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                                                                     Exhibit 4.4

THIS WARRANT AND THE SHARES ISSUABLE HEREUNDER HAVE NOT BEEN REGISTERED UNDER
THE SECURITIES ACT OF 1933, AS AMENDED OR ANY APPLICABLE STATE SECURITIES LAW,
AND MAY NOT BE SOLD, PLEDGED, OR OTHERWISE TRANSFERRED WITHOUT AN EFFECTIVE
REGISTRATION THEREOF UNDER SUCH ACT OR PURSUANT TO RULE 144 AND AN EXEMPTION
UNDER APPLICABLE STATE LAW OR AN OPINION OF COUNSEL REASONABLY SATISFACTORY TO
THE CORPORATION AND ITS COUNSEL, THAT SUCH REGISTRATION IS NOT REQUIRED.

                            WARRANT TO PURCHASE STOCK

Corporation:                    PTC Therapeutics, Inc., a Delaware corporation
Number of Shares:               26,000
Class of Stock:                 Common
Initial Exercise Price:         $2.50 per share
Issue Date:                     March 15, 2001
Expiration Date:                March 14, 2008

      THIS WARRANT CERTIFIES THAT, for the agreed upon value of $1.00 and for
other good and valuable consideration, SILICON VALLEY BANK ("Holder") is
entitled to purchase the number of fully paid and nonassessable shares of the
class of securities (the "Shares") of the corporation (the "Company") at the
initial exercise price per Share (the "Warrant Price") all as set forth above
and as adjusted pursuant to Article 2 of this Warrant, subject to the provisions
and upon the terms and conditions set forth in this Warrant.

ARTICLE 1. EXERCISE.

            1.1 Method of Exercise. Holder may exercise this Warrant by
delivering a duly executed Notice of Exercise in substantially the form attached
as Appendix 1 to the principal office of the Company. Unless Holder is
exercising the conversion right set forth in Section 1.2, Holder shall also
deliver to the Company a check for the aggregate Warrant Price for the Shares
being purchased.

            1.2 Conversion Right. In lieu of exercising this Warrant as
specified in Section 1.1, Holder may from time to time convert this Warrant, in
whole or in part, into a number of Shares determined by dividing (a) the
aggregate fair market value of the Shares or other securities otherwise issuable
upon exercise of this Warrant minus the aggregate Warrant Price of such Shares
by (b) the fair market value of one Share. The fair market value of the Shares
shall be determined pursuant Section 1.3.

            1.3 Fair Market Value. If the Shares are traded in a public market,
the fair market value of the Shares shall be the closing price of the Shares (or
the closing price of the Company's stock into which the Shares are convertible)
reported for the business day immediately before Holder delivers its Notice of
Exercise to the Company. If the Shares are not

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traded in a public market, the Board of Directors of the Company shall determine
fair market value in its reasonable good faith judgment.

            1.4 Delivery of Certificate and New Warrant. Promptly after Holder
exercises or converts this Warrant, the Company shall deliver to Holder
certificates for the Shares acquired and, if this Warrant has not been fully
exercised or converted and has not expired, a new Warrant representing the
Shares not so acquired.

            1.5 Replacement of Warrants. On receipt 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, on delivery of an
indemnity agreement reasonably satisfactory in form and amount to the Company
or, in the case of mutilation, or surrender and cancellation of this Warrant,
the Company shall execute and deliver, in lieu of this Warrant, a new warrant of
like tenor.

            1.6 Assumption on Sale, Merger, or Consolidation of the Company.

                  1.6.1 "Acquisition". For the purpose of this Warrant,
"Acquisition" means any sale, license, or other disposition of all or
substantially all of the assets of the Company, or any reorganization,
consolidation, or merger of the Company where the holders of the Company's
securities before the transaction beneficially own less than 50% of the
outstanding voting securities of the surviving entity after the transaction.

                  1.6.2 Assumption of Warrant. Upon the closing of any
Acquisition, the successor entity shall assume the obligations of this Warrant,
and this Warrant shall be exercisable for the same securities, cash, and
property as would be payable for the Shares issuable upon exercise of the
unexercised portion of this Warrant as if such Shares were outstanding on the
record date for the Acquisition and subsequent closing. The Initial Exercise
Price and/or number of Shares shall be adjusted accordingly.

ARTICLE 2. ADJUSTMENTS TO THE SHARES.

            2.1 Stock Dividends, Splits, Etc. If the Company declares or pays a
dividend on its common stock (or the Shares if the Shares are securities other
than common stock) payable in common stock, or other securities or if the
Company, subdivides the outstanding common stock into a greater amount of common
stock, or, if the Shares are securities other than common stock, subdivides the
Shares in a transaction that increases the amount of common stock into which the
Shares are convertible, then upon exercise of this Warrant, for each Share
acquired, Holder shall receive, without cost to Holder, the total number and
kind of securities to which Holder would have been entitled had Holder owned the
Shares of record as of the date the dividend or subdivision occurred. If the
outstanding shares are combined or consolidated, by reclassification or
otherwise, into a lesser number of shares, the Initial Exercise Price shall be
proportionately increased.

            2.2 Reclassification, Exchange, Combinations or Substitution. Upon
any reclassification, exchange, substitution, or other event that results in a
change of the number and/or class of the securities issuable upon exercise or
conversion of this Warrant, Holder shall be entitled to receive, upon exercise
or conversion of this Warrant, the number and kind of

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securities and property that Holder would have received for the Shares if this
Warrant had been exercised immediately before such reclassification, exchange,
substitution, or other event. Such an event shall include any automatic
conversion of the outstanding or issuable securities of the Company of the same
class or series as the Shares to common stock pursuant to the terms of the
Company's Articles of Incorporation upon the closing of a registered public
offering of the Company's common stock. The Company or its successor shall
promptly issue to Holder a new Warrant for such new securities or other
property. The new Warrant shall provide for adjustments which shall be as nearly
equivalent as may be practicable to the adjustments provided for in this Article
2 including, without limitation, adjustments to the Initial Exercise Price and
to the number of securities or property issuable upon exercise of the new
Warrant. The provisions of this Section 2.2 shall similarly apply to successive
reclassifications, exchanges, substitutions, or other events.

            2.3 Adjustments for Diluting Issuances. The Warrant Price and the
number of Shares issuable upon exercise of this Warrant shall be subject to
adjustment, from time to time in the manner set forth in the Company's
Certificate of Incorporation. The provisions set forth for the Shares in the
Company's Certificate of Incorporation relating to the above in effect as of the
issue Date may not be amended, modified or waived, without the prior written
consent of Holder unless such amendment, modification or waiver affects Holder
in the same manner as they affect all other shareholders of the same series of
shares granted to the Holder.

            2.4 No Impairment. The Company shall not, by amendment of its
Articles of Incorporation or through a reorganization, transfer of assets,
consolidation, merger, dissolution, issue, or sale of securities or any other
voluntary action, avoid or seek to avoid the observance or performance of any of
the terms to be observed or performed under this Warrant by the Company, but
shall at all times in good faith assist in carrying out of all the provisions of
this Article 2 and in taking all such action as may be necessary or appropriate
to protect Holder's rights under this Article against impairment.

            2.5 Fractional Shares. No fractional Shares shall be issuable upon
exercise or conversion of the Warrant and the number of Shares to be issued
shall be rounded down to the nearest whole Share. If a fractional share interest
arises upon any exercise or conversion of the Warrant, the Company shall
eliminate such fractional share interest by paying Holder the amount computed by
multiplying the fractional interest by the fair market value of a full Share.

            2.6 Certificate as to Adjustments. Upon each adjustment of the
Warrant Price, the Company shall promptly notify Holder in writing, and, at the
Company's expense, promptly compute such adjustment, and furnish Holder with a
certificate of its Chief Financial Officer setting forth such adjustment and the
facts upon which such adjustment is based. The Company shall, upon written
request, furnish Holder a certificate setting forth the Warrant Price in effect
upon the date thereof and the series of adjustments leading to such Warrant
Price.

ARTICLE 3. REPRESENTATIONS AND COVENANTS OF THE COMPANY.

            3.1 Representations and Warranties. The Company represents and
warrants to the Holder as follows:

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                  (a) The initial Warrant Price referenced on the first page of
this Warrant is not greater than (i) the price per share at which the Shares
were last issued in an arms-length transaction in which at least $500,000 of the
Shares were sold and (ii) the fair market value of the Shares as of the date of
this Warrant.

                  (b) All Shares which may be issued upon the exercise of the
purchase right represented by this Warrant, and all securities, if any, issuable
upon conversion of the Shares, shall, upon issuance, be duly authorized, validly
issued, fully paid and nonassessable, and free of any liens and encumbrances
except for restrictions on transfer provided for herein or under applicable
federal and state securities laws.

                  (c) The Capitalization Table previously provided to Holder
remains true and complete as of the Issue Date.

            3.2 Notice of Certain Events. If the Company proposes at any time
(a) to declare any dividend or distribution upon its common stock, whether in
cash, property, stock, or other securities and whether or not a regular cash
dividend; (b) to offer for subscription pro rata to the holders of any class or
series of its stock any additional shares of stock of any class or series or
other rights; (c) to effect any reclassification or recapitalization of common
stock; (d) to merge or consolidate with or into any other corporation, or sell,
lease, license, or convey all or substantially all of its assets, or to
liquidate, dissolve or wind up; or (e) offer holders of registration rights the
opportunity to participate in an underwritten public offering of the company's
securities for cash, then, in connection with each such event, the Company shall
give Holder (1) at least 10 days prior written notice of the date on which a
record will be taken for such dividend, distribution, or subscription rights
(and specifying the date on which the holders of common stock will be entitled
thereto) or for determining rights to vote, if any, in respect of the matters
referred to in (c) and (d) above; (2) in the case of the matters referred to in
(c) and (d) above at least 10 days prior written notice of the date when the
same will take place (and specifying the date on which the holders of common
stock will be entitled to exchange their common stock for securities or other
property deliverable upon the occurrence of, such event); and (3) in the case of
the matter referred to in (e) above, the same notice as is given to the holders
of such registration rights.

            3.3 Registration Under Securities Act of 1933, as amended. If the
Company (i) initiates any public offering of its common stock within one year of
the Issue Date or (ii) amends its Second Amended and Restated Investor Rights
Agreement ("Investor Rights Agreement"), the Company agrees to amend the
Investor Rights Agreement so that the Shares shall be subject to the
registration rights set forth in the Investor Rights Agreement. Once the Shares
are subject to the Investor Rights Agreement, the provisions set forth in the
Investor Right Agreement relating to the above may not be amended, modified or
waived without the prior written consent of Holder unless such amendment,
modification or waiver affects Holder in the same manner as they affect all
other shareholders of the same series of shares granted to the Holder.

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ARTICLE 4. REPRESENTATIONS, WARRANTIES OF THE HOLDER. The holder represents and
warrants to the company as follows:

            4.1 Purchase for Own Account. Except for transfers to Holder's
affiliates, this Warrant and the securities to be acquired upon exercise of this
Warrant by the Holder will be acquired for investment for the Holder's account,
not as a nominee or agent, and not with a view to the public resale or
distribution within the meaning of the 1933 Act, and the Holder has no present
intention of selling, granting any participation in, or otherwise distributing
the same. If not an individual, the Holder also represents that the Holder has
not been formed for the specific purpose of acquiring this Warrant or the
Shares.

            4.2 Disclosure of Information. The Holder has received or has had
full access to all the information it considers necessary or appropriate to make
an informed investment decision with respect to the acquisition of this Warrant
and its underlying securities. The Holder further has had an opportunity to ask
questions and receive answers from the Company regarding the terms and
conditions of the offering of this Warrant and its underlying securities and to
obtain additional information (to the extent the Company possessed such
information or could acquire it without unreasonable effort or expense)
necessary to verify any information furnished to the Holder or to which the
Holder has access.

            4.3 Investment Experience. The Holder understands that the purchase
of this Warrant and its underlying securities involves substantial risk. The
Holder: (i) has experience as an investor in securities of companies in the
development stage and acknowledges that the Holder is able to fend for itself,
can bear the economic risk of such Holder's investment in this Warrant and its
underlying securities and has such knowledge and experience in financial or
business matters that the Holder is capable of evaluating the merits and risks
of its investment in this Warrant and its underlying securities and/or (ii) has
a preexisting personal or business relationship with the Company and certain of
its officers, directors or controlling persons of a nature and duration that
enables the Holder to be aware of the character, business acumen and financial
circumstances of such persons.

            4.4 Accredited Investor Status. The Holder is an "accredited
investor" within the meaning of Regulation D promulgated under the 1933 Act.

ARTICLE 5. MISCELLANEOUS.

            5.1 Term. This Warrant is exercisable in whole or in part at any
time and from time to time on or before the Expiration Date.

            5.2 Legends. This Warrant and the Shares (and the securities
issuable, directly or indirectly, upon conversion of the Shares, if any) shall
be imprinted with a legend in substantially the following form:

            THIS SECURITY HAS NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF
            1933, AS AMENDED OR UNDER ANY APPLICABLE STATE LAWS, AND MAY NOT BE
            SOLD, PLEDGED OR OTHERWISE TRANSFERRED WITHOUT AN EFFECTIVE
            REGISTRATION THERE OF

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            UNDER SUCH ACT AND AN EXEMPTION UNDER APPLICABLE STATE LAW OR
            PURSUANT TO RULE 144 OR AN OPINION OF COUNSEL REASONABLY
            SATISFACTORY TO THE CORPORATION AND ITS COUNSEL THAT SUCH
            REGISTRATION IS NOT REQUIRED.

            5.3 Compliance with Securities Laws on Transfer. This Warrant and
the Shares issuable upon exercise of this Warrant (and the securities issuable,
directly or indirectly, upon conversion of the Shares, if any) may not be
transferred or assigned in whole or in part without compliance with applicable
federal and state securities laws by the transferor and the transferee
(including, without limitation, the delivery of investment representation
letters and legal opinions reasonably satisfactory to the Company, as reasonably
requested by the Company). The Company shall not require Holder to provide an
opinion of counsel if the transfer is to an affiliate of Holder or if there is
no material question as to the availability of current information as referenced
in Rule 144(c), Holder represents that it has complied with Rule 144(d) and (e)
in reasonable detail, the selling broker represents that it has complied with
Rule 144(f), and the Company is provided with a copy of Holder's notice of
proposed sale.

            5.4 Transfer Procedure. Subject to the provisions of Section 5.3,
Holder may transfer all or part of this Warrant or the Shares issuable upon
exercise of this Warrant (or the securities issuable, directly or indirectly,
upon conversion of the Shares, if any) at any time to Silicon Valley Bancshares
or The Silicon Valley Bank Foundation, or to any affiliate of Holder, or, to any
other transferee by giving the Company notice of the portion of the Warrant
being transferred with the name, address and taxpayer identification number of
the transferee and surrendering this Warrant to the Company for reissuance to
the transferee(s) (and Holder if applicable). The Company may refuse to transfer
this Warrant to any person who directly competes with the Company, unless the
stock of the Company is publicly traded.

            5.5 Notices. All notices and other communications from the Company
to the Holder, or vice versa, shall be deemed delivered and effective when given
personally or mailed by first-class registered or certified mail, postage
prepaid, at such address as may have been furnished to the Company or the
Holder, as the case may be, in writing by the Company or such holder from time
to time. All notices to the Holder shall be addressed as follows:

            Silicon Valley Bank
            Attn:  Treasury Department
            3003 Tasman Drive, HG 110
            Santa Clara, CA 95054

            5.6 Waiver. This Warrant and any term hereof may be changed, waived,
discharged or terminated only by an instrument in writing signed by the party
against which enforcement of such change, waiver, discharge or termination is
sought.

            5.7 Attorney's Fees. In the event of any dispute between the parties
concerning the terms and provisions of this Warrant, the party prevailing in
such dispute shall be entitled to collect from the other party all costs
incurred in such dispute, including reasonable attorney's fees.

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            5.8 Governing Law. This Warrant shall be governed by and construed
in accordance with the laws of the State of New Jersey, without giving effect to
its principles regarding conflicts of law.

                                PTC THERAPEUTICS, INC.

                                By:    /s/ STUART PELTZ
                                   ---------------------------------------------
                                       Name: STUART PELTZ
                                       Title: Chief Executive Officer

                                "HOLDER"
                                SILICON VALLEY BANK

                                By:    /s/ ASH MILANI
                                   ---------------------------------------------
                                       Name: Ash Milani
                                       Title: Vice President

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                                   APPENDIX 1

                               NOTICE OF EXERCISE

      1. Holder elects to purchase _________________ shares of the Common/Series
_________________ Preferred [strike one] Stock of PTC Therapeutics, Inc.
pursuant to the terms of the attached Warrant, and tenders payment of the
purchase price of the shares in full.

      1. Holder elects to convert the attached Warrant into Shares/cash (strike
one] in the manner specified in the Warrant. This conversion is exercised for
_________________ of the Shares covered by the Warrant.

[Strike paragraph that does not apply.]

      2. Please issue a certificate or certificates representing the shares in
the name specified below:

                       _________________________________
                                  Holders Name

                       _________________________________

                       _________________________________
                                    (Address)

      3. The undersigned represents it is acquiring the shares solely for its
own account and not as a nominee for any other party and not with a view toward
the resale or distribution except in compliance with applicable securities laws.

                                       HOLDER:

                                       _________________________________________

                                       By:
                                             ___________________________________

                                       Name:
                                             ___________________________________

                                       Title:
                                             ___________________________________

____________________________________
               (Date)

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