not be sold pursuant to Rule 144 adopted under the Securities Act unless certain conditions are met and until the undersigned has held the shares for the number of years prescribed by Rule
144, that among the conditions for use of the Rule is the availability of current information to the public about the Company and the Company has not made such information available and has no present plans to do so; and (vi) the undersigned
agrees not to make any disposition of all or any part of the aforesaid shares of Exercise Shares unless and until there is then in effect a registration statement under the Securities Act covering such proposed disposition and such disposition is
made in accordance with said registration statement, or, if reasonably requested by the Company, the undersigned has provided the Company with an opinion of counsel satisfactory to the Company, stating that such registration is not required. 

  
 A-1 

 (4) By execution and delivery of this notice, the undersigned hereby agrees to become a party to
(i) that certain Amended and Restated Investors’ Rights Agreement dated as of August 14, 2015 (the “Investors’ Rights Agreement”) by and among the Company and the Investors (as defined in the
Investors’ Rights Agreement), (ii) that certain Amended and Restated Voting Agreement dated as of August 14, 2015 (the “Voting Agreement”) by and among the Company and the Investors (as defined in the Voting
Agreement) and (iii) that certain Amended and Restated Right of First Refusal and Co-Sale Agreement, dated as of August 14, 2015 (the “ROFR Agreement”) by and among the
Company and the Investors and Key Holders (as defined in the ROFR Agreement) and the undersigned is entitled to all of the benefits under and subject to all of the obligations, restrictions and limitations set forth in the Investors’ Rights
Agreement, Voting Agreement and ROFR Agreement that are applicable to the Investors. This notice shall take effect and shall become a part of said Investors’ Rights Agreement, Voting Agreement and ROFR Agreement immediately upon execution. 

 

					
	 	 		 	 
	 (Date)
	 		 	 (Signature)

		 		 	
		 		 	 (Print name)

  
 2.EX-4.4

 Exhibit 4.4 

THIS WARRANT AND THE SECURITIES ISSUABLE UPON THE EXERCISE HEREOF HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED. THEY MAY NOT BE
SOLD, OFFERED FOR SALE, PLEDGED, HYPOTHECATED, OR OTHERWISE TRANSFERRED EXCEPT PURSUANT TO AN EFFECTIVE REGISTRATION STATEMENT UNDER THE SECURITIES ACT OF 1933, AS AMENDED OR AN OPINION OF COUNSEL SATISFACTORY TO THE COMPANY THAT REGISTRATION IS NOT
REQUIRED. 
 X4 PHARMACEUTICALS, INC. 

WARRANT TO PURCHASE SERIES B PREFERRED STOCK 
  

			
	No. BW-            	  	For the Purchase
	Issue Date:                     , 2017	  	of _____ shares of
		  	Series B Preferred Stock

 Void after
                    , 2020 

THIS WARRANT TO PURCHASE SERIES B PREFERRED STOCK (this “Warrant”) is issued to [_________] (the “Holder”)
by X4 Pharmaceuticals, Inc. (the “Company”), as of November 1, 2017 pursuant to the terms of the Series B Preferred Stock Purchase Agreement, dated November 1, 2017 by and among the Company and the Investors therewith (the
“Purchase Agreement”). 
 1. Purchase of Exercise Shares. Subject to the terms and conditions hereinafter set forth,
the Holder is entitled, upon surrender of this Warrant at the principal office of the Company (or at such other place as the Company shall notify the holder hereof in writing), to purchase from the Company the Exercise Shares at the Exercise
Price (each subject to adjustment as provided herein) at any time or from time to time at or before the earlier of 5:00 p.m. eastern time on the Expiration Date (as defined below). This Warrant shall be exercisable, in whole or in part, prior to the
Expiration Date. 
 2. Definitions. 

(a) “Conversion Shares” shall mean the shares of Common Stock issuable upon conversion of the Exercise Shares. 

(b) “Exercise Price” shall mean $1.88 per Exercise Share, subject to adjustment pursuant to
Section 7 below. 
 (c) “Exercise Shares” shall mean [_______] shares1 of the Company’s Series B Preferred Stock issuable upon exercise of this Warrant, subject to adjustment pursuant to Section 7 below. The Exercise Shares
shall have the rights and preferences associated with them as set forth in the Restated Certificate. 
  

	1 	 10% of the Series B stock purchased. 

 (d) “Expiration Date” shall mean November 1, 2020. 

(e) “Majority of the Conversion Shares” shall mean a majority of the Conversion Shares issuable upon exercise and
conversion of all the Warrants issued pursuant to the Purchase Agreement. 
 (f) “Series B Preferred Stock” shall
mean the Company’s Series B Preferred Stock, par value $0.001 per share. 
 (g) “Shares” shall mean the
Exercise Shares and Conversion Shares. 
 (h) “Restated Certificate” shall mean the Third Amended and Restated
Certificate of Incorporation of the Company, as amended and/or amended and restated from time to time. 
 3. Exercise of Warrant.
While this Warrant remains outstanding and exercisable, the Holder may exercise, in whole or in part, the purchase rights evidenced hereby. Such exercise shall be effected by: (a) the surrender of the Warrant, together with a notice of exercise
to the Secretary of the Company at its principal offices; and (b) the payment to the Company of an amount equal to the aggregate Exercise Price for the number of Exercise Shares being purchased. Payment of the purchase price shall be in cash or
by certified or official bank check payable to the order of the Company 
 4. Net Exercise. Notwithstanding any provisions herein to
the contrary, in lieu of exercising this Warrant by delivering payment of the purchase price to the Company as provided in Section 3(b), the Holder may elect to receive shares equal to the value of this Warrant (or the
portion thereof being canceled) by surrender of this Warrant at the principal office of the Company together with notice of such election at any time or from time to time prior to the Expiration Date, in which event the Company shall issue to the
holder hereof a number of Exercise Shares computed using the following formula: 
         Y (A -
B) 
 X =             A 

Where 
  

	 	X —	 the number of Exercise Shares to be issued to the Holder. 

 

	 	Y —	 The number of Exercise Shares purchasable under this Warrant. 

 

	 	A —	 The fair market value of one Exercise Share as of the date of exercise. 

 

	 	B —	 The Exercise Price (as adjusted to the date of such calculations). 

  
 -2- 

 For purposes of the above calculation, the fair market value of one Exercise Share as of a
particular date (the “Determination Date”) shall mean: 
 (A) In the event that all of the
Company’s shares of Preferred Stock shall prior to exercise or exchange of this Warrant have been converted into shares of the Common Stock of the Company, $0.0001 par value per share (the “Common Stock”), as a result of
the conversion of all of the issued and outstanding shares of Preferred Stock in connection with the closing of a firm commitment underwritten public offering (as such term is described in the Restated Certificate), then the fair market value per
share of each Exercise Share shall be determined by taking the product of “x” and “y,” with “x” equal to the number of shares of Common Stock into which one share of the Series B Preferred Stock, could be converted on
the Determination Date, and “y” equal to the fair market value of Common Stock on the Determination Date. For purposes of making this calculation, the fair market value of the Company’s Common Stock shall be determined as follows:

 (1) If the Common Stock is traded on an exchange or is quoted on the NASDAQ Stock Market (“NASDAQ”), then the
closing price on the day before the Determination Date; 
 (2) If Common Stock is not traded on an exchange or on the NASDAQ
but is traded in the over-the-counter market, then the closing price on the day before the Determination Date; or 

(3) If the Determination Date is the date on which the Common Stock is first sold to the public by the Company in a firm
commitment public offering under the Securities Act of 1933, as amended (the “1933 Act”), then the initial public offering price (before deducting commissions, discounts or expenses) at which the Common Stock is sold in such
offering; 
 (B) In the event that the Determination Date is the date of a liquidation, dissolution or winding up, or any
Deemed Liquidation Event (as defined in the Restated Certificate), then the fair market value per share of each Exercise Share shall be determined by aggregating all amounts to be payable per share to holders of the Exercise Shares in the event of
such liquidation, dissolution or winding up or Deemed Liquidation Event, plus all other amounts to be payable per share in respect of the Exercise Shares in liquidation, assuming for the purposes of this subsection that all of the Exercise Shares
issuable upon exercise of all of the Warrants are outstanding at the Determination Date; or 
 (C) In all other cases, the
fair market value per share of the Exercise Shares shall be determined in good faith by the unanimous approval of Company’s Board of Directors upon review of relevant factors. 

5. Certificates for Shares. Upon the exercise of the purchase rights evidenced by this Warrant, one or more certificates for the number
of Exercise Shares (or Conversation Shares, as the case may required) so purchased shall be issued as soon as practicable thereafter, and in any event within thirty (30) days of the delivery of the Notice of Exercise. 

  
 -3- 

 6. Issuance of Shares. The Company covenants that the Exercise Shares, when issued
pursuant to the exercise of this Warrant, and the Conversion Shares, when issued pursuant to the conversion of the Exercise Shares and the terms and conditions of the Restated Certificate will be duly and validly issued, fully paid and nonassessable
and free from all taxes, liens, and charges with respect to the issuance thereof. 
 7. Adjustment of Exercise Price and Number of
Shares. The number of and kind of Exercise Shares purchasable upon exercise of this Warrant and the Exercise Price shall be subject to adjustment from time to time as follows (it being agreed that similar adjustments with respect to the number
and kind of Conversion Shares receivable upon conversion of the Exercise Shares and the conversion price of the Conversion Shares shall be subject to adjustment from time to time as provided for in the Restated Certificate): 

(a) Subdivisions, Combinations and Other Issuances. If the Company shall at any time prior to the expiration of this Warrant subdivide
the Exercise Shares, by split-up or otherwise, or combine its Exercise Shares, or issue additional shares of Exercise Shares as a dividend, the number of Exercise Shares issuable on the exercise of this
Warrant shall forthwith be proportionately increased in the case of a subdivision or stock dividend, or proportionately decreased in the case of a combination. Appropriate adjustments shall also be made to the purchase price payable per share, but
the aggregate purchase price payable for the total number of Exercise Shares purchasable under this Warrant (as adjusted) shall remain the same. Any adjustment under this Section 7(a) shall become effective at the close of
business on the date the subdivision or combination becomes effective, or as of the record date of such dividend, or in the event that no record date is fixed, upon the making of such dividend. 

(b) Reclassification, Reorganization and Consolidation. In the event of changes in the series or class of equity securities of the
Company comprising the Exercise Shares by reason of stock dividends, splits, recapitalizations, reclassifications, combinations or exchanges of shares, separations, reorganizations, liquidations, or the like, the number and series or class of
Exercise Shares available under the Warrant in the aggregate and the Exercise Price shall be correspondingly adjusted to give the Holder of the Warrant, on exercise for the same Aggregate Exercise Price, the total number, class, and kind of shares
as the Holder would have owned had the Warrant been exercised prior to the event and had the Holder continued to hold such shares until after the event requiring adjustment; provided, however, that such adjustment shall not be made
with respect to, and this Warrant shall terminate if not exercised prior to, the events set forth in Section 7 below. For purposes of this Section 7, the “Aggregate Exercise
Price” shall mean the aggregate Exercise Price payable in connection with the exercise in full of this Warrant. 
 (c)
Merger, Consolidation or Sale of Assets. If there shall be a merger or consolidation of the Company with or into another corporation (other than a merger or reorganization involving only a change in the state of incorporation of the Company
or the acquisition by the Company of other businesses where the Company survives as a going concern), or the sale of all or substantially all of the Company’s capital stock or assets to any 

  
 -4- 

 
other person, then as a part of such transaction, provision shall be made so that the Holder shall thereafter be entitled to receive the number of shares of stock or other securities or property
(including cash) of the Company, or of the successor corporation resulting from the merger, consolidation or sale, to which the Holder would have been entitled if the Holder had exercised its rights pursuant to this Warrant immediately prior
thereto. In any such case, appropriate adjustment shall be made in the application of the provisions of this Section 7(c) to the end that the provisions of this Section 7(c) shall be applicable
after that event in as nearly equivalent a manner as may be practicable 
 (d) Notice of Adjustment; Expiration. When any adjustment
is required to be made in the number or kind of shares purchasable upon exercise of the Warrant or conversion of the Exercise Shares or in the Exercise Price, the Company shall promptly notify the holder of such event and of the number of Exercise
Shares, Conversion Shares or other securities or property thereafter purchasable upon exercise of this Warrant, as applicable. 
 (e)
Adjustment Pursuant to Restated Certificate. If the Series B Preferred Stock shall be adjusted pursuant to the Restated Certificate in a manner not set forth above in clauses (a) through (d) of this Section 7,
the Exercise Shares shall be adjusted in the same manner as the Series B Preferred Stock in accordance with the Restated Certificate. 
 8.
No Fractional Shares or Scrip. No fractional shares or scrip representing fractional shares shall be issued upon the exercise of this Warrant, but in lieu of such fractional shares the Company shall make a cash payment therefor on the basis
of the Exercise Price then in effect. 
 9. Representations of the Company. The Company represents that all corporate actions on the
part of the Company, its officers, directors and stockholders necessary for the sale and issuance of this Warrant, the Exercise Shares and the Conversion Shares have been taken. 

10. Restrictive Legend. 

The Shares (unless registered under the 1933 Act) shall be stamped or imprinted with a legend in substantially the following form: 

THE SECURITIES REPRESENTED HEREBY HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AND HAVE BEEN ACQUIRED FOR INVESTMENT AND NOT WITH
A VIEW TO, OR IN CONNECTION WITH, THE SALE OR DISTRIBUTION THEREOF. NO SUCH TRANSFER MAY BE EFFECTED WITHOUT AN EFFECTIVE REGISTRATION STATEMENT RELATED THERETO OR AN OPINION OF COUNSEL IN A FORM SATISFACTORY TO THE COMPANY THAT SUCH REGISTRATION IS
NOT REQUIRED UNDER THE SECURITIES ACT OF 1933, AS AMENDED. 
 11. Warrants Transferable. With respect to any offer, sale or other
disposition of this Warrant, Holder will give written notice to the Company prior thereto, describing briefly the manner thereof, together with a written opinion of Holder’s counsel, or other evidence if reasonably satisfactory to the Company,
to the effect that such offer, sale or other distribution 

  
 -5- 

 
may be effected without registration or qualification (under any federal or state law then in effect). Upon receiving such written notice and reasonably satisfactory opinion, if so requested, or
other evidence, the Company, as promptly as practicable, shall notify Holder that Holder may sell or otherwise dispose of this Warrant in accordance with the terms of the notice delivered to the Company. If a determination has been made pursuant to
this Section 11 that the opinion of counsel for Holder, or other evidence, is not reasonably satisfactory to the Company, the Company shall so notify Holder promptly after such determination has been made. Each Warrant thus
transferred shall bear a legend as to the applicable restrictions on transferability in order to ensure compliance with the 1933 Act, unless in the opinion of counsel for the Company such legend is not required in order to ensure compliance with the
1933 Act. The Company may issue stop transfer instructions to its transfer agent in connection with such restrictions. Subject to the foregoing transfers of this Warrant shall be registered upon registration books maintained for such purpose by or
on behalf of the Company. 
 12. Early Termination. In the event of, at any time prior to the Expiration Date: (a) the
consummation of an initial public offering of securities of the Company registered under the 1933 Act, (b) the automatic conversion of the Series B Preferred Stock pursuant to Section 5 of the Restated Certificate or (c) a Deemed
Liquidation Event, the Company shall provide to the Holder fifteen (15) days advance written notice of the estimated closing date of such public offering, the effective date of such automatic conversion of the Series B Preferred Stock or the
closing of such Deemed Liquidation Event. Unless the Holder exercises this Warrant pursuant to the terms of Section 3 within five (5) days of the receipt of such advance written notice, immediately prior to the date
such public offering is closed, the effective date of such automatic conversion of the Series B Preferred Stock or the closing of such Deemed Liquidation Event, this Warrant shall (i) be automatically deemed exercised in full pursuant to
Section 4 if the fair market value of each Exercise Share is greater than the Exercise Price or (ii) be automatically terminated if the fair market value each Exercise Share is equal to or less than the Exercise Price.

 13. Market Stand-Off Agreement. The Holder hereby agrees that it will not, without the
prior written consent of the managing underwriter, during the period beginning on and including the date that the Company’s Common Stock is first quoted on a national exchange and continuing through the close of trading on the date that is 180
days thereafter, or such other period as may be requested by the Company or an underwriter to comply with regulatory restrictions on (1) the publication or other distribution of research reports and (2) analyst recommendations and options,
including, but not limited to, the restrictions contained in FINRA Rule 2711(f)(4) or NYSE Rule 472(f)(4), or any successor provisions or amendments thereto), (i) lend; offer; pledge; sell; contract to sell; sell any option or contract to purchase;
purchase any option or contract to sell; grant any option, right, or warrant to purchase; or otherwise transfer or dispose of, directly or indirectly, any shares of Common Stock or any securities convertible into or exercisable or exchangeable
(directly or indirectly) for Common Stock (whether such shares or any such securities are then owned by the Holder or are thereafter acquired) or (ii) enter into any swap or other arrangement that transfers to another, in whole or in part, any
of the economic consequences of ownership of such securities, whether any such transaction described in clause (i) or (ii) above is to be settled by delivery of Common Stock or other securities, in cash, or otherwise. The foregoing provisions
of this Section shall apply only to an initial public offering, shall not apply to the sale of any shares to an underwriter pursuant to an underwriting agreement, or the transfer of any shares to any trust for the direct or indirect benefit of the
Holder or the 

  
 -6- 

 
immediate family of the Holder, provided that the trustee of the trust agrees to be bound in writing by the restrictions set forth herein, and provided further that any such transfer shall not
involve a disposition for value (an “Estate Planning Transfer”), and shall be applicable to the Holder only if all officers and directors are subject to the same restrictions and the Company uses commercially reasonable efforts to obtain a
similar agreement from all stockholders individually owning more than one percent (1%) of the Company’s outstanding Common Stock (after giving effect to conversion into Common Stock of all outstanding Preferred Stock). The underwriters in
connection with such registration are intended third party beneficiaries of this Section and shall have the right, power, and authority to enforce the provisions hereof as though they were a party hereto. Each Holder further agrees to execute such
agreements as may be reasonably requested by the underwriters in connection with such registration that are consistent with this Section or that are necessary to give further effect thereto. Any discretionary waiver or termination of the
restrictions of any or all of such agreements by the Company or the underwriters shall apply pro rata to all Holders subject to such agreements, based on the number of shares subject to such agreements. Holder agrees that any transferee of the
Warrant (or other securities) of the Company held by Holder shall be bound by this Section 13. 
 14. Rights of
Stockholders. The Holder shall not be entitled, as a Warrant holder, to vote or receive dividends or be deemed the holder of the Exercise Shares or any other securities of the Company which may at any time be issuable on the exercise hereof for
any purpose, nor shall anything contained herein be construed to confer upon the Holder, as such, any of the rights of a stockholder of the Company or any right to vote for the election of directors or upon any matter submitted to stockholders at
any meeting thereof, or to give or withhold consent to any corporate action (whether upon any recapitalization, issuance of stock, reclassification of stock, change of par value, consolidation, merger, conveyance, or otherwise) or to receive notice
of meetings, or to receive dividends or subscription rights or otherwise until the Warrant shall have been exercised and the Exercise Shares purchasable upon the exercise hereof shall have become deliverable, as provided herein. 

15. Taxes. The issuance of the Exercise Shares upon the exercise of this Warrant, and the delivery of certificates or other instruments
representing such Exercise Shares, shall be made without charge to the Holder for any tax or other charge of whatever nature in respect of such issuance and the Company shall bear any such taxes in respect of such issuance. 

16. Lost Warrant, etc. Upon receipt by the Company of evidence reasonably satisfactory to the Company of the loss, theft, destruction
or mutilation of this Warrant, and, in the case of loss, theft or destruction, of any indemnification undertaking by the Holder to the Company in customary form and, in the case of mutilation, upon surrender and cancellation of this Warrant, the
Company shall execute and deliver to the Holder a new Warrant representing the right to purchase the Exercise Shares then underlying this Warrant. Whenever the Company is required to issue a new Warrant pursuant to the terms of this Warrant, such
new Warrant (i) shall be of like tenor with this Warrant, (ii) shall represent, as indicated on the face of such new Warrant, the right to purchase the Exercise Shares then underlying this Warrant (or in the case of a new Warrant being
issued pursuant to Section 11, the Exercise Shares designated by the Holder which, when added to the number of Exercise Shares underlying the other new Warrants issued in connection with such issuance, does not exceed the number of Exercise
Shares then underlying this Warrant), (iii) shall have an issuance date, as indicated on the face of such new Warrant, which is the same as the issuance date of this Warrant, and (iv) shall have the same rights and conditions as this Warrant.

  
 -7- 

 17. Notices. All notices and other communications given or made pursuant to this
Agreement shall be in writing and shall be deemed effectively given upon the earlier of actual receipt, or (a) personal delivery to the party to be notified, (b) when sent, if sent by electronic mail or facsimile during normal business
hours of the recipient, and if not sent during normal business hours, then on the recipient’s next business day, (c) five (5) days after having been sent by registered or certified mail, return receipt requested, postage prepaid, or
(d) one (1) business day after deposit with a nationally recognized overnight courier, freight prepaid, specifying next business day delivery, with written verification of receipt, and shall be addressed (i) if to the Holder, at the
Holder’s address as set forth on the Schedule of Investors to the Purchase Agreement, and (ii) if to the Company, at 955 Massachusetts Avenue, Cambridge, Massachusetts, 02139, or at such other address as a party may designate by ten days
advance written notice to the other party pursuant to the provisions above. 
 18. Waiver and Amendment. Any term of this Warrant may
be amended or waived with the written consent of the Company and Holders of at least a Majority of the Conversion Shares, provided that all Warrants are similarly affected. Upon the effectuation of such amendment or waiver in conformance with
this Section 16, the Company shall promptly give written notice thereof to the record holders of the Warrants who have not previously consented thereto in writing. 

19. Governing Law. This Warrant and all actions arising out of or in connection with this Agreement shall be governed by and construed
in accordance with the laws of the State of Delaware, without regard to the conflicts of law provisions of the State of Delaware or of any other state. 

20. Rights and Obligations Survive Exercise of Warrant. Unless otherwise provided herein, the rights and obligations of the Company, of
the Holder and of the holder of the Exercise Shares issued upon exercise of this Warrant, shall survive the exercise of this Warrant. 
 21.
Severability. If one or more provisions of this Warrant are held to be unenforceable under applicable law, such provision(s) shall be excluded from this Warrant and the balance of this Warrant shall be interpreted as if such provision(s) were
so excluded and shall be enforceable in accordance with its terms. 
 [Signature Page Follows] 

  
 -8- 

 The Company has caused this Warrant to be issued as of the date first written above. 

 

			
	X4 PHARMACEUTICALS, INC.
		
	By: 	 	 
		 	Paula Ragan, Ph.D., President and
		 	Chief Executive Officer

 [Signature Page to Series B Preferred Stock Warrant] 

 EXHIBIT A 

NOTICE OF EXERCISE 
  

	TO:	 X4 PHARMACEUTICALS, INC. 

	 	 Attention: Chief Executive Officer 

1. The undersigned hereby elects to purchase __________ shares of ____________ pursuant to the terms of the attached Warrant (the
“Shares”). 
 2. Method of Exercise (Please initial the applicable blank): 

 

	 	—	 The undersigned elects to exercise the attached Warrant by means of a cash payment, and tenders herewith
payment in full for the purchase price of the shares being purchased. 

  

	 	—	 The undersigned elects to exercise the attached Warrant by means of the net exercise provisions of
Section 4 of the Warrant. 

 3. Please issue a certificate or certificates representing said
Shares in the name of the undersigned or in such other name as is specified below: 
  

 
 (Name) 

 
  

 
  

(Address) 
 4. The undersigned
hereby represents and warrants that the aforesaid Shares are being acquired for the account of the undersigned for investment and not with a view to, or for resale, in connection with the distribution thereof, and that the undersigned has no present
intention of distributing or reselling such shares and all representations and warranties of the undersigned set forth in Sections 3 of the Purchase Agreement (as defined in the Warrant) are true and correct as of the date hereof. 

 

					
		  		  	(Signature)
			
		  		  	
		  		  	(Name)
			
		  		  	
	(Date)	  		  	(Title)

 EXHIBIT B 

FORM OF TRANSFER 
 (To be
signed only upon transfer of Warrant) 
 FOR VALUE RECEIVED, the undersigned hereby sells, assigns and transfers unto
_______________________________________________ the right represented by the attached Warrant to purchase ____________ shares of ________________________ of X4 Pharmaceuticals, Inc. to which the attached Warrant relates, and appoints ______________
Attorney to transfer such right on the books of __________, with full power of substitution in the premises. 
 Dated: ____________________ 

 

			
	(Signature must conform in all respects to name of Holder as specified on the face of the Warrant)
		
	Address:	 	   

	  
	 	   

	  
	 	   

  

	
	Signed in the presence of:

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