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                                                                     EXHIBIT 4.2

                           FIFTH AMENDED AND RESTATED

                            INVESTOR RIGHTS AGREEMENT

      FIFTH AMENDED AND RESTATED INVESTOR RIGHTS AGREEMENT (this "Agreement")
made as of September 21, 2005 (the "Effective Date") by and among (i) PTC
Therapeutics, Inc., a Delaware corporation (the "Company"), (ii) the persons
identified on the signature page of this Agreement as Founders (the "Founders"),
(iii) those Holders of shares of Series A Stock and Series B Stock (each as
defined below) of the Company listed on Exhibit A attached hereto (the "Series A
and Series B Investors"), (iv) those Holders of shares of Series C Stock (as
defined below) of the Company listed on Exhibit B attached hereto (the "Series C
Purchasers"), (v) the Holders of shares of Series D Stock (as defined below) of
the Company listed on Exhibit C attached hereto (the "Series D Purchasers"),
(vi) those Holders of Series E Stock (as defined below) of the Company listed on
Exhibit D attached hereto (the "Series E Purchasers"), (vii) the purchasers of
Series E-2 Stock (as defined below) of the Company listed on Exhibit E attached
hereto (the "Series E-2 Purchasers") and (viii) the holders of warrants to
purchase Preferred Stock and/or Common Stock of the Company listed on Exhibit F
attached hereto (the "Warrant Purchasers"). The Series A and Series B Investors,
the Series C Purchasers, the Series D Purchasers, the Series E Purchasers and
the Series E-2 Purchasers may be referred to herein individually as an
"Investor" and collectively as the "Investors." The Founders and the Investors
may be referred to herein individually as a "Stockholder" and collectively as
the "Stockholders."

      WHEREAS, the Company proposes to issue and sell up to an aggregate of
4,132,232 shares of Series E-2 Convertible Preferred Stock, par value $.001 per
share (the "Series E-2 Stock"), to the Series E-2 Purchasers pursuant to that
certain Subscription Agreement dated as of the date hereof among the Company and
the Series E-2 Purchasers (the "Subscription Agreement");

      WHEREAS, in connection with their respective purchases of shares of the
Series C Stock, the Series D Stock, the Series E Stock and the Series E-2 Stock,
the Series C Purchasers, the Series D Purchasers, the Series E Purchasers and
the Series E-2 Purchasers have requested that the Company extend to them
registration rights and certain other rights and covenants as set forth herein,
and the Series C Purchasers, the Series D Purchasers, the Series E Purchasers
and the Series E-2 Purchasers have requested that the Founders enter into
certain restrictions on the transfer of Common Stock (as hereinafter defined)
owned or controlled by such Founders;

      WHEREAS, in connection with their purchase of certain warrants, the
Warrant Purchasers have requested that the Company extend to them registration
rights as set forth herein;

      WHEREAS, the Board of Directors of the Company has determined that it is
in the best interests of the Company that the Company enter into this Agreement;
and

      WHEREAS, the Company and the holders of two-thirds of the outstanding
shares of the Company's Series A Convertible Preferred Stock, par value $.001
per share (the

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"Series A Stock"), the Company's Series B Convertible Preferred Stock, par value
$.001 per share (the "Series B Stock"), the Company's Series C Convertible
Preferred Stock, par value $.001 per share (the "Series C Stock"), the holders
of the Company's Series D Convertible Preferred Stock, par value $.001 per share
(the "Series D Stock") and the Company's Series E Convertible Preferred Stock,
par value $.001 per share (the "Series E Stock") (the Series A Stock, the Series
B Stock, the Series C Stock, the Series D Stock, the Series E Stock and the
Series E-2 Stock, collectively being referred to herein as the "Preferred
Stock") have consented to the terms of this Agreement and the amendment and
restatement of that certain Fourth Amended and Restated Investor Rights
Agreement, dated as of December 17, 2003, among the Company and the Investors
named therein (the "Fourth Amended Rights Agreement"), which amended and
restated that certain Third Amended and Restated Investor Rights Agreement,
dated as of August 17, 2001, among the Company and the Investors named therein
(the "Third Amended Rights Agreement"), which amended and restated that certain
Second Amended and Restated Investor Rights Agreement dated as of May 26, 2000
among the Company and the Investors named therein (the "Second Amended Rights
Agreement") which amended and restated that certain Amended and Restated
Investor Rights Agreement dated as of February 9, 2000 among the Company and the
Investors named therein (the "Amended Rights Agreement"), which amended and
restated that certain Investor Rights Agreement dated as of August 19, 1998
among the Company and the Series A and Series B Investors named therein (the
"Original Rights Agreement"), as set forth herein and have evidenced such
consent by executing and delivering either (i) an omnibus signature page in the
form attached hereto as Exhibit H-1 or (ii) a written consent of stockholders in
the form attached hereto as Exhibit H-2 to this Agreement.

      NOW, THEREFORE, in consideration of the covenants and agreements set forth
herein and for other good and valuable consideration, the receipt and
sufficiency of which are hereby mutually acknowledged, the parties hereto amend
and restate in its entirety the Third Amended Rights Agreement and covenant and
agree as follows:

1.    GENERAL PROVISIONS

1.1. SHARES SUBJECT TO THIS AGREEMENT. The Stockholders expressly agree that the
terms and restrictions of this Agreement shall apply to all shares of capital
stock which any of them now owns or hereafter acquires by any means, including
without limitation by purchase, assignment or operation of law, or as a result
of any stock dividend, stock split, reorganization, reclassification, whether
voluntary or involuntary, or other similar transaction, and to any shares of
capital stock of any successor in interest of the Company, whether by sale,
merger, consolidation or other similar transaction, or by purchase, assignment
or operation of law (the "Shares").

1.2. NO PARTNERSHIP RELATIONSHIP. Notwithstanding, but not in limitation of, any
other provision of this Agreement, the parties understand and agree that the
creation, management and operation of the Company shall not create or imply a
general partnership between or among the Stockholders and shall not make any
Stockholder the agent or partner of any other Stockholder for any purpose.

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1.3. CERTAIN DEFINITIONS. As used in this Agreement, the following terms shall
have the following respective meanings:

      "Additional Closing Date" shall have the meaning assigned to such term in
the Subscription Agreement.

      "Affiliate" has the meaning ascribed to that term in Rule 12b-2 under the
Exchange Act, or any successor rule.

      "Commission" shall mean the Securities and Exchange Commission and any
successor agency of the Federal government administering the Securities Act and
the Exchange Act.

      "Common Stock" shall mean (i) the common stock, $.001 par value per share,
of the Company, (ii) any other capital stock of the Company, however designated,
authorized on or after the date hereof, which shall neither be limited to a
fixed sum or percentage of par value in respect of the rights of the holders
thereof to participate in dividends nor entitled to a preference in the
distribution of assets upon the voluntary or involuntary liquidation,
dissolution or winding up of the Company; and (iii) any other securities into
which or for which any of the securities described in (i) or (ii) may be
converted or exchanged pursuant to a plan of recapitalization, reorganization,
merger, consolidation, sale of assets or other similar transaction.

      "Company Stock Option Plan" shall mean the Company's 1998 Employee,
Director and Consultant Stock Option Plan (as amended from time-to-time); or
such other arrangements, contracts, or plans as are recommended by management
and approved by the Board of Directors or compensation committee established by
the Board of Directors.

      "Exchange Act" shall mean the Securities Exchange Act of 1934, as amended,
and any similar or successor Federal statute, and the rules and regulations of
the Commission thereunder, all as the same shall be in effect from time to time.

      "Holder" shall mean the Investors and Warrant Purchasers holding
Registrable Securities or securities convertible into Registrable Securities and
any person holding such securities to whom the rights under this Agreement have
been transferred in accordance with Section 4.12 hereof.

      "Initial Closing Date" shall have the meaning assigned to such term in the
Subscription Agreement.

      "Initial Public Offering" shall mean either (i) the first underwritten
public offering of Common Stock of the Company, offered on a firm commitment
basis pursuant to a registration statement filed with the Commission under the
Securities Act on Form S-1 or its then equivalent with net proceeds to the
Company of not less than $50,000,000 and a per share price of not less than an
amount equal to two times the Adjusted Series E-2 Price (subject to equitable
adjustment in the event of any stock dividend, stock split, combination,
reorganization, recapitalization or similar event involving a change in the
Common Stock) or (ii) any other public offering upon the written election of the

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Company and Holders of at least sixty-six and two-thirds percent (66 2/3 %) of
the outstanding shares of Preferred Stock, on an as-converted basis.

      "Person" means an individual, corporation, company, partnership, joint
venture, trust or unincorporated organization, or a government or any agency or
political subdivision thereof.

      The terms "register", "registered" and "registration" shall refer to a
registration effected by preparing and filing a registration statement in
compliance with the Securities Act and applicable rules and regulations
thereunder, and the declaration or ordering of the effectiveness of such
registration statement, or, as the context may require, under the Exchange Act
or applicable state securities laws.

      "Registrable Securities" shall mean any (i) shares of Common Stock issued
or issuable pursuant to the conversion of the Preferred Stock; (ii) shares of
Common Stock issued or issuable pursuant to the exercise of the Warrants (and
the conversion of any Preferred Stock issued or issuable pursuant thereto)
pursuant to the terms set forth therein and (iii) shares of Common Stock issued
or issuable pursuant to the conversion of the Preferred Stock, the exercise of
the Warrants or the exercise or conversion of any warrant, right or other
security issued in exchange for or in replacement of the Preferred Stock or
issued with respect to the Preferred Stock or issued upon any stock split, stock
dividend or other distribution, recapitalization, reorganization, merger,
consolidation, sale of assets or similar event. As to any particular Registrable
Securities, once issued, such securities shall cease to be Registrable
Securities when (a) a registration statement with respect to the sale of such
securities shall have become effective under the Securities Act and such
securities shall have been disposed of in accordance with such registration
statement, (b) they shall have been sold as permitted by Rule 144 (or any
successor provision) under the Securities Act and the purchaser thereof does not
receive "restricted securities" as defined in Rule 144, (c) they shall have been
otherwise transferred, new certificates for them not bearing a legend
restricting further transfer shall have been delivered by the Company and
subsequent public distribution of them shall not, in the opinion of counsel for
the holders, require registration of them under the Securities Act or (d) they
shall have ceased to be outstanding.

      "Registration Expenses" shall mean the expenses so described in Section
4.7.

      "Securities Act" shall mean the Securities Act of 1933, as amended, and
any similar or successor Federal statute, and the rules and regulations of the
Commission thereunder, all as the same shall be in effect from time to time.

      "Selling Expenses" shall mean the expenses so described in Section 4.7.

      "Subsidiary" or "Subsidiaries" shall mean any corporation, partnership,
trust or other entity of which the Company and/or any of its other Subsidiaries
directly or indirectly owns at the time a majority of the outstanding shares of
any class of equity security of such corporation, partnership, trust or other
entity.

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      "Warrants" shall mean those certain warrants to purchase capital stock
issued by the Company to the Warrant Purchasers listed on Exhibit E attached
hereto.

2.    PERCENTAGE MAINTENANCE RIGHTS

2.1. NOTICE OF NEW ISSUANCE. Except with respect to "Exempt Issuances" as
defined in Section 2.3, in the event that the Company issues any (i) shares of
Common Stock, (ii) warrants, options or other rights to purchase Common Stock
(collectively, "Rights"), or (iii) any debentures or other securities
convertible into or exchangeable for shares of Common Stock (collectively,
"Convertible Securities"), the Company will deliver to those Investors then
holding more than half of one percent (0.5%) of the shares of the Preferred
Stock (each a "Qualified Stockholder") a notice (the "Offer Notice") upon the
completion of such issuance (the "New Issuance"), stating the price and other
terms and conditions thereof.

2.2. RIGHT TO PURCHASE SHARES, RIGHTS OR CONVERTIBLE SECURITIES. In the event of
a New Issuance (other than an Exempt Issuance), any Qualified Stockholder shall
have the right to purchase such number of shares of Common Stock, Rights or
Convertible Securities at the price and on the terms upon which the New Issuance
was made, such price to be paid in full in cash or by check at the time of
issuance of such securities to such Qualified Stockholders so that, after giving
effect to the issuance to the Qualified Stockholders and the conversion,
exercise and exchange into or for (whether directly or indirectly) shares of
Common Stock of all such Rights and Convertible Securities, each Qualified
Stockholder who exercises such right will continue to maintain its same
proportionate ownership of Common Stock as of the date immediately preceding the
New Issuance, treating each Qualified Stockholder, for the purpose of such
computation, as the Holder of the number of shares of Common Stock which would
be issuable to it upon conversion, exercise and exchange of all Rights and
Convertible Securities held by it on the date immediately preceding the New
Issuance and assuming the like conversion, exercise and exchange of all such
securities held by other persons. The rights set forth in this Article 2 shall
be exercised by the Qualified Stockholders, if at all, by written notice to the
Company delivered not later than thirty (30) days after the receipt by the
Investors of the Offer Notice in accordance with the terms and conditions stated
therein, and such right shall expire with respect to a Qualified Stockholder at
the end of the thirtieth day after the day of the receipt by such Qualified
Stockholders of the Offer Notice. The Company shall, promptly following the
expiration of such thirty (30) day period, notify all Qualified Stockholders who
have exercised their purchase rights under this Section 2.2 of the aggregate
number of shares of Common Stock, Rights and Convertible Securities to be issued
pursuant to this Section 2.2, and afford each Qualified Stockholder the
opportunity to adjust the number of such securities it purchases under this
Section 2.2 in order to conform its proportionate ownership after the New
Issuance and the exercise of rights under this Section 2.2 to its proportionate
ownership as of the date immediately preceding the New Issuance.

2.3. EXEMPT ISSUANCES. The issuances referred to in Section 2.1 which will not
give the Investors the rights described in Section 2.2 (the "Exempt Issuances")
are issuances in which shares of Common Stock or Rights or Convertible
Securities of the Company are

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issued or deemed issued (i) as a dividend or distribution payable pro rata to
all holders of Common Stock or other securities of the Company; (ii) up to
2,326,008 shares of Common Stock (subject to equitable adjustment in the event
of any stock dividend, stock split, combination, reorganization,
recapitalization or similar event involving a change in the Common Stock),
including Options therefor, pursuant to the Corporation's 1998 Employee Director
and Consultant Stock Option Plan (as amended from time-to-time); and shares of
Common Stock or Options therefor in excess of such 2,326,008 shares that are
issued pursuant to the Corporation's 1998 Employee, Director and Consultant
Stock Option Plan (as amended from time-to-time), or such other arrangements,
contracts, or plans that are recommended by management and approved by the Board
of Directors or compensation committee established by the Board of Directors,
upon written consent of the holders of a majority of the Preferred Stock; (iii)
upon conversion or exercise of any Rights (including shares issued in connection
with any subsequent conversion of the shares issued upon the exercise of such
Rights) or Convertible Securities outstanding on the date hereof; (iv) as up to
4,132,232 shares of Series E-2 Stock in connection with the Subscription
Agreement (including shares issued in connection with any subsequent conversion
of such Series E-2 Stock); (vi) as up to that number of shares equal to 1% of
the aggregate number of shares (subject to equitable adjustment in the event of
any stock dividend, stock split, combination, reorganization, recapitalization
or similar event involving a change in the Common Stock) of outstanding Common
Stock on an as-converted, fully-diluted basis (after giving effect to the
issuance of the Series E-2 Stock pursuant to the Subscription Agreement)
measured as of the date of the latest issuance of Series E-2 Stock issued after
the date hereof, in each case, in accordance with the terms of the Subscription
Agreement, to institutional lenders in connection with the establishment or
maintenance by the Company of credit facilities, including equipment lease
facilities, approved in each case by a majority of the Board of Directors of the
Company; (vii) pursuant to a registered public offering, the closing of which is
on or after the date hereof; (viii) as up to that number of shares equal to 1%
of the aggregate number of shares (subject to equitable adjustment in the event
of any stock dividend, stock split, combination, reorganization,
recapitalization or similar event involving a change in the Common Stock) of
outstanding Common Stock on an as-converted, fully-diluted basis (after giving
effect to the issuance of the Series E-2 Stock pursuant to the Subscription
Agreement) measured as of the date of the latest issuance of Series E-2 Stock
issued after the date hereof, in each case, in accordance with the terms of the
Subscription Agreement in connection with the sale of Common Stock or
Convertible Securities of the Company to any licensor of technology or patent
rights to the Company or to any collaborative partner or licensee with respect
to the development or commercialization of products; or (ix) as up to that
number of shares equal to 4% of the aggregate number of shares (subject to
equitable adjustment in the event of any stock dividend, stock split,
combination, reorganization, recapitalization or similar event involving a
change in the Common Stock) of outstanding Common Stock on an as-converted,
fully-diluted basis (after giving effect to the issuance of the Series E-2 Stock
pursuant to the Subscription Agreement) measured as of the date of the latest
issuance of Series E-2 Stock issued after the date hereof, in each case, in
accordance with the terms of the Subscription Agreement in connection with the
acquisition by the Company of another business entity by merger,

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purchase of all or substantially all of its assets or acquisition of all or
substantially all of the equity interest of such business entity.

3.    RESTRICTIONS ON TRANSFER

3.1. NON-COMPLYING TRANSFERS PROHIBITED. No Founder may sell, assign, transfer,
exchange, gift, devise, pledge, hypothecate, encumber or otherwise alienate or
dispose of any Shares owned by such Founder, or any right or interest therein,
whether voluntarily or involuntarily, by operation of law or otherwise, except
in accordance with this Agreement. Notwithstanding the foregoing, a Founder may
transfer any or all of his Shares (i) to his spouse or children or to a trust
established for the benefit of his spouse, children or himself, or (ii) under
his will (each such transferee, a "Permitted Transferee"); provided that such
Shares shall remain subject to this Agreement and such permitted transferee
shall, as a condition to such transfer, deliver to the Company a written
instrument confirming that such transferee shall be bound by all of the terms
and conditions of this Agreement. The restrictions on the transfer of shares by
the Founders are in addition to, and not in lieu of, any restrictions on such
Shares contained in each of the Stock Restriction Agreements between the
Founders and the Company dated August 19, 1998.

3.2.  RIGHTS OF FIRST REFUSAL ON VOLUNTARY TRANSFERS.

      3.2.1. RIGHT OF FIRST REFUSAL OF THE COMPANY. Except in cases explicitly
      permitted by Section 3.1, any Founder who intends to sell, assign,
      transfer or otherwise voluntarily alienate or dispose of any Shares (the
      "Selling Stockholder") shall, prior to any such transfer, give written
      notice (the "Selling Stockholder's Notice") of such intention to the
      Company and to each Qualified Stockholder. The Selling Stockholder's
      Notice shall include the name of the proposed transferee, the proposed
      purchase price per Share, the terms of payment of such purchase price and
      all other matters relating to such sale and shall be accompanied by a copy
      of a binding written agreement of the proposed transferee to purchase such
      Shares from the Selling Stockholder. The Selling Stockholder's Notice
      shall constitute a binding offer by the Selling Stockholder to sell to the
      Company all or any part of such number of Shares (the "Offered Shares")
      then owned by the Selling Stockholder as are proposed to be sold in the
      Selling Stockholder's Notice at the monetary price per Share designated in
      the Selling Stockholder's Notice, payable as provided in Section 3.2.3
      hereof. Not later than thirty (30) days after receipt of the Selling
      Stockholder's Notice, the Company shall deliver written notice (the
      "Company Notice") to the Selling Stockholder stating whether the Company
      has accepted the offer stated in the Selling Stockholder's Notice (in
      whole or in part). The closing of any purchase of the Offered Shares by
      the Company shall take place on the later of (i) fifteen (15) days after
      the end of the thirty (30) day period set forth above and (ii) the date on
      which the Buying Stockholders (as hereinafter defined in Section 3.2.2)
      consummate any purchase of Offered Shares pursuant to Section 3.2.2 below.

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      3.2.2. RIGHT OF FIRST REFUSAL OF THE STOCKHOLDERS. If the Company does not
      accept the offer to purchase all of the Offered Shares within the thirty
      (30) day period provided in Section 3.2.1, no later than the end of such
      thirty (30) day period the Company shall give notice (the "Second Company
      Notice") of that fact to each Qualified Stockholder, and each Qualified
      Stockholder shall have the right to purchase all or any part of its
      Proportionate Percentage (as defined below) of the Offered Shares not
      purchased by the Company (the "Remaining Shares"), at the monetary price
      per Share designated in the Selling Stockholder's Notice, payable as
      provided in Section 3.2.3. Not later than fifteen (15) days after delivery
      of the Second Company Notice, each Qualified Stockholder shall deliver to
      the Company, the other Qualified Stockholders and the Selling Stockholder
      a written notice stating whether such Qualified Stockholder has accepted
      the offer stated in the Selling Stockholder's Notice with respect to its
      Proportionate Percentage of the Remaining Shares. Promptly following the
      expiration of such fifteen (15) day period, the Company shall notify each
      Qualified Stockholder that has exercised its rights under this Section
      3.2.2 (each a "Buying Stockholder") of the number of Remaining Shares (if
      any) that other Qualified Stockholders have not elected to purchase. If
      one or more of the Qualified Stockholders elects not to purchase all of
      the Remaining Shares which it is entitled to purchase pursuant to this
      Section 3.2.2, the Buying Stockholders, by written notice to the Company
      and the Selling Stockholder within five (5) days after the delivery of the
      Company's notice pursuant to the proceeding sentence, may elect to
      purchase all or a part of such unpurchased Remaining Shares without the
      consent of any non-purchasing Qualified Stockholders, pro rata between or
      among them or in such other manner as they may agree. The closing of any
      purchase of the Remaining Shares by the Buying Stockholders shall take
      place no later than fifteen (15) days after the end of the fifteen (15)
      day period set forth above. As used in this Section 3.2.2, "Proportionate
      Percentage" shall mean with respect to each Qualified Stockholder a
      fraction, the numerator of which is the number of Shares owned by such
      Qualified Stockholder (calculated on a fully diluted basis), and the
      denominator of which is the total number of Shares owned by all the
      Qualified Stockholders (calculated on a fully-diluted basis).

      3.2.3. CLOSING. The place for the closing of any purchase and sale
      described in Section 3.2.1 or Section 3.2.2 shall be the principal office
      of the Company or at such other place as the parties shall agree. At the
      closing, the Selling Stockholder shall accept payment on the terms offered
      by the proposed transferee named in the Selling Stockholder's Notice,
      provided, however, that the Company and the Buying Stockholders shall not
      be required to meet any non-monetary terms of the proposed transfer,
      including, without limitation, delivery of other securities in exchange
      for the Shares proposed to be sold. At the closing, the Selling
      Stockholder shall deliver to the Company or the Buying Stockholders, as
      the case may be, in exchange for Shares purchased and sold at the closing,
      certificates for the number of Shares stated in the Selling Stockholder's
      Notice, accompanied by duly executed instruments of transfer.

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      3.2.4. TRANSFERS TO THIRD PARTIES. If the Company and the Buying
      Stockholders in aggregate fail to accept the offer stated in the Selling
      Stockholder's Notice with respect to all of the Offered Shares, they shall
      not have the right to purchase any Offered Shares, and the Selling
      Stockholder shall, subject to Section 3.3 herein, be free to sell all, but
      not less than all, of the Offered Shares to the designated transferee at a
      price and on terms no less favorable to the Selling Stockholder than
      described in the Selling Stockholder's Notice, provided, however, that
      such sale is consummated within ninety (90) days after the giving of the
      Selling Stockholder's Notice pursuant to Section 3.2.1. As a condition
      precedent to the effectiveness of a transfer pursuant to this Section
      3.2.4, the proposed transferee(s) shall agree in writing prior to such
      transfer to become a party to this Agreement and shall thereafter be
      permitted to transfer Shares only in accordance with this Agreement.

3.3.  PARTICIPATION IN SALES.

      3.3.1. TAKE-ALONG RIGHT. In the event that a Founder (the "Offeree")
      receives a bona fide offer from a third party or parties other than the
      Company or any other Stockholder (the "Third Party Buyer") to purchase any
      of the Shares owned by the Offeree (the "Take-Along Shares"), for a
      specified price payable in cash or otherwise and on specified terms and
      conditions (the "Offer"), and the Offeree proposes to sell or otherwise
      transfer the Take-Along Shares to the Third Party Buyer pursuant to the
      Offer, each Qualified Stockholder shall have the right to sell to the
      Third Party Buyer, at the same price per Share and on the same terms and
      conditions as stated in the Offer, up to the number of Shares equal to the
      Take-Along Shares multiplied by a fraction, the numerator of which is the
      aggregate number of Shares owned by such Qualified Stockholder (calculated
      on a fully diluted, as converted to Common Stock basis) and the
      denominator shall be the aggregate number of Shares held by all
      stockholders (calculated on a fully-diluted, as converted to Common Stock
      basis), provided, however, that if the prospective third-party purchaser
      objects to the delivery of any series of Preferred Stock in lieu of Common
      Stock, such selling Qualified Stockholder shall convert such Preferred
      Stock into Common Stock and deliver Common Stock as provided in this
      Section 3.3.1. The Company agrees to make any such conversion concurrent
      with the actual transfer of such shares to the purchaser and contingent on
      such transfer; and provided further, that such rights of the Qualified
      Stockholders granted pursuant to this 3.3.1 shall not apply to the
      transfer or transfers by a Founder of up to 100,000 Shares in the
      aggregate (subject to appropriate adjustment for stock splits, stock
      dividends, combinations and similar recapitalization events) (a "Permitted
      Transfer"), provided that, such Shares transferred pursuant to a Permitted
      Transfer shall remain subject to this Agreement and such permitted
      transferee shall, as a condition to such transfer, deliver to the Company
      a written instrument confirming that such transferee shall be bound by all
      of the terms and conditions of this Agreement.

      3.3.2. NOTICES OF OFFER AND INTENT TO PARTICIPATE. If a Qualified
      Stockholder wishes to participate in any sale pursuant to Section 3.3.1 it
      shall notify the

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      Offeree in writing of such intention and the number of Shares it wishes to
      sell pursuant to this Section 3.3 not later than fifteen (15) days after
      delivery of the Second Company Notice (as described in Section 3.2.2
      above). If the Offeree does not receive such notice from a Qualified
      Stockholder within such fifteen (15) day period, the Offeree shall be free
      to consummate the proposed transaction without any obligation to include
      such Qualified Stockholder's Shares in such transaction. The non-exercise
      or partial exercise of the rights of any Qualified Stockholder hereunder
      to participate in one or more sales of Shares made by a Founder shall not
      adversely affect such Qualified Stockholder's right to participate in
      subsequent sales of Shares by a Founder.

      3.3.3. SALE OF TAKE-ALONG SHARES. The Offeree and each Qualified
      Stockholder who has provided timely notice in accordance with Section
      3.3.2 above shall sell to the Third Party Buyer all, or at the option of
      the Third Party Buyer, any part of the Shares proposed to be sold by them
      at not less than the price and upon other terms and conditions, if any,
      not more favorable to the Third Party Buyer than those stated in the
      Offer, provided, however, that any purchase of less than all of such
      Shares by the Third Party Buyer shall be made from the Offeree and each
      Qualified Stockholder pro rata based upon the relative amount of the
      Shares that the Offeree and such Qualified Stockholder is entitled to sell
      pursuant to Section 3.3.1. The stock certificate or certificates that the
      selling Qualified Stockholder delivers to the Offeree pursuant to this
      Section 3.3 shall be transferred to the prospective purchaser in
      consummation of the sale of the Shares pursuant to the terms and
      conditions specified in the transfer notice, and the Offeree shall
      concurrently therewith remit to such selling Qualified Purchaser that
      portion of the sale proceeds to which such selling Qualified Purchaser is
      entitled by reason of its participation in such sale. To the extent that
      any prospective purchaser or purchasers prohibits such assignment or
      otherwise refuses to purchase shares or other securities from a selling
      Qualified Stockholder exercising its take along rights hereunder, the
      Offeree shall not sell to such prospective purchaser or purchasers any
      Shares unless and until, simultaneously with such sale, the Offeree shall
      purchase such shares or other securities from such selling Qualified
      Stockholder for the same consideration and on the same terms and
      conditions as the proposed transfer described in the transfer notice.

      3.3.4. PROHIBITED TRANSFERS.

            (a) In the event any Founder should sell any Shares in contravention
      of the take-along rights of the Qualified Stockholders under Section 3 (a
      "Prohibited Transfer"), the Qualified Stockholders, in addition to such
      other remedies as may be available at law, in equity or hereunder, shall
      have the put option provided below, and such Founder
      shall be bound by the applicable provisions of such option.

            (b) In the event of a Prohibited Transfer, each Qualified
      Stockholder shall have the right to sell to such Founder the type and
      number of the Shares equal to the number of shares each Qualified
      Stockholder would have been

                                       10
<PAGE>

      entitled to transfer to the third-party transferee(s) under Section 3.3
      hereof had the Prohibited Transfer been effected pursuant to and in
      compliance with the terms hereof. Such sale shall be made on the following
      terms and conditions:

                  (i) The price per share at which the shares are to be sold to
            such Founder shall be equal to the price per share paid by the
            third-party transferee(s) to such Founder in the Prohibited
            Transfer. Such Founder shall also reimburse each Qualified
            Stockholder for any and all fees and expense, including legal fees
            and expenses, incurred pursuant to the exercise or the attempted
            exercise of the Qualified Stockholder's rights under Section 3.3.4.

                  (ii) Within 45 days after the later of the dates on which the
            Qualified Stockholder (A) received notice of the Prohibited Transfer
            or (B) otherwise became aware of the Prohibited Transfer, each
            Qualified Stockholder shall, if exercising the option created
            hereby, deliver to such Founder the certificate or certificates
            representing shares to be sold, each certificate to be properly
            endorsed for transfer.

                  (iii) Such Founder shall, upon receipt of the certificate or
            certificates for the shares to be sold by a Qualified Stockholder,
            pursuant to this Section 3.3.4, pay the purchase price therefor and
            the amount of reimbursable fees and expenses, as specified in
            Section 3.3.4(b)(i), in cash or by other means acceptable to such
            Qualified Stockholder.

                  (iv) Notwithstanding the foregoing, any attempt by a Founder
            to transfer the Shares in violation of this Agreement shall be null
            and void ab initio and the Company agrees it will not effect such a
            transfer nor will it treat any alleged transferee(s) as the owner of
            such shares without the written consent of a majority in interest of
            the Qualified Stockholders.

            (c) The Qualified Stockholders, severally but not jointly, will
      indemnify the Company against all liabilities incurred by the Company
      relating to tax obligations owed by the Company pursuant to the exercise
      by such Qualified Stockholders of the option set forth in this Section
      3.3.4.

3.4. COMPANY RECORDS. The Company shall not transfer on its books any of the
shares of Common Stock held by any Stockholder or Permitted Transferee without
first ascertaining compliance with all of the applicable provisions of this
Agreement with respect to such transfer. Each Founder agrees that, in order to
ensure compliance with the restrictions referred to herein, the Company may
issue appropriate "stop-transfer" instructions to its transfer agent, if any,
and that, if the Company transfers its own securities, it may make appropriate
notations to the same effect in its own records. The Company shall not be
required (i) to transfer on its books any Shares that have been sold or
otherwise transferred in violation of any of the provisions of this Section 3.3,
or (ii) to treat as owner of such Shares or to accord the right to vote or pay
dividends to any purchaser or other transferee to whom such Shares shall have
been transferred.

                                       11
<PAGE>

3.5. LEGEND-REQUIREMENT. All certificates evidencing Shares subject to Section
3.3 herein shall, during the term of this Agreement, bear such restrictive
legends as the Company and the Company's counsel deem necessary or advisable
under applicable law or pursuant to this Agreement, including without limitation
the following (or a substantially similar legend):

            "THE SECURITIES REPRESENTED HEREBY ARE SUBJECT TO A RIGHT OF CO-SALE
      BY CERTAIN SHAREHOLDERS OF THE COMPANY, PURSUANT TO AN AGREEMENT RELATING
      TO SUCH SECURITIES, AND MAY NOT BE SOLD OR OTHERWISE TRANSFERRED EXCEPT IN
      COMPLIANCE WITH THE TERMS OF SUCH AGREEMENT."

4.    TRANSFER OF REGISTRABLE SECURITIES; REGISTRATION

4.1. RESTRICTIVE LEGEND. Each certificate representing shares of Preferred Stock
or Registrable Securities shall, except as otherwise provided in this Section
4.1 or in Section 4.2, be stamped or otherwise imprinted with a legend
substantially in the following form (in addition to any legend required under
applicable state securities laws):

            "The securities represented by this certificate have not been
            registered under the Securities Act of 1933, as amended, or any
            other 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."

      Upon request of a Holder of such a certificate, the Company shall remove
the foregoing legend (as well as the legend set forth in Section 4.2 of the
Subscription Agreement) from the certificate or issue to such Holder a new
certificate therefor free of any transfer legend, if there is an effective
registration statement covering the securities represented by such certificate
or, with such request, the Company shall have received either the opinion
referred to in Section 4.2(i) or the "no-action" letter referred to in Section
4.2(ii).

4.2. NOTICE OF PROPOSED TRANSFER. Prior to any proposed sale, pledge,
hypothecation or other transfer of any shares of Preferred Stock or Registrable
Securities (other than under the circumstances described in Section 4.3, 4.4 or
4.5), the Holder thereof shall give written notice to the Company of its
intention to effect such sale, pledge, hypothecation or other transfer. Each
such notice shall describe the manner of the proposed sale, pledge,
hypothecation or other transfer and, if requested by the Company

                                       12
<PAGE>

shall be accompanied by either (i) an opinion of counsel reasonably satisfactory
to the Company stating that that the proposed sale, pledge, hypothecation or
other transfer may be effected without registration under the Securities Act or
(ii) a "no action" letter from the Commission to the effect that the
distribution of such securities without registration will not result in a
recommendation by the staff of the Commission that action be taken with respect
thereto, whereupon the Holder of such stock shall be entitled to transfer such
stock in accordance with the terms of its notice; provided, however, that no
such opinion of counsel shall be required for (x) a distribution to one or more
partners of the transferor (in the case of a transferor that is a partnership),
stockholders (in the case of a transferor that is a corporation) or members (in
the case of a transferor that is a limited liability company) in each case in
respect of the beneficial interest of such partner, stockholder or member or (y)
Affiliates, except in the case of clause (y), as the Company shall otherwise
reasonably request. Each certificate for Preferred Stock or Registrable
Securities transferred as above provided shall bear the appropriate restrictive
legend set forth in Section 4.1, except that such certificate shall not bear
such legend if (i) such transfer is in accordance with the provisions of Rule
144 (or any other rule permitting public sale without registration under the
Securities Act) or (ii) the opinion of counsel or "no-action" letter referred to
above is to the further effect that the transferee and any subsequent transferee
(other than an affiliate of the Company) would be entitled to transfer such
securities in a public sale without registration under the Securities Act or
that such legend is not required to establish compliance with any provisions of
the Securities Act. Notwithstanding any other provision hereof, the restrictions
provided for in this Section 4.2 shall not apply to securities which are not
required to bear the legend prescribed by Section 4.1 in accordance with the
provisions of that Section. If the Company does not accept an opinion of counsel
required hereby signed by the original Holder's counsel, the Company will pay
the reasonable fees and disbursements of other counsel in connection with all
opinions rendered by them pursuant to this Section 4.2.

4.3.  REGISTRATION ON FORM S-1.

            (a) If at any time, Holders of more than 20% of the outstanding
      shares of Registrable Securities request that the Company file a
      registration statement for at least 20% of the shares of the outstanding
      Registrable Securities, (such Holders, the "Initiating Holders")
      anticipated aggregate proceeds of which, net of underwriting discounts and
      commissions, would exceed $10,000,000, the Company shall:

                  (i) within ten days of the receipt by the Company of such
            notice, give written notice of the proposed registration,
            qualification or compliance to all other Holders; and

                  (ii) as soon as practicable use its best efforts to effect
            such registration, qualification or compliance (including, without
            limitation, appropriate qualification under applicable blue sky or
            other state securities laws and appropriate compliance with
            applicable regulations issued under the Securities Act and any other
            governmental requirements or regulations) as may be so requested and
            as would permit or facilitate the

                                       13
<PAGE>

            sale and distribution of all or such portion of such Registrable
            Securities as are specified in such request, together with all or
            such portion of the Registrable Securities of any Holder or Holders
            joining in such request as are specified in a written request
            received by the Company within 20 days after receipt of such written
            notice from the Company;

            provided, however, that the Company shall not be obligated to take
      any action to effect any such registration, qualification or compliance
      pursuant to this Section 4.3:

                  (A) In any particular jurisdiction in which the Company would
            be required to execute a general consent to service of process in
            effecting such registration, qualification or compliance unless the
            Company is already subject to service in such jurisdiction and
            except as may be required by the Securities Act;

                  (B) Prior to the date six (6) months following the effective
            date of the Company's first registered public offering of its stock,
            pursuant to a firm commitment underwritten offering;

                  (C) After the Company has effected four (4) such registrations
            pursuant to this Section 4.3(a) and such registrations have been
            declared or ordered effective and the securities offered thereunder
            have been sold;

                  (D) If the Company shall furnish to such Holders a certificate
            signed by the President of the Company stating that in the good
            faith judgment of the Board of Directors it would be seriously
            detrimental to the Company or its stockholders for a registration
            statement to be filed in the near future, then the Company's
            obligation to use its best efforts to register, qualify or comply
            under this Section 4.3 shall be deferred for a period not to exceed
            90 days from the date of receipt of written request from the
            Initiating Holders; provided, however, that the Company shall not
            exercise such right more than once in any twelve-month period;
            provided, further, that it shall be considered seriously detrimental
            to the Company and its stockholders for a registration statement to
            be filed in the near future if the Company shall be preparing in
            good faith to commence any offering at the time any such request by
            the Initiating Holders is made.

                  (E) If such Holders propose to dispose of shares of
            Registrable Securities that may be immediately registered on Form
            S-3 pursuant to a request made under the provisions of Section 4.4
            hereof.

Subject to the foregoing clauses (A) through (E), the Company shall file a
registration statement covering the Registrable Securities so requested to be
registered as soon as practicable after receipt of the request or requests of
the Initiating Holders.

            (b) Underwriting. If the Initiating Holders request a registration
      pursuant to this Section 4.3 for a registered public offering and intend
      to distribute

                                       14
<PAGE>

      the Registrable Securities covered by their request by means of an
      underwriting, they shall so advise the Company as a part of their request
      made pursuant to this Section 4.3 and the Company shall so advise the
      Holders as part of the notice given pursuant to Section 4.3(a)(i). In such
      event, the right of any Holder to registration pursuant to Section 4.3
      shall be conditioned upon such Holder's participation in the underwriting
      arrangements required by this Section 4.3, and the inclusion of such
      Holder's Registrable Securities in the underwriting to the extent
      requested shall be limited to the extent provided herein.

      A registration statement filed pursuant to this Section 4.3 may, subject
to the following provisions, include (i) shares of Common Stock for sale by the
Company for its own account, (ii) shares of Common Stock held by officers or
directors of the Company and (iii) shares of Common Stock held by persons who by
virtue of agreements with the Company are entitled to include such shares in
such registration (the "Other Shareholders"), in each case for sale in
accordance with the method of disposition specified by the requesting Holders.
If such registration shall be underwritten, the Company shall (together with all
Holders proposing to distribute their securities through such underwriting)
enter into an underwriting agreement in customary form with the managing
underwriter of recognized national standing selected for such underwriting by
the Company and reasonably acceptable to a majority of the Holders proposing to
distribute their securities through such underwriting; provided, that, the
Holders shall not be required to make any representation or warranty in the
underwriting agreement in connection with such offering other than as to (i)
their ownership and authority to transfer, free of liens, claims and
encumbrances (other than those that arise under the terms of this Agreement) and
(ii) the accuracy and completeness of any information furnished by them for
inclusion in the registration statement. If and to the extent that the managing
underwriter determines that marketing factors require a limitation on the number
of shares to be included in such registration, then the shares of Common Stock
held by officers or directors (other than Registrable Securities) of the Company
or by Other Shareholders (other than Registrable Securities) and shares of
Common Stock to be sold by the Company for its own account shall be excluded
from such registration to the extent so required by such managing underwriter,
and unless the Holders of such shares and the Company have otherwise agreed in
writing, such exclusion shall be applied first to the shares held by the
directors and officers and the Other Shareholders to the extent required by the
managing underwriter, then to the shares of Common Stock of the Company to be
included for its own account to the extent required by the managing underwriter.
If the managing underwriter determines that marketing factors require a further
limitation of the number of Registrable Securities to be registered under this
Section 4.3, then Registrable Securities shall be excluded in such manner that
the securities to be sold shall be allocated among the selling Holders pro rata
based on their ownership of Registrable Securities. In any event all securities
to be sold other than Registrable Securities will be excluded prior to any
exclusion of Registrable Securities. No Registrable Securities or any other
security excluded from the underwriting by reason of the underwriter's marketing
limitation shall be included in such registration.

      If any Holder of Registrable Securities or Other Shareholder disapproves
of the terms of the underwriting, such Holder or Other Shareholder may elect to
withdraw

                                       15
<PAGE>

therefrom by written notice to the Company, the managing underwriter and the
Initiating Holders. The Registrable Securities and/or other securities so
withdrawn shall also be withdrawn from registration. All such withdrawn
Registrable Securities and/or other securities shall remain subject to any
agreements executed in connection with Section 4.11.

4.4.  REGISTRATION ON FORM S-3.

            (a) In addition to the rights provided in Sections 4.3 and 4.5,
      subject to a limit of one (1) registration hereunder in any six (6) month
      period, if at any time (i) any Holder or Holders of Registrable Securities
      request that the Company file a registration statement on Form S-3 or any
      comparable or successor form thereto for a public offering of all or any
      portion of the shares of Registrable Securities held by such requesting
      Holder or Holders, the reasonably anticipated aggregate price to the
      public of which would exceed US $5,000,000 and (ii) the Company is a
      registrant entitled to use Form S-3 or any comparable or successor form
      thereto to register such shares, then the Company shall use its best
      efforts to register under the Securities Act on Form S-3 or any comparable
      or successor form thereto, for public sale in accordance with the method
      of disposition specified in such notice, the number of shares of
      Registrable Securities specified in such notice. Whenever the Company is
      required by this Section 4.4 to use its best efforts to effect the
      registration of Registrable Securities, each of the procedures and
      requirements set forth below, including but not limited to the requirement
      that the Company notify all Holders of Registrable Securities from whom
      notice has not been received and provide them with the opportunity to
      participate in the offering, shall apply to such registration.

            (b) The Company shall use its best efforts to qualify for
      registration on Form S-3 or any comparable or successor form or forms; and
      to that end the Company shall register (whether or not required by law to
      do so) the Common Stock under the Exchange Act in accordance with the
      provisions of that Act following the effective date of the first
      registration of any securities of the Company on Form S-1 or any
      comparable or successor form.

            (c) Following receipt of any notice under this Section 4.4, the
      Company shall immediately notify all Holders of Registrable Securities
      from whom notice has not been received and such Holders shall then be
      entitled within thirty (30) days after receipt of such notice from the
      Company to request the Company to include in the requested registration
      all or any portion of their shares of Registrable Securities. The Company
      shall use its best efforts to register under the Securities Act, for
      public sale in accordance with the method of disposition specified in the
      notice from requesting Holders described in paragraph (a) above, the
      number of shares of Registrable Securities specified in such notice (and
      in all notices received by the Company from other Holders within thirty
      (30) days after the receipt of such notice by such Holders). The Company
      shall be obligated to register the Registrable Securities pursuant to this
      Section 4.4 on four (4) occasions; provided, however, that such obligation
      shall be deemed satisfied only

                                       16
<PAGE>

      when a registration statement covering all shares of Registrable
      Securities specified in notices received as aforesaid (except to the
      extent reduced by the managing underwriter, if any, pursuant to Section
      4.4(e)), for sale in accordance with the method of disposition specified
      by the requesting Holders, shall have become effective.

            (d) If the Holders requesting such registration intend to distribute
      the Registrable Securities covered by their request by means of an
      underwriting, they shall so advise the Company as a part of their request
      made pursuant to this Section 4.4 and the Company shall include such
      information in the written notice referred to in paragraph (c) above. The
      right of any Holder to registration pursuant to this Section 4.4 shall be
      conditioned upon such Holder's agreeing to participate in such
      underwriting and to permit inclusion of such Holder's Registrable
      Securities in the underwriting; provided, that, the Holders shall not be
      required to make any representation or warranty in the underwriting
      agreement in connection with such offering other than as to (i) their
      ownership and authority to transfer, free of liens, claims and
      encumbrances (other than those that arise under the terms of this
      Agreement), and (ii) the accuracy and completeness of any information
      furnished by them for inclusion in the registration statement. If such
      method of disposition is an underwritten public offering, the Holders of
      at least a majority in interest of the shares of Registrable Securities to
      be sold in such offering may designate the managing underwriter of such
      offering, which managing underwriter shall be reasonably acceptable to the
      Company. A Holder may elect to include in such underwriting all or a part
      of the Registrable Securities it holds.

            (e) A registration statement filed pursuant to this Section 4.4 may,
      subject to the following provisions, include (i) shares of Common Stock
      for sale by the Company for its own account, (ii) shares of Common Stock
      held by officers or directors of the Company and (iii) shares of Common
      Stock held by persons who by virtue of agreements with the Company are
      entitled to include such shares in such registration (the "Other
      Shareholders"), in each case for sale in accordance with the method of
      disposition specified by the requesting Holders. If such registration
      shall be underwritten, the Company, such officers and directors and Other
      Shareholders proposing to distribute their shares through such
      underwriting shall enter into an underwriting agreement in customary form
      with the representative of the underwriter or underwriters selected for
      such underwriting on terms no less favorable to such officers, directors
      or Other Shareholders than the terms afforded the Holders of Registrable
      Securities. If and to the extent that the managing underwriter determines
      that marketing factors require a limitation on the number of shares to be
      included in such registration, then the shares of Common Stock held by
      officers or directors (other than Registrable Securities) of the Company
      or by Other Shareholders (other than Registrable Securities) and shares of
      Common Stock to be sold by the Company for its own account shall be
      excluded from such registration to the extent so required by such managing
      underwriter, and unless the Holders of such shares and the Company have
      otherwise agreed in writing, such exclusion shall be applied

                                       17
<PAGE>

      first to the shares held by the directors and officers and the Other
      Shareholders to the extent required by the managing underwriter, then to
      the shares of Common Stock of the Company to be included for its own
      account to the extent required by the managing underwriter. If the
      managing underwriter determines that marketing factors require a further
      limitation of the number of Registrable Securities to be registered under
      this Section 4.4, then Registrable Securities shall be excluded in such
      manner that the securities to be sold shall be allocated among the selling
      Holders pro rata based on their ownership of Registrable Securities. In
      any event all securities to be sold other than Registrable Securities will
      be excluded prior to any exclusion of Registrable Securities. No
      Registrable Securities or any other security excluded from the
      underwriting by reason of the underwriter's marketing limitation shall be
      included in such registration.

      If any Holder of Registrable Securities, officer, director or Other
Shareholder who has requested inclusion in such registration as provided above,
disapproves of the terms of the underwriting, such Holder, officer, director or
Other Shareholder may elect to withdraw therefrom by written notice to the
Company and the managing underwriter. The securities so withdrawn shall also be
withdrawn from registration.

4.5. INCIDENTAL REGISTRATION. If the Company at any time (other than pursuant to
Sections 4.3 and 4.4) proposes to register any of its securities under the
Securities Act for sale to the public, whether for its own account or for the
account of other security Holders or both (except with respect to registration
statements on Forms S-4, S-8 or any successor to such forms or another form not
available for registering the Registrable Securities for sale to the public)
each such time it will promptly give written notice to all Holders of the
Registrable Securities of its intention so to do. Upon the written request of
any such Holder, received by the Company within thirty (30) days after the
giving of any such notice by the Company, to register any or all of its
Registrable Securities, the Company will use its best efforts to cause the
Registrable Securities as to which registration shall have been so requested to
be included in the securities to be covered by the registration statement
proposed to be filed by the Company, all to the extent requisite to permit the
sale or other disposition by the Holder (in accordance with its written request)
of such Registrable Securities so registered. If the registration of which the
Company gives notice is for a registered public offering involving an
underwriting, the Company shall so advise the Holders of Registrable Securities
as a part of the written notice given pursuant to this Section 4.5. In such
event the right of any Holder of Registrable Securities to registration pursuant
to this Section 4.5 shall be conditioned upon such Holder's participation in
such underwriting to the extent provided herein. All Holders of Registrable
Securities proposing to distribute their securities through such underwriting
shall (together with the Company and the Other Shareholders distributing their
securities through such underwriting) enter into an underwriting agreement in
customary form with the underwriter or underwriters selected for underwriting by
the Company; provided, that, the Holders shall not be required to make any
representation or warranty in the underwriting agreement in connection with such
offering other than as to (i) their ownership and authority to transfer, free of
liens, claims and encumbrances (other than those that arise under the terms of
this Agreement), and (ii) the accuracy and completeness of any information
furnished by them for inclusion in the registration

                                       18
<PAGE>

statement. Notwithstanding any other provision of the Agreement, if the
underwriter determines in good faith that marketing factors require a limitation
of the number of shares to be underwritten, the number of shares that may be
included in the underwriting shall be allocated, first, to the Company; second,
to the Holders on a pro rata basis based on the total number of Registrable
Securities requested to be registered by such Holders; and third, to the Other
Shareholders on a pro rata basis. No such reduction shall (i) reduce the
securities being offered by the Company for its own account to be included in
the registration and underwriting, or (ii) reduce the amount of securities of
the selling Holders included in the registration below twenty-five percent (25%)
of the total amount of securities included in such registration, unless such
offering is the initial public offering and such registration does not include
shares of any other selling shareholders, in which event any or all of the
Registrable Securities of the Holders may be excluded in accordance with the
immediately preceding sentence. Notwithstanding the foregoing provisions, the
Company may withdraw any registration statement referred to in this Section 4.4
without thereby incurring any liability to the Holders of Registrable
Securities. If any Holder of Registrable Securities or Other Shareholder
disapproves of the terms of any such underwriting, it may elect to withdraw
therefrom by written notice to the Company and the underwriter. Any Registrable
Securities or other securities excluded or withdrawn from such underwriting
shall be withdrawn from such registration.

4.6. REGISTRATION PROCEDURES. If and whenever the Company is required by the
provisions of Section 4.3, 4.4 or 4.5 to use its best efforts to effect the
registration of any Registrable Securities under the Securities Act, the Company
will, as expeditiously as possible:

            (a) prepare and file with the Commission a registration statement
      with respect to such securities including executing an undertaking to file
      post-effective amendments and use its best efforts to cause such
      registration statement to become and remain effective for the period of
      the distribution contemplated thereby;

            (b) prepare and file with the Commission such amendments and
      supplements to such registration statement and the prospectus used in
      connection therewith as may be necessary to keep such registration
      statement effective for the period specified herein and comply with the
      provisions of the Securities Act with respect to the disposition of all
      Registrable Securities covered by such registration statement in
      accordance with the sellers' intended method of disposition set forth in
      such registration statement for such period;

            (c) furnish to each seller of Registrable Securities and to each
      underwriter such number of copies of the registration statement and each
      such amendment and supplement thereto (in each case including all
      exhibits) and the prospectus included therein (including each preliminary
      prospectus) as such persons reasonably may request in order to facilitate
      the public sale or other disposition of the Registrable Securities covered
      by such registration statement;

                                       19
<PAGE>

            (d) use its best efforts to register or qualify the Registrable
      Securities covered by such registration statement under the securities or
      "blue sky" laws of such jurisdictions as the sellers of Registrable
      Securities or, in the case of an underwritten public offering, the
      managing underwriter reasonably shall request, provided, however, that the
      Company shall not for any such purpose be required to qualify generally to
      transact business as a foreign corporation in any jurisdiction where it is
      not so qualified or to consent to general service of process in any such
      jurisdiction, unless the Company is already subject to service in such
      jurisdiction;

            (e) use its best efforts to list the Registrable Securities covered
      by such registration statement with any securities exchange on which the
      Common Stock of the Company is then listed;

            (f) comply with all applicable rules and regulations under the
      Securities Act and Exchange Act;

            (g) immediately notify each seller of Registrable Securities and
      each underwriter under such registration statement, at any time when a
      prospectus relating thereto is required to be delivered under the
      Securities Act, of the happening of any event of which the Company has
      knowledge as a result of which the prospectus contained in such
      registration statement, as then in effect, includes an untrue statement of
      a material fact or omits to state a material fact required to be stated
      therein or necessary to make the statements therein not misleading in
      light of the circumstances then existing, and promptly prepare and furnish
      to such seller a reasonable number of copies of a prospectus supplemented
      or amended so that, as thereafter delivered to the purchasers of such
      Registrable Securities, such prospectus shall not include an untrue
      statement of a material fact or omit to state a material fact required to
      be stated therein or necessary to make the statements therein not
      misleading in light of the circumstances then existing;

            (h) if the offering is underwritten on a "best efforts" basis and at
      the request of any seller of Registrable Securities, use its best efforts
      to furnish on the date that Registrable Securities are delivered to the
      underwriters for sale pursuant to such registration: (i) an opinion dated
      such date of counsel representing the Company for the purposes of such
      registration, addressed to the underwriters to such effects as reasonably
      may be requested by counsel for the underwriters and (ii) a letter dated
      such date from the independent public accountants retained by the Company,
      addressed to the underwriters stating that they are independent public
      accountants within the meaning of the Securities Act and that, in the
      opinion of such accountants, the financial statements of the Company
      included in the registration statement or the prospectus, or any amendment
      or supplement thereof, comply as to form in all material respects with the
      applicable accounting requirements of the Securities Act, and such letter
      shall additionally cover such other financial matters (including
      information as to the period ending no more than five (5) business days
      prior to the date of such letter) with respect to such registration as
      such underwriters reasonably may request;

                                       20
<PAGE>

            (i) make available for inspection by each seller of Registrable
      Securities, any underwriter participating in any distribution pursuant to
      such registration statement, and any attorney, accountant or other agent
      retained by such seller or underwriter, reasonable access to all financial
      and other records, pertinent corporate documents and properties of the
      Company, as such parties may reasonably request, and cause the Company's
      officers, directors and employees to supply all information reasonably
      requested by any such seller, underwriter, attorney, accountant or agent
      in connection with such registration statement;

            (j) cooperate with the selling Holders of Registrable Securities and
      the managing underwriter, if any, to facilitate the timely preparation and
      delivery of certificates representing Registrable Securities to be sold,
      such certificates to be in such denominations and registered in such names
      as such Holders or the managing underwriter may request at least two
      business days prior to any sale of Registrable Securities;

            (k) permit any Holder of Registrable Securities which Holder, in the
      sole and exclusive judgment, exercised in good faith, of such Holder,
      might be deemed to be a controlling person of the Company, to participate
      in good faith in the preparation of such registration or comparable
      statement and to require the insertion therein of material, furnished to
      the Company in writing, which in the reasonable judgment of such Holder
      and its counsel should be included; and

            (l) in connection with such registration, use reasonable efforts to
      cause the senior management of the Company to participate in any "road
      show" presentations to prospective investors for such period of time as is
      reasonably requested by the managing underwriters.

      For purposes of this Agreement, the period of distribution of Registrable
Securities in a firm commitment underwritten public offering shall be deemed to
extend until each underwriter has completed the distribution of all securities
purchased by it, and the period of distribution of Registrable Securities in any
other registration shall be deemed to extend until the earlier of the sale of
all Registrable Securities covered thereby or 180 days after the effective date
thereof; provided, however, in the case of any registration of Registrable
Securities on Form S-3 or a comparable or successor form which are intended to
be offered on a continuous or delayed basis, such 180 day-period shall be
extended, if necessary, to keep the registration statement effective until all
such Registrable Securities are sold, provided that Rule 415, or any successor
rule under the Securities Act, permits an offering on a continuous or delayed
basis; and provided, further, that applicable rules under the Securities Act
governing the obligation to file a post-effective amendment, permit, in lieu of
filing a post-effective amendment which (y) includes any prospectus required by
Section 10(a)(3) of the Securities Act or (z) reflects facts or events
representing a material or fundamental change in the information set forth in
the registration statement, the incorporation by reference of information
required to be included in (y) and (z) above contained in periodic reports filed
pursuant to Section 13 or 15(d) of the Exchange Act in the registration
statement.

                                       21
<PAGE>

      In connection with each registration hereunder, the sellers of Registrable
Securities will furnish to the Company in writing such information requested by
the Company with respect to themselves and the proposed distribution by them as
shall be reasonably necessary in order to assure compliance with Federal and
applicable state securities laws.

4.7.  EXPENSES.

            (a) All expenses incurred by the Company in complying with Sections
      4.3, 4.4 and 4.5, including, without limitation, all registration and
      filing fees, printing expenses, fees and disbursements of counsel and
      independent public accountants for the Company, fees and expenses
      (including counsel fees) incurred in connection with complying with state
      securities or "blue sky" laws, fees of the National Association of
      Securities Dealers, Inc., transfer taxes, fees of transfer agents and
      registrars, costs of any insurance which might be obtained by the Company
      with respect to the offering by the Company, and up to $150,000 in
      reasonable fees and disbursements of one counsel selected by a majority in
      interest of the sellers of Registrable Securities, but excluding any
      Selling Expenses, are called "Registration Expenses". All underwriting
      discounts and selling commissions applicable to the sale of Registrable
      Securities are called "Selling Expenses."

            (b) The Company will pay all Registration Expenses in connection
      with each registration statement under Sections 4.3, 4.4 and 4.5. All
      Selling Expenses in connection with each registration statement under
      Section 4.3, 4.4 or 4.5 shall be borne by the participating sellers in
      proportion to the number of shares registered by each, or by such
      participating sellers other than the Company (except to the extent the
      Company shall be a seller) as they may agree.

4.8.  INDEMNIFICATION AND CONTRIBUTION.

            (a) In the event of a registration of any of the Registrable
      Securities under the Securities Act pursuant to Section 4.3, 4.4 or 4.5,
      the Company will indemnify and hold harmless each Holder of Registrable
      Securities, its officers, directors and partners, each underwriter of such
      Registrable Securities thereunder and each other person, if any, who
      controls such Holder or underwriter within the meaning of the Securities
      Act, against any losses, claims, damages or liabilities, joint or several,
      to which such Holder, officer, director, partner, underwriter or
      controlling person may become subject under the Securities Act or
      otherwise, insofar as such losses, claims, damages or liabilities (or
      actions in respect thereof) arise out of or are based upon (i) any untrue
      statement or alleged untrue statement of any material fact contained in
      any registration statement under which such Registrable Securities were
      registered under the Securities Act pursuant to Section 4.3, 4.4 or 4.5,
      any preliminary prospectus or final prospectus contained therein, or any
      amendment or supplement thereto, or (ii) any omission or alleged omission
      to state in any such registration statement, prospectus, amendment or
      supplement, a material fact required to be stated therein or necessary to
      make the

                                       22
<PAGE>

      statements therein not misleading, and will reimburse each such seller,
      and such officer, director and partner, each such underwriter and each
      such controlling person for any legal or other expenses reasonably
      incurred by them in connection with investigating or defending any such
      loss, claim, damage, liability or action, promptly after being so
      incurred, provided, however, that the Company will not be liable in any
      such case if and to the extent that any such loss, claim, damage or
      liability arises out of or is based upon an untrue statement or alleged
      untrue statement or omission or alleged omission so made in conformity
      with written information furnished by any such Holder, any such
      underwriter or any such controlling person in writing specifically for use
      in such registration statement or prospectus.

            (b) In the event of a registration of any of the Registrable
      Securities under the Securities Act pursuant to Section 4.3, 4.4 or 4.5,
      each seller of such Registrable Securities thereunder, severally and not
      jointly, will indemnify and hold harmless the Company, each person, if
      any, who controls the Company within the meaning of the Securities Act,
      each officer of the Company who signs the registration statement, each
      director of the Company, each other seller of Registrable Securities, each
      underwriter and each person who controls any underwriter within the
      meaning of the Securities Act, against all losses, claims, damages or
      liabilities, joint or several, to which the Company or such officer,
      director, other seller, underwriter or controlling person may become
      subject under the Securities Act or otherwise, insofar as such losses,
      claims, damages or liabilities (or actions in respect thereof) arise out
      of or are based upon (i) any untrue statement or alleged untrue statement
      of any material fact contained in any registration statement under which
      such Registrable Securities were registered under the Securities Act
      pursuant to Section 4.3, 4.4 or 4.5, any preliminary prospectus or final
      prospectus contained therein, or any amendment or supplement thereto, or
      (ii) any omission or alleged omission to state therein a material fact
      required to be stated therein or necessary to make the statements therein
      not misleading (if and only to the extent that any such statement or
      alleged statement or omission or alleged omission was made in reliance
      upon and in conformity with written information furnished to the Company
      by such seller specifically stating that it is for use in the preparation
      of such registration statement, preliminary prospectus, final prospectus,
      summary prospectus, amendment or supplement) and will reimburse the
      Company and each such officer, director, other seller, underwriter and
      controlling person for any legal or other expenses reasonably incurred by
      them in connection with investigating or defending any such loss, claim,
      damage, liability or action, promptly after being so incurred; provided,
      however, that the liability of such indemnifying party under this Section
      4.8(b) shall be limited to the amount of the net proceeds received by such
      indemnifying party in the offering giving rise to such liability. Such
      indemnity shall remain in full force and effect, regardless of any
      investigation made by or on behalf of the Company or any such director,
      officer or controlling person and shall survive the transfer of such
      securities by such seller.

                                       23
<PAGE>

            (c) Promptly after receipt by an indemnified party hereunder of
      notice of the commencement of any action, such indemnified party shall, if
      a claim in respect thereof is to be made against the indemnifying party
      hereunder, notify the indemnifying party in writing thereof, but the
      omission so to notify the indemnifying party shall not relieve it from any
      liability which it may have to such indemnified party other than under
      this Section 4.8 and shall only relieve it from any liability which it may
      have to such indemnified party under this Section 4.8 if and to the extent
      the indemnifying party is actually prejudiced by such omission. In case
      any such action shall be brought against any indemnified party and it
      shall notify the indemnifying party of the commencement thereof, the
      indemnifying party shall be entitled to participate in and, to the extent
      it shall wish, to assume and undertake the defense thereof with counsel
      satisfactory to such indemnified party, and, after notice from the
      indemnifying party to such indemnified party of its election so to assume
      and undertake the defense thereof, the indemnifying party shall not be
      liable to such indemnified party under this Section 4.8 for any legal
      expenses subsequently incurred by such indemnified party in connection
      with the defense thereof other than reasonable costs of investigation and
      of liaison with counsel so selected, provided, however, that, if the
      defendants in any such action include both the indemnified party and the
      indemnifying party and the indemnified party shall have reasonably
      concluded that there may be reasonable defenses available to it which are
      different from or additional to those available to the indemnifying party
      or that the interests of the indemnified party reasonably may be deemed to
      conflict with the interests of the indemnifying party, the indemnified
      party shall have the right to select a separate counsel and to assume such
      legal defenses and otherwise to participate in the defense of such action,
      with the expenses and fees of such separate counsel and other expenses
      related to such participation to be reimbursed by the indemnifying party
      as incurred. No indemnifying party, in the defense of any such claim or
      action, shall, except with the consent of each indemnified party, consent
      to entry of any judgment or enter into any settlement which does not
      include as an unconditional term thereof the giving by the claimant or
      plaintiff to such indemnified party of a release from all liability in
      respect to such claim or action. Each indemnified party shall furnish such
      information regarding itself or the claim in question as an indemnifying
      party may reasonably request in writing and as shall be reasonably
      required in connection with defense of such claim and litigation resulting
      therefrom.

            (d) In order to provide for just and equitable contribution to joint
      liability under the Securities Act in any case in which either (i) any
      Holder of Registrable Securities exercising rights under this Agreement,
      or any controlling person of any such Holder, makes a claim for
      indemnification pursuant to this Section 4.8 but it is judicially
      determined (by the entry of a final judgment or decree by a court of
      competent jurisdiction and the expiration of time to appeal or the denial
      of the last right of appeal) that such indemnification may not be enforced
      in such case notwithstanding the fact that this Section 4.8 provides for
      indemnification in such case, or (ii) contribution under the Securities
      Act may be required on the part of any such selling Holder or any such
      controlling person in

                                       24
<PAGE>

      circumstances for which indemnification is provided under this Section
      4.8; then, and in each such case, the Company and such Holder will
      contribute to the aggregate losses, claims, damages or liabilities to
      which they may be subject (after contribution from others) in such
      proportion as is appropriate to reflect the relative fault of the Company
      on the one hand and of the Holder on the other in connection with the
      matter that resulted in such loss, claim, damage or liability, as well as
      any other relevant equitable considerations. The relative fault of the
      Company and of the Holder will be determined by a court of law by
      reference to, among other things, whether the untrue or alleged untrue
      statement of a material fact or the omission to state a material fact
      relates to information supplied by the Company or by the Holder and the
      parties' relative intent, knowledge, access to information and opportunity
      to correct or prevent such statement or omission; provided, however, that,
      in any such case, (A) no such Holder of Registrable Securities will be
      required to contribute any amount in excess of the net proceeds received
      from the sale of all such Registrable Securities offered by it pursuant to
      such registration statement; and (B) no person or entity guilty of
      fraudulent misrepresentation (within the meaning of Section 11(f) of the
      Securities Act) will be entitled to contribution from any person or entity
      who was not guilty of such fraudulent misrepresentation. Such Holders'
      obligations to contribute as provided in this Section 4.8(d) are several
      in proportion to the relative value of their respective Registrable
      Securities covered by such registration statement and not joint. In
      addition, no person or entity shall be obligated to contribute hereunder
      any amounts in payment for any settlement of any action or claim effected
      without such person's or entity's consent, which consent shall not be
      unreasonably withheld.

            (e) The indemnities and obligations provided in this Section 4.8
      shall survive the transfer of any Registrable Securities by such Holder.

4.9. CHANGES IN COMMON STOCK. If, and as often as, there is any change in the
Common Stock by way of a stock split, stock dividend, combination or
reclassification, or through a merger, consolidation, reorganization or
recapitalization, or by any other means, appropriate adjustment shall be made in
the provisions hereof so that the rights and privileges granted hereby shall
continue with respect to the Common Stock as so changed.

4.10. RULE 144 REPORTING. With a view to making available the benefits of
certain rules and regulations of the Commission which may at any time permit the
sale of the Registrable Securities to the public without registration, except as
provided in paragraph (c) below, at all times after ninety (90) days after any
registration statement covering a public offering of securities of the Company
under the Securities Act shall have become effective, the Company agrees to:

            (a) make and keep public information available, as those terms are
      understood and defined in Rule 144 under the Securities Act (or any
      successor rule);

                                       25
<PAGE>

            (b) use its best efforts to file with the Commission in a timely
      manner all reports and other documents required of the Company under the
      Securities Act and the Exchange Act; and

            (c) furnish to each Holder of Registrable Securities forthwith upon
      request (i) a written statement by the Company as to its compliance with
      the reporting requirements of such Rule 144 (or any successor rule), (ii)
      at any time after it has become subject to such reporting requirements, of
      the Securities Act and the Exchange Act, a copy of the most recent annual
      or quarterly report of the Company and (iii) such other reports and
      documents so filed by the Company as such Holder may reasonably request in
      availing itself of any rule or regulation of the Commission allowing such
      Holder to sell any Registrable Securities without registration.

4.11. "MARKET STAND-OFF" AGREEMENT. Each of the Holders of Registrable
Securities agrees, severally and not jointly, if reasonably requested by the
Company and an underwriter of Common Stock (or other securities) of the Company,
not to sell or otherwise transfer or dispose of any Common Stock (or other
securities) of the Company held by such Holder of Registrable Securities during
a period not to exceed one hundred and eighty (180) days following the effective
date of the Company's Initial Public Offering (or such additional period, not to
exceed 17 days, as may be reasonably requested by the Company in order to permit
the publication or distribution of a research report or make a public appearance
concerning the Company prior to and after the expiration, waiver or termination
of a lock-up agreement executed in connection with the Initial Public Offering),
and to enter into an agreement to such effect; provided, that (y) all persons
including Shares in such offering and (z) all of the Company's officers and
directors and holders of at least 5% of the Company's Common Stock also enter
into agreements to such effect. Nothing in this Section 4.11 shall require any
Investor to make any factual representations other than the Investor's
commitment not to sell or otherwise transfer or dispose of any capital stock of
the Company as provided herein.

      The Company may impose stop-transfer instructions with respect to the
shares (or securities) subject to the foregoing restriction until the end of
said period.

4.12. ASSIGNMENT OF REGISTRATION RIGHTS. The rights to cause the Company to
register Registrable Securities pursuant to this Section 4 may be assigned (but
only with all related obligations) by a Holder of Registrable Securities who is
a Qualified Stockholder to a transferee or assignee of such securities who is
not engaged in a business activity competitive with the Company (as reasonably
determined by the Company's Board of Directors) and who, after such assignment
or transfer, holds (i) at least one half of one percent (0.5%) of the shares of
the Preferred Stock (and is, therefore, a Qualified Shareholder) (subject to
appropriate adjustment for stock splits, stock dividends, combinations and
similar recapitalization events), (ii) who, after any such transfer, as a single
entity, holds all Registrable Securities of the Company held by the transferring
party immediately prior to such transfer, or (iii) who is an Affiliate, partner
or member of such Holder, provided that in each of the foregoing cases the
Company is, within a reasonable time after such transfer, furnished with written
notice of the name and address

                                       26
<PAGE>

of such transferee or assignee and the securities with respect to which such
registration rights are being assigned; and provided, further, that such
assignment shall be effective only if (i) immediately following such transfer
the further disposition of such securities by the transferee or assignee is
restricted under the Act and (ii) the transferee or assignee shall acknowledge
in writing that the transferred or assigned Registrable Securities shall remain
subject to this Agreement. For the purposes of determining the number of shares
of Registrable Securities held by a transferee or assignee, the holdings of
transferees and assignees of a partnership who are partners or retired partners
of such partnership (including spouses and ancestors, lineal descendants and
siblings of such partners or spouses who acquire Registrable Securities by gift,
will or intestate succession) shall be aggregated together and with the
partnership; provided, that all assignees and transferees who would not qualify
individually for assignment of registration rights shall have a single
attorney-in-fact for the purpose of exercising any rights, receiving notices or
taking any action under this Section 4.

4.13. LIMITATION ON SUBSEQUENT REGISTRATION RIGHTS. Except for any registration
rights granted to the parties pursuant to this Agreement and for the
registration rights that are both in effect as of the Effective Date and listed
on Schedule 4.13 hereto, the Company shall not grant to any third party any
registration rights senior to, or that conflict with, any of those contained
herein, so long as any of the registration rights under this Agreement remains
in effect; provided, however, that any additional entity may be added as an
"Other Shareholder" solely for purposes of Section 4.5 and as a "Holder" solely
for purposes of such provisions of Sections 4.6 thru 4.13 that such entity may
agree to become subject as a result of its negotiations with the Company without
an amendment to this Agreement upon the execution and delivery by such entity of
a Joinder Agreement substantially in the form of Exhibit G attached hereto. The
execution of any such Joinder Agreement(s) shall not constitute an amendment for
purposes of Section 7.13 hereof.

5.    AFFIRMATIVE COVENANTS OF THE COMPANY

The Company covenants and agrees that, from and after the date of the Initial
Closing under the Subscription Agreement and thereafter so long as any Investor
owns Preferred Stock or Registrable Securities, it will perform and observe the
following covenants and provisions, and will cause each Subsidiary, if and when
such Subsidiary exists, to perform and observe the following covenants and
provisions as applicable to such Subsidiary.

5.1. FINANCIAL STATEMENTS; OTHER REPORTS. The Company will deliver to each
Qualified Stockholder:

            (a) as soon as available and in any event within 45 days after the
      end of each of the first three quarters of each fiscal year of the
      Company, a consolidated balance sheet of the Company and its Subsidiaries
      as of the end of such quarter and the related statements of income and
      stockholders' equity and of cash flows of the Company for the period
      commencing at the end of the previous fiscal year and ending with the end
      of such quarter, setting forth in each case in

                                       27
<PAGE>

      comparative form the corresponding figures for the corresponding period of
      the preceding fiscal year and the budget for such current year, all in
      reasonable detail and prepared in accordance with generally accepted
      accounting principles consistently applied, and duly certified (subject to
      year-end audit adjustments) by the chief financial officer of the Company;

            (b) the Company shall use its best efforts to provide within 90
      days, but in no event more than 120 days, after the end of each fiscal
      year of the Company, a copy of the annual audit report for such year for
      the Company, including therein a consolidated balance sheet of the Company
      and its Subsidiaries as of the end of such fiscal year and statements of
      income and stockholders' equity and of cash flows of the Company for such
      fiscal year, setting forth in each case in comparative form the
      corresponding figures for the preceding fiscal year, all duly certified by
      independent public accountants of recognized standing acceptable to the
      Qualified Stockholders;

            (c) promptly after sending or making available, and only to the
      extent available and requested in writing, such reports and financial
      statements as the Company shall send or make available to the management
      of the Company from time to time;

            (d) promptly upon receipt thereof, any written report submitted to
      the Company by independent public accountants in connection with an annual
      or interim audit of the books of the Company and its Subsidiaries made by
      such accountants;

            (e) promptly after sending, making available, or filing the same,
      such reports and financial statements as the Company shall send or make
      available to the stockholders of the Company; and

            (f) as soon as available in the form approved by the Board of
      Directors, and in any event before the beginning of the fiscal year to
      which it applies, the annual budget and business plan of the Company.

      Neither the foregoing provisions of this Section nor any other provision
of this Agreement shall be in limitation of any rights which a Qualified
Stockholder may have with respect to the books and records of the Company and
its Subsidiaries, or to inspect their properties or discuss their affairs,
finances and accounts, under the laws of the jurisdictions in which they are
incorporated.

5.2. STOCK DISPOSITION AGREEMENT. The Company and each employee, officer,
director or consultant of the Company who hereafter acquires shares of the
Company's capital stock from the Company, or any option or right to acquire
shares of the Company's capital stock from the Company, shall enter into a stock
disposition agreement, or similar agreement, in such form as approved by the
Board of Directors, and with such amendments or modifications as may be entered
into or effected with the approval of the Board of Directors.

                                       28
<PAGE>

5.3. CONFIDENTIALITY AND ASSIGNMENT OF INVENTION PROVISIONS. Other than as may
be approved by the Board of Directors from time-to-time on a case-by-case basis,
the Company shall require all officers, employees, contractors and consultants
who may have access to Company Intellectual Property to execute and deliver an
Inventions Agreement. The terms "Company Intellectual Property" and "Inventions
Agreement" shall have the meanings assigned to such terms in the Subscription
Agreement.

5.4. USE OF NAME. Except where required by law or regulation, the Company shall
not use the name of any Investor in any press release or other public disclosure
without the advance consent of the relevant Investor, such consent not to be
unreasonably withheld or delayed. The foregoing notwithstanding, the previous
sentence of this Section 5.3 shall not be interpreted to require the Company to
obtain the advance consent of any Investor in order to identify to potential
third party investors in a private offering in compliance with the Securities
Act the fact that such Investor is an investor in the Company.

6.    PRIOR AGREEMENTS

By execution of this Agreement, the Company, each Series A and Series B
Investor, Founder, Series C Purchaser, Series D Purchaser, Series E Purchaser
and Series E-2 Purchaser agrees as follows:

            (a) the Fourth Amended Rights Agreement, Third Amended Rights
      Agreement, the Second Amended Rights Agreement, the Amended Rights
      Agreement, the Original Rights Agreement are hereby terminated and
      replaced in their entirety by this Fifth Amended and Restated Investor
      Rights Agreement; and

            (b) each Series A and Series B Investor, Series C Purchaser, Series
      D Purchaser and Series E Purchaser agrees and consents to waive any and
      all percentage maintenance rights with respect to the offer and sale by
      the Company of shares of its Series E-2 Stock to the Series E-2 Purchasers
      contemplated by the Subscription Agreement including, without limitation,
      such rights set forth in Section 2 "Percentage Maintenance Rights" of that
      certain Fourth Amended Rights Agreement.

7.    MISCELLANEOUS

7.1. NOTICES. All notices, requests, consents and other communications hereunder
shall be in writing, shall be addressed to the receiving party's address set
forth below or to such other address as a party may designate by notice
hereunder, and shall be either (i) delivered by hand, (ii) made by telecopy or
facsimile transmission, (iii) sent by overnight courier, or (iv) sent by
registered or certified mail, return receipt requested, postage prepaid;
provided, however, that if the notice being provided under this Section 7.1 is
to an address outside of the United States, such notice shall be sent using the
methods specified in (ii) and/or (iii) above.

                                       29
<PAGE>

If to the Company:                  PTC Therapeutics, Inc.
                                    100 Corporate Court
                                    Middlesex Business Center
                                    South Plainfield, NJ  07080-2449
                                    Attn.:  Legal Dep't

With a copy to:                     Wilmer Cutler Pickering Hale and Dorr LLP
                                    60 State Street
                                    Boston, Massachusetts  02109

                                    Attn:  David E. Redlick, Esq.
And an email copy to                legal@ptcbio.com

If to the Holders:                  To the addresses set forth on
                                    Exhibit A, Exhibit B, Exhibit C,
                                    Exhibit D, Exhibit E or
                                    Exhibit F attached hereto

All notices, requests, consents and other communications hereunder shall be
deemed to have been given either (i) if by hand, at the time of the delivery
thereof to the receiving party at the address of such party set forth above,
(ii) if made by telecopy, facsimile, or email transmission, at the time that
receipt thereof has been acknowledged by electronic confirmation or otherwise,
(iii) if sent by overnight courier, on the next business day following the day
such notice is delivered to the courier service, or, in the case of
international deliveries, two business days following the day such notice is
delivered to the courier service or (iv) if sent by registered or certified
mail, on the fifth business day following the day such mailing is made.

7.2. ENTIRE AGREEMENT. This Agreement, together with those certain Stock
Restriction Agreements by and between the Company and each of the Founders dated
August 19, 1998, embodies the entire agreement and understanding between the
parties hereto with respect to the subject matter hereof and supersedes all
prior oral or written agreements and understandings relating to the subject
matter hereof, including the Fourth Amended Rights Agreement, the Third Amended
Rights Agreement, the Second Amended Rights Agreement, the Amended Rights
Agreement and the Original Rights Agreement. No statement, representation,
warranty, covenant or agreement of any kind not expressly set forth in this
Agreement shall affect, or be used to interpret, change or restrict, the express
terms and provisions of this Agreement.

7.3. MODIFICATIONS AND AMENDMENTS. This Agreement may not be amended or
modified, and no provision hereof may be waived, without the written consent of
the Company, Holders of at least sixty-six and two-thirds percent (66 2/3 %) of
the outstanding shares of Preferred Stock, on an as-converted, fully-diluted
basis (after giving effect to the issuance of the Series E-2 Stock pursuant to
the Subscription Agreement); provided, that, (i) if any such amendment,
modification or waiver adversely affects the interests of any Stockholder or
group of Stockholders differently from the interests of any other Stockholder,
such amendment, modification or waiver shall also require the written consent of
such Stockholder or of a majority of such group of Stockholders, as the case may
be; provided, however, that the determination of whether

                                       30
<PAGE>

any Stockholder or group of Stockholders is affected differently shall be made
without regard to the number of shares of Preferred Stock held by such
Stockholder or group of Stockholders and (ii) no such amendment, modification or
waiver shall impose any new or increased obligation on any Stockholder without
the written agreement of the affected Stockholder. Any waiver or consent
hereunder shall be effective only in the specific instance and for the purpose
for which it was given and shall not constitute a continuing waiver or consent.

7.4. ASSIGNMENT. Except by operation of law, the rights and obligations of the
Company under this Agreement may not be assigned by the Company without the
prior written consent of at least a majority of the Holders of Registrable
Securities, unless specifically permitted by the terms hereof. Any purported or
attempted assignment in violation of this Section 7.4 shall be null and void.

7.5. BENEFIT. All statements, representations, warranties, covenants and
agreements in this Agreement shall be binding on the parties hereto and shall
inure to the benefit of the respective successors and permitted assigns of each
party hereto. Nothing in this Agreement shall be construed to create any rights
or obligations except among the parties hereto and no person or entity shall be
regarded as a third-party beneficiary of this Agreement.

7.6. AGGREGATION OF STOCK. All shares held or acquired by affiliated entities or
persons or successor entities of a Stockholder shall be aggregated together with
the shares held by such Stockholder for the purposes of determining the
availability of rights under this Agreement.

7.7. GOVERNING LAW. This Agreement and the rights and obligations of the parties
hereunder shall be construed in accordance with and governed by the law of the
State of Delaware, without giving effect to the conflict of law principles
thereof.

7.8. SEVERABILITY. In the event that any court of competent jurisdiction shall
determine that any provision, or any portion thereof, contained in this
Agreement shall be unenforceable in any respect, then such provision shall be
deemed limited to the extent that such court deems it enforceable and as so
limited shall remain in full force and effect. In the event that such court
shall deem any such provision, or portion thereof, wholly unenforceable, the
remaining provisions of this Agreement shall nevertheless remain in full force
and effect.

7.9. INTERPRETATION. The parties hereto acknowledge and agree that: (i) each
party and its counsel reviewed and negotiated the terms and provisions of this
Agreement and have contributed to its revision; (ii) the rule of construction to
the effect that any ambiguities are resolved against the drafting party shall
not be employed in the interpretation of this Agreement; and (iii) the terms and
provisions of this Agreement shall be construed fairly as to all parties hereto
and not in favor of or against any party, regardless of which party was
generally responsible for the preparation of this Agreement.

                                       31
<PAGE>

7.10. HEADINGS AND CAPTIONS. The headings and captions of the various
subdivisions of this Agreement are for convenience of reference only and shall
in no way modify or affect the meaning or construction of any of the terms or
provisions hereof.

7.11. ENFORCEMENT. Each of the parties hereto acknowledges and agrees that the
rights acquired by each party hereunder are unique and that irreparable damage
would occur in the event that any of the provisions of this Agreement to be
performed by the other parties were not performed in accordance with their
specific terms or were otherwise breached. Accordingly, in addition to any other
remedy to which the parties hereto are entitled at law or in equity, each party
hereto shall be entitled to an injunction or injunctions to prevent breaches of
this Agreement by any other party and to enforce specifically the terms and
provisions hereof in any federal or state court to which the parties have agreed
hereunder to submit to jurisdiction.

7.12. NO WAIVER OF RIGHTS, POWERS AND REMEDIES. No failure or delay by a party
hereto in exercising any right, power or remedy under this Agreement, and no
course of dealing among the parties hereto, shall operate as a waiver of any
such right, power or remedy of the party. No single or partial exercise of any
right, power or remedy under this Agreement by a party hereto, nor any
abandonment or discontinuance of steps to enforce any such right, power or
remedy, shall preclude such party from any other or further exercise thereof or
the exercise of any other right, power or remedy hereunder. The election of any
remedy by a party hereto shall not constitute a waiver of the right of such
party to pursue other available remedies. No notice to or demand on a party not
expressly required under this Agreement shall entitle the party receiving such
notice or demand to any other or further notice or demand in similar or other
circumstances or constitute a waiver of the rights of the party giving such
notice or demand to any other or further action in any circumstances without
such notice or demand.

7.13. COUNTERPARTS. This Agreement may be executed by facsimile signature and in
one or more counterparts, and by different parties hereto on separate
counterparts, each of which shall be deemed an original, but all of which
together shall constitute one and the same instrument.

7.14. TERMINATION. The respective rights and obligations of the parties under
the following provisions of this Agreement shall terminate upon the consummation
of an Initial Public Offering: Section 1.1 (but only with respect to Shares
acquired after the Initial Public Offering); Article 2; Article 3 and Article 5.

7.15. LEGAL REPRESENTATIONS OF THE PARTIES. Each Holder and Founder acknowledges
that (i) Credit Suisse First Boston Equity Partners L.P. ("CSFB") retained Paul,
Weiss, Rifkind, Wharton & Garrison LLP ("PWRW&G") to represent CSFB in
connection with this Agreement, (ii) PWRW&G represents CSFB and any other Holder
who has specifically retained PWRW&G in writing for representation with respect
to this Agreement (CSFB and all such Holders, collectively, the "PWRW&G
Clients") and (iii) no Holder, other than the PWRW&G Clients, nor any Founder
has relied on PWRW&G for legal counsel in connection with the negotiation,
preparation, execution, delivery or performance of this Agreement.

                                       32
<PAGE>

      IN WITNESS WHEREOF, the parties hereto have executed this Fifth Amended
and Restated Investor Rights Agreement or caused this Fifth Amended and Restated
Investor Rights Agreement to be executed by their duly authorized
representatives, as of the date first written above.

                                              PTC THERAPEUTICS, INC.

                                              By:   /S/ STUART W. PELTZ
                                                    -------------------
                                                    Name: Stuart W. Peltz
                                                    Title: President and CEO

                                              FOUNDERS

                                              By:   /S/ STUART W. PELTZ
                                                    -------------------
                                                    Stuart W. Peltz

                                              By:   /S/ ALLAN S. JACOBSON
                                                    ---------------------
                                                    Allan S. Jacobson

AGREED TO AND ACKNOWLEDGED BY:

/S/ DENISE BROUILLETTE JACOBSON
-------------------------------
Denise Brouillette Jacobson

[Investors Executed by Means of Omnibus Signature Pages or Stockholder Consents,
     the forms of which are attached hereto as Exhibit H-1and Exhibit H-2,
                                 respectively]<PAGE>
                                                                     Exhibit 4.3

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 295,000
                                             shares of Series C Preferred Stock,
                                                       par value $.001 per share

No. W-01                                                            May 26, 2000

      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 May 26, 2010 (the "Exercise Period"),
up to two hundred ninety-five thousand (295,000) fully-paid and non-assessable
shares of Series C Preferred Stock, par value $.001 per share, of the Company
(the "Series C 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 C Preferred Stock as provided in Section 3 hereof (the Series C
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
$2.50 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 May 26, 2000.

      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 part, at the principal office of the
Company together with the notice of exercise attached hereto

<PAGE>

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 Certificate, 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.

      3. Adjustments Upon Mandatory Conversion of Series C Preferred Stock. Upon
any mandatory conversion of the Series C Preferred Stock pursuant to the
Company's Certificate of

                                       2
<PAGE>

Incorporation, as amended or amended and restated from time to time, this
Warrant shall cease to be exercisable for shares of Series C Preferred Stock and
shall become exercisable for that number of shares of Common Stock into which
the shares of Series C 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 C 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 C Preferred Stock, or issues a
stock dividend on the outstanding shares of Series C 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 C 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 C Preferred Stock as provided in Section 3, each reference to Series
C 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.

      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

                                       3
<PAGE>

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 C Preferred Stock and Common Stock. The Company
covenants and agrees that the shares of Series C 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 C 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 C 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

                                       4
<PAGE>

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 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 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.

      12.3. 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."

                                       5
<PAGE>

      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.

      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

                                       6
<PAGE>

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

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

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,

                                       7
<PAGE>

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.

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

                                     PTC THERAPEUTICS, INC.

                                     By:  /s/ Stuart Peltz
                                          ____________________________

                                       8
<PAGE>

                                    EXHIBIT A

                    NOTICE OF EXERCISE OR NET ISSUE EXERCISE

                                           Date:
                                                 _______________________________

PTC Therapeutics, Inc.
100 Corporate Court
Middlesex Business Center
South Plainfield, NJ 07080
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 C 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
  _______________________________

                                       9

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