Document:

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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

                                       2
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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.

                                       4
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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)

                                       8<PAGE>
                                                                     Exhibit 4.5

THIS WARRANT AND THE SHARES OF CAPITAL STOCK ISSUED UPON ANY EXERCISE HEREOF
HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED (THE
"SECURITIES ACT"), OR ANY APPLICABLE STATE SECURITIES LAWS AND MAY NOT BE SOLD
OR OTHERWISE TRANSFERRED BY ANY PERSON, INCLUDING A PLEDGEE, UNLESS (1) EITHER
(A) A REGISTRATION WITH RESPECT TO THERETO SHALL BE EFFECTIVE UNDER THE
SECURITIES ACT, OR (B) THE COMPANY SHALL HAVE RECEIVED AN OPINION OF COUNSEL
SATISFACTORY TO THE COMPANY THAT AN EXEMPTION FROM REGISTRATION UNDER THE
SECURITIES ACT IS AVAILABLE, AND (2) THERE SHALL HAVE BEEN COMPLIANCE WITH ALL
APPLICABLE STATE SECURITIES OR "BLUE SKY" LAWS.

                                     For the purchase of up to 507,692 shares of
                                 shares of Series D Convertible Preferred Stock,
                                                       par value $.001 per share

No. W-02                                                         August 17, 2001

      PTC THERAPEUTICS, INC., a Delaware corporation (the "Company"), hereby
certifies that, for good and valuable consideration, Three Crowns Capital
(Bermuda) Ltd. or its permitted assigns (the "Warrant Holder") is entitled,
subject to the terms set forth below, to purchase from the Company at any time
on or before 5:00 p.m., eastern time, on August 17, 2011 (the "Exercise
Period"), up to 507,692 fully-paid and non-assessable shares of Series D
Convertible Preferred Stock, par value $.001 per share, of the Company (the
"Series D Preferred Stock"), or shares of the Common Stock, par value $.001 per
share, of the Company (the "Common Stock") upon a mandatory conversion of the
Series D Preferred Stock as provided in Section 3 hereof (the Series D Preferred
Stock and the Common Stock are hereinafter collectively referred to as
"Shares"), at a price per share (the "Exercise Price") which shall initially be
$3.25 per share and which shall be subject to adjustment as herein provided.

      1. Exercise of Warrants.

      1.1. Exercise. Subject to the terms and conditions of this Warrant, this
Warrant shall become exercisable by the Warrant Holder on August 17, 2001.

      1.2. Procedure to Exercise. This Warrant may be exercised by the Warrant
Holder, in whole or in part, by surrendering this Warrant, with the purchase
form appended hereto as Exhibit A duly executed by such Warrant Holder or by
such Warrant Holder's duly authorized attorney, at the principal office of the
Company, or at such other office or agency as the Company may designate,
accompanied by payment in full by cash, check or wire transfer of the amount
obtained by multiplying the number of Shares in the notice of exercise by the
Exercise Price (the "Purchase Price").

      1.3. Net Issue Exercise. In lieu of exercising this Warrant in the manner
provided above in Section 1.2, the Warrant Holder may elect to receive shares
equal to the value of this Warrant (or the portion thereof being converted) by
surrender of this Warrant, in whole or in

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part, at the principal office of the Company together with the notice of
exercise attached hereto as Exhibit A, in which event the Company shall issue to
the Warrant Holder the number of Shares computed using the following formula (a
"Net Issue Exercise"):

                                  X =  Y(A - B)
                                       -------
                                          A

Where:     X =   The number of Shares to be issued to the Warrant Holder.

           Y =   The number of Shares purchasable under this Warrant (at the
                 date of such calculation) that are being converted, in whole
                 or in part, hereunder.

           A =   The Fair Market Value of one Share (at the date of such
                 calculation).

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

      For purposes of this Section 1.3, if any Shares of the Warrant Holder or
any other shareholder of the Company are registered or publicly traded, then the
Fair Market Value of one Share shall mean the average of the closing bid and
asked prices of the Common Stock quoted in the over the counter market summary
or the closing price quoted by the Nasdaq National Market or any exchange on
which the Common Stock is listed, whichever is applicable, as published in the
Western Edition of The Wall Street Journal for the business day prior to the
date of determination of fair market value. If the Shares are not traded on the
Nasdaq National Market or on an exchange, the Fair Market Value of one Share
shall be determined in good faith by the Company's Board of Directors.

      1.4. Effectiveness. Each exercise or Net Issue Exercise of this Warrant
shall be deemed to have been effected immediately prior to the close of business
on the day on which this Warrant shall have been surrendered to the Company as
provided in Sections 1.2 or 1.3 above. At such time, the person or persons in
whose name or names any certificates for the Shares shall be issuable upon such
exercise or Net Issue Exercise as provided in Section 2 below shall be deemed to
have become the holder or holders of record of the Shares represented by such
certificates.

      2. Delivery of Stock Certificates, Etc. As soon as practicable after the
exercise or Net Issue Exercise of this Warrant in full or in part, and in any
event within 10 days thereafter, the Company at its expense will cause to be
issued in the name of, and delivered to, the Warrant Holder, or as such Warrant
Holder (upon payment by such Warrant Holder of any applicable transfer taxes)
may direct (i) a certificate or certificates for the number of fully paid and
non-assessable Shares to which the Warrant Holder shall be entitled upon such
exercise or Net Issue Exercise, and (ii) in case such exercise or Net Issue
Exercise is in part only, a new warrant or warrants (dated the date hereof) of
like tenor, calling in the aggregate on the face or faces thereof for the number
of Shares equal (without giving effect to any adjustment therein) to the number
of Shares called for on the face of this Warrant minus the number of Shares
purchased by the Warrant Holder upon such exercise or Net Issuance Exercise as
provided in Sections 1.2 or 1.3 herein.

                                       2
<PAGE>

      3. Adjustments Upon Mandatory Conversion of Series D Preferred Stock. Upon
any mandatory conversion of the Series D Preferred Stock pursuant to the
Company's Certificate of Incorporation, as amended or amended and restated from
time to time, this Warrant shall cease to be exercisable for shares of Series D
Preferred Stock and shall become exercisable for that number of shares of Common
Stock into which the shares of Series D Preferred Stock purchasable hereunder
would have been convertible immediately prior to such mandatory conversion, and
such that payment of the Exercise Price, or any multiple thereof, shall entitle
the Warrant Holder to receive the number of shares of Common Stock as would have
been issued upon conversion of each share of Series D Preferred Stock
purchasable hereunder immediately prior to such mandatory conversion.

      4. Stock Splits, Stock Dividends and Combinations. If the Company at any
time subdivides the outstanding shares of Series D Preferred Stock, or issues a
stock dividend on the outstanding shares of Series D Preferred Stock, the
Exercise Price in effect immediately prior to such subdivision or the issuance
of such stock dividend shall be proportionately decreased, and the number of
Shares shall be proportionately increased, and if the Company at any time
combines the outstanding shares of Series D Preferred Stock, the Exercise Price
in effect immediately prior to such combination shall be proportionately
increased, and the number of shares shall be proportionately decreased,
effective at the close of business on the date of such subdivision, stock
dividend or combination, as the case may be. Upon any mandatory conversion of
the Series D Preferred Stock as provided in Section 3, each reference to Series
D Preferred Stock in this Section 4 shall be deemed to be Common Stock.

      5. Conversions; Reorganizations; Reclassifications; Merger; Sales. In case
of any capital reorganization or any reclassification of the capital stock of
the Company or in case of the consolidation or merger of the Company with or
into another-corporation or the conveyance of all or substantially all of the
assets of the Company to another corporation, this Warrant shall thereafter be
exercisable for the number of shares of stock or other securities or property to
which a holder of the number of Shares deliverable upon exercise of the Warrant
would have been entitled to upon such conversion, reorganization,
reclassification, consolidation, merger or conveyance and, in any such case,
appropriate adjustment as determined by the Board of Directors of the Company
shall be made in the application of the provisions herein set forth with respect
to the rights and interests thereafter of the Warrant Holder to the end that the
provisions set forth herein (including provisions with respect to changes in and
other adjustments of the Exercise Price and the number of Shares) shall
thereafter be applicable, as nearly as reasonably may be, in relation to any
shares of stock or other property thereafter deliverable upon the exercise of
the Warrant.

      6. Statement of Adjustment. Whenever the Exercise Price shall be adjusted
as provided herein, the Company shall promptly file with the Secretary of the
Company or at such other place as shall be designated by the Company, a
statement, signed by its chief financial officer, showing in detail the facts
requiring such adjustment, the Exercise Price in effect before and after such
adjustment and the kind and amount of shares of capital stock, securities or
other property thereafter to be received upon the exercise of this Warrant. The
Company shall also cause a copy of such statement to be sent in the manner
specified in Section 16.3 to the Warrant Holder.

                                       3
<PAGE>

      7. Notice of Adjustment. In the event the Company shall propose to take
any action of the types described in Sections 4 or 5, the Company shall give
notice to the Warrant Holder in the manner set forth in Section 16.3, which
notice shall specify the record date, if any, with respect to any such action
and the date on which such action is to take place. Such notice shall also set
forth such facts with respect thereto as shall be reasonably necessary to
indicate the effect of such action (to the extent such effect may be known at
the date of such notice) on the Exercise Price and the number, kind or class of
shares or other securities or property which shall be deliverable or purchasable
upon the occurrence of such action or deliverable upon the exercise hereof. In
the case of any action which would require the fixing of a record date, such
notice shall be given at least ten (10) days prior to the date so fixed, and in
case of all other actions, such notice shall be given at least twenty (20) days
prior to the taking of such proposed action. Failure to give such notice, or any
defect therein, shall not affect the legality or validity of any such action.

      8. Taxes. The Company shall pay all documentary, stamp or other
transactional taxes attributable to the issuance or delivery of shares of
capital stock of the Company upon the exercise or conversion hereof.

      9. No Fractional Shares. Each adjustment in the number of Shares
purchasable hereunder shall be calculated, to the nearest whole share with
fractional shares disregarded.

      10. Covenants as to Series D Preferred Stock and Common Stock. The Company
covenants and agrees that the shares of Series D Preferred Stock issuable
hereunder, and the Common Stock issuable upon conversion thereof, and the Common
Stock issuable hereunder, as the case may be, will, upon issuance in accordance
with the terms hereof, be validly issued and outstanding, fully paid and
nonassessable, with no personal liability attaching to the ownership thereof,
and free from all taxes, liens and charges with respect to the issuance thereof
imposed by or through the Company. The Company further covenants and agrees that
the Company will at all times have authorized and reserved, free from preemptive
rights imposed by or through the Company, a sufficient number of shares of
Series D Preferred Stock and Common Stock to provide for the exercise of the
rights represented by this Warrant. The Company further covenants and agrees
that if any shares of capital stock to be reserved for the purpose of the
issuance of shares upon the exercise of this Warrant require registration or
qualification with or approval by the Securities and Exchange Commission or any
state regulatory agency under Federal or state law before such shares may be
validly issued or delivered upon exercise, then the Company will in good faith
and as expeditiously as possible endeavor to secure such registration,
qualification or approval, as the case may be. Holders of Series D Preferred
Stock and Common Stock issuable upon exercise of this Warrant or conversion of
any such shares, as the case may be, shall be entitled to all the rights and
privileges of the Purchase Agreement and the Registration Rights Agreement dated
as of the date hereof between the Company and the Investors (as defined therein)
with respect to such shares.

      11. No Impairment. The Company will not, by amendment of its Certificate
of Incorporation or through any reorganization, transfer of assets,
consolidation, merger, dissolution, or any other similar voluntary action, avoid
or seek to avoid the observance or performance of any of the terms of this
Warrant, but will at all times in good faith assist in the carrying out of all
such terms and in the taking of all such action as may be necessary or
appropriate in order to protect the rights of the Warrant Holder against
impairment due to such event. Without limiting the generality of the foregoing,
the Company (a) will not increase the par value of any shares of stock
receivable on the exercise of the Warrant above the amount payable therefor on
such exercise, (b) will take all action that may be necessary or

                                       4
<PAGE>

appropriate in order that the Company may validly and legally issue fully paid
and nonassessable shares of stock, free from all taxes, liens and charges with
respect to the issue thereof, on the exercise of all of the Warrants from time
to time outstanding and (c) will not consolidate with or merge into any other
person or permit any such person to consolidate with or merge into the Company
(if the Company is not the surviving person), unless such other person shall
expressly assume in writing and will be bound by all the terms of this Warrant.

      12. Compliance with Securities Act.

      12.1. Unregistered Securities. The Warrant Holder acknowledges that this
Warrant and the Shares have not been registered under the Securities Act of
1933, as amended, and the rules and regulations thereunder, or any successor
legislation (the "Securities Act"), and agrees not to sell, pledge, distribute,
offer for sale, transfer or otherwise dispose of this Warrant or any Shares in
the absence of (i) an effective registration statement under the Securities Act
covering this Warrant or such Shares and registration or qualification of this
Warrant or such Shares under any applicable "blue sky" or state securities law
then in effect, or (ii) an opinion of counsel, satisfactory to the Company, that
such registration and qualification are not required. The Company may delay
issuance of the Shares until completion of any action or obtaining of any
consent, which the Company deems necessary under any applicable law (including
without limitation state securities or "blue sky" laws).

      12.2. Investment Representation. The Warrant Holder represents to the
Company that this Warrant is being acquired for the Warrant Holder's own account
and for the purpose of investment and not with a present view to, or for sale in
connection with, the distribution thereof, nor with any present intention of
distributing or selling the Warrant or Common Stock issuable upon exercise of
the Warrant. The Warrant Holder acknowledges that it has been afforded the
opportunity to meet with the management of the Company and to ask questions of,
and receive answers from, such management and the Company's counsel about the
business and affairs of the Company and concerning the terms and conditions of
the offering of this Warrant, and to obtain any additional information, to the
extent that the Company possessed such information or could acquire it without
unreasonable effort or expense, necessary to verify the accuracy of the
information otherwise obtained by or furnished to the Warrant Holder. The Holder
has received all information which the Warrant Holder considered necessary to
form a decision concerning the purchase of this Warrant, and no valid request to
the Company by the Warrant Holder hereof for information of any kind about the
Company has been refused or denied by the Company or remains unfulfilled as of
the date hereof. The Warrant Holder attests that it may be considered to be a
sophisticated investor, is familiar with the risks inherent in speculative
investments such as in the Company, has such knowledge and experience in
financial business matters that he is capable of evaluating the merits and risk
of the investment in this Warrant and the Shares, and is able to bear the
economic risk of the investment. The Warrant Holder confirms that he is an
"accredited investor" as such term is defined in rule 501(a) of the Securities
Act.

                                       5
<PAGE>

      12.3. Investment Letter. Without limiting the generality of Section 12.1,
unless the offer and sale of any shares of Shares shall have been effectively
registered under the Securities Act, the Company shall be under no obligation to
issue the Shares unless and until the Warrant Holder shall have executed an
investment letter in form and substance satisfactory to the Company, including a
warranty at the time of such exercise that the Warrant Holder is acquiring such
shares for its own account, for investment and not with a view to, or for sale
in connection with, the distribution of any such shares.

      12.4. Legend. Certificates delivered to the Warrant Holder pursuant to
Section 2 shall bear the following legend or a legend in substantially similar
form:

            "THE SECURITIES REPRESENTED BY THIS CERTIFICATE HAVE NOT BEEN
            REGISTERED UNDER THE SECURITIES ACT OF 1933 OR ANY OF THE SECURITIES
            LAWS. THESE SECURITIES HAVE BEEN ACQUIRED FOR INVESTMENT AND NOT
            WITH A VIEW TO DISTRIBUTION OR RESALE. SUCH SECURITIES MAY NOT BE
            OFFERED FOR SALE, SOLD, DELIVERED AFTER SALE, TRANSFERRED, PLEDGED
            OR HYPOTHECATED IN THE ABSENCE OF AN EFFECTIVE REGISTRATION
            STATEMENT COVERING SUCH SECURITIES UNDER THE SECURITIES ACT OF 1933
            AND ANY OTHER APPLICABLE SECURITIES LAWS, UNLESS THE HOLDER SHALL
            HAVE OBTAINED AN OPINION OF COUNSEL REASONABLY SATISFACTORY TO THE
            CORPORATION THAT SUCH REGISTRATION IS NOT REQUIRED."

      13. Transferability. Without the prior written consent of the Company, the
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. Any attempted transfer, assignment, pledge, hypothecation or
other disposition of the Warrant or of any rights granted hereunder contrary to
the provisions of this Section 13, or the levy of any attachment or similar
process upon the Warrant or such rights, shall be null and void.

      14. 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 in its discretion 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.

      15. Register of Warrants. The Company shall maintain, at the principal
office of the Company (or such other office as it may designate by notice to the
Warrant Holder), a register for the Warrants in which the Company shall record
the name and address of the person in whose name a Warrant has been issued, as
well as the name and address of each transferee and each prior owner of such
Warrant.

                                       6
<PAGE>
      16. Miscellaneous.

      16.1. Waivers and Amendments. This Warrant or any provisions hereof may be
changed, waived, discharged or terminated only by a statement in writing signed
by the Company and by Warrant Holders who hold or have the right to acquire at
least two-thirds of the Shares at such time issued or issuable upon exercise of
the Warrants, provided that no change, addition, omission or waiver shall be
made without the written consent of the Warrant Holder(s) which affects (i) the
number of Shares issuable on exercise of this Warrant, (ii) the Exercise Price
or (iii) any other provision other than in a manner in which all the Warrants
are affected.

      16.2. Governing Law. This Agreement shall be governed by and construed and
enforced in accordance with the laws of the State of Delaware without giving
effect to the conflicts of laws principles thereof.

      16.3. Notices. All notices or other communications required or permitted
hereunder shall be in writing and shall be hand delivered, sent by facsimile or
other electronic medium, or mailed, postage prepaid, or sent by reputable
overnight courier, delivery charges prepaid, addressed as follows or to such
other address as may be furnished in writing to the other parties hereto:

      If to the Warrant Holder:    Three Crowns Capital (Bermuda) Ltd.
                                   Clarendon House
                                   PO Box HM 666
                                   2 Church Street
                                   Hamilton HM KX
                                   Bermuda
                                   441-292-4720 (Fax)
                                   Attention: Peter Svennilson

      With a copy to:              Conyers Dill & Pearman
                                   Clarendon House
                                   2 Church Street
                                   Hamilton HM 11
                                   Bermuda
                                   441-292-4720 (Fax)
                                   Attention: Peter A.S. Pearman

      And:                         Nixon Peabody LLP
                                   101 Federal Street
                                   Boston, MA 02110
                                   617-345-1300
                                   Attention:  Carter S. Boom, Jr.

      If to the Company:           PTC Therapeutics, Inc.
                                   Two Chestnut Street
                                   Grafton, MA 01519
                                   508-856-5920 (Fax)
                                   Attention:  President

                                       7
<PAGE>

      With a copy to:        Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C.
                             One Financial Center
                             Boston, MA 02111
                             617-542-6000
                             617-542-2241 (Fax)
                             Attention:  Jeffrey M. Wiesen, Esq.

      All such notices and communications shall be deemed to have been duly
given (i) five (5) business days after being deposited in the mail, postage
prepaid if mailed, (ii) one (1) business day after being sent by overnight
courier, (iii) when receipt acknowledged if telecopied and (iv) upon receipt if
delivered by hand.

      16.4. Headings. The headings in this Warrant are for convenience of
reference only and shall not limit or otherwise affect the terms hereof.

      16.5. Remedies. The Company stipulates that the remedies at law of the
holder of this Warrant in the event of any default or threatened default by the
Company in the performance of or compliance with any of the terms of this
Warrant are not and will not be adequate, and that such terms may be
specifically enforced by a decree for the specific performance of any agreement
contained herein or by an injunction against a violation of any of the terms
hereof or otherwise.

      16.6. Closing of Books. The Company will at no time close its transfer
books against the transfer of any Warrant or of any Shares issued or issuable
upon the exercise of the Warrant in a manner which interferes with the timely
exercise of this Warrant.

      16.7. No Rights or Liabilities as a Stockholder. This Warrant shall not
entitle the Warrant Holder to any voting rights or other rights as a stockholder
of the Company prior to the exercise or conversion of the Warrant, provided that
nothing herein shall be construed to limit or impair other rights that the
Warrant Holder may have under this Warrant or otherwise. No provision of this
Warrant, in the absence of affirmative action by the Warrant Holder to purchase
the Shares, and no mere enumeration herein of the rights or privileges of the
Warrant Holder, shall give rise to any liability of such Warrant Holder for the
Exercise Price or as a stockholder of the Company, whether such liability is
asserted by the Company or by creditors of the Company.

                                       8
<PAGE>

      IN WITNESS WHEREOF, the undersigned has caused this Warrant to be executed
as an instrument under seal.

                                        PTC THERAPEUTICS, INC.

                                        BY: /S/ STUART W. PELTZ
                                            --------------------------
                                        Name:   Stuart W. Peltz, Ph.D.
                                        Title:  President

AGREED AND ACKNOWLEDGED:

THREE CROWNS CAPITAL (BERMUDA) LTD.

By:  /s/ P. SVENNILSON
     --------------------------------
     Name:  P. Svennilson
     Title:

                                       10
<PAGE>

                                    EXHIBIT A

                    NOTICE OF EXERCISE OR NET ISSUE EXERCISE

                                                     Date:
                                                          ______________________

PTC Therapeutics, Inc.
Two Chestnut Street
Grafton, MA 01519
Attention: President

Gentlemen:

      The undersigned hereby elects to exercise or Net Issue Exercise the
enclosed Warrant issued to it by PTC Therapeutics, Inc. (the "Company") and
dated as of ____________, 20__.

      The undersigned elects to:

      [ ]   Exercise the Warrant and to purchase thereunder __ shares of the
            Series D Preferred Stock of the Company (the "Shares") at an
            exercise price of ___ per Share for an aggregate purchase price of
            ___________ Dollars ($___) (the "Purchase Price"). Pursuant to the
            terms of the Warrant, the undersigned has delivered the Purchase
            Price herewith in full.

      [ ]   Net Issue Exercise ____% of the value of the Warrant at the current
            Exercise Price (as defined in the Warrant) of $ ____ per Share.

                                          Very truly yours,

                                          ______________________________________

Receipt Acknowledged:

PTC Therapeutics, Inc.

By:
   ______________________________
on
   ______________________________

                                       10

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