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                                                                Exhibit 10.4

                           SIXTH AMENDED AND RESTATED
                          REGISTRATION RIGHTS AGREEMENT

         AGREEMENT, made as of the 7th day of April, 2000, by and among Exact
Laboratories, Inc. (the "Company"), Stanley N. Lapidus (the "Founder"), and the
persons listed as Investors on the attached SCHEDULE I (collectively, the
"Investors" and individually, the "Investor").

         WHEREAS, the Founder is the beneficial holder of an aggregate of
435,000 shares (the "Founder's Shares") of Common Stock, $.01 par value per
share, of the Company (the "Common Stock"); and certain of the Investors are the
holders of an aggregate of 902,414 shares of Series A Convertible Preferred
Stock, $.01 par value per share (the "Series A Preferred Stock"), 996,196 shares
of Series B Convertible Preferred Stock (the "Series B Preferred Stock") of the
Company, and 1,007,186 shares of Series C Convertible Preferred Stock (the
"Series C Preferred Stock").

         WHEREAS, certain of the Investors are acquiring shares of Series D
Convertible Preferred Stock (the "Series D Preferred Stock") (such shares,
together with the 902,414 shares of Series A Preferred Stock and 996,196 shares
of Series B Preferred Stock, 1,007,186 shares of Series C Preferred Stock,
previously referred to, being referred to collectively as the "Preferred
Shares") pursuant to the Series D Convertible Preferred Stock Purchase Agreement
(the "Stock Purchase Agreement") between the Company and certain of the
Investors of even date herewith, with the result that each Investor will own the
number of shares of Series A Preferred Stock, Series B Preferred Stock, Series C
Preferred Stock and/or Series D Preferred Stock set forth in SCHEDULE I
attached; and

         WHEREAS, pursuant to Section 13(d) of the Fifth Amended and Restated
Registration Rights Agreement dated as of March 17, 1998 among the Company, the
Founder, and certain of the Investors (the "Registration Rights Agreement") the
parties thereto desire to amend and restate the Registration Rights Agreement in
its entirety by entering into this Sixth Amended and Restated Registration
Rights Agreement.

         NOW THEREFORE, in consideration of the foregoing, and the agreements
set forth below, the parties hereby agree with each other as follows:

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

         "COMMISSION" shall mean the Securities and Exchange Commission, or any
      other federal agency at the time administering the Securities Act.

         "COMMON STOCK" shall mean the Common Stock, $ .01 par value, of the
      Company, as constituted as of the date of this Agreement.

         "CONVERSION SHARES" shall mean shares of Common Stock issued or
      issuable upon conversion of the Preferred Shares.

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         "EXCHANGE ACT" shall mean the Securities Exchange Act of 1934, as
      amended, or any similar federal statute, and the rules and regulations of
      the Commission thereunder, all as the same shall be in effect at the time.

         "REGISTRATION EXPENSES" shall mean the expenses so described in
      Section 8.

         "RESTRICTED STOCK" shall mean the Conversion Shares and the Founder's
      Shares, excluding Conversion Shares and Founder's Shares which have been
      (a) registered under the Securities Act pursuant to an effective
      registration statement filed thereunder and disposed of in accordance with
      the registration statement covering them or (b) publicly sold pursuant to
      Rule 144 under the Securities Act.

         "SECURITIES ACT" shall mean the Securities Act of 1933, as amended, or
      any similar federal statute, and the rules and regulations of the
      Commission thereunder, all as the same shall be in effect at the time.

         "SELLING EXPENSES" shall mean the expenses so described in Section 8.

               2. RESTRICTIVE LEGEND. Each certificate representing Preferred
Shares or Conversion Shares shall, except as otherwise provided in this Section
2 or in Section 3, be stamped or otherwise imprinted with a legend substantially
in the following form:

                  "THIS SECURITY HAS NOT BEEN REGISTERED UNDER THE SECURITIES
              ACT OF 1933 OR ANY STATE SECURITIES LAWS AND MAY NOT BE
              TRANSFERRED OR OTHERWISE DISPOSED OF UNLESS IT HAS BEEN REGISTERED
              UNDER SUCH ACT AND ALL SUCH APPLICABLE LAWS OR AN EXEMPTION FROM
              REGISTRATION IS AVAILABLE."

A certificate shall not bear such legend if in the opinion of counsel
satisfactory to the Company (it being agreed that Testa, Hurwitz & Thibeault,
LLP or Schulte Roth & Zabel LLP shall be satisfactory) the securities
represented thereby may be publicly sold without registration under the
Securities Act and any applicable state securities laws.

               3. NOTICE OF PROPOSED TRANSFER. Prior to any proposed transfer of
any Preferred Shares or Conversion Shares (other than under the circumstances
described in Sections 4, 5 or 6), the holder thereof shall give written notice
to the Company of its intention to effect such transfer. Each such notice shall
describe the manner of the proposed transfer and, if requested by the Company,
shall be accompanied by an opinion of counsel satisfactory to the Company (it
being agreed that Testa, Hurwitz & Thibeault, LLP shall be satisfactory) to the
effect that the proposed transfer may be effected without registration under the
Securities Act and any applicable state securities laws, 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 a transfer to one or more members or partners of the

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transferor (in the case of a transferor that is a limited liability company or
partnership) or to an affiliate, entity controlled by an affiliate (as such term
is defined in Rule 405 promulgated under the Securities Act of 1933, as amended)
or subsidiary of such holder or to a successor by operation of law. Each
certificate for Preferred Shares or Conversion Shares transferred as above
provided shall bear the legend set forth in Section 2, 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
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. The restrictions provided for in this Section 3 shall not apply
to securities which are not required to bear the legend prescribed by Section 2
in accordance with the provisions of that Section.

               4. REQUIRED REGISTRATION. (a) At any time after the earliest of
(i) six months after any registration statement covering an initial public
offering of securities of the Company under the Securities Act shall have become
effective or (ii) six months after the Company shall have become a reporting
company under Section 12 of the Exchange Act, the holders of Conversion Shares
constituting at least 30% of the total shares of Conversion Shares then
outstanding may request the Company to register under the Securities Act all or
any portion of the shares of Conversion Shares held by such requesting holder or
holders for sale in the manner specified in such notice, PROVIDED that the
shares of Conversion Shares for which registration has been requested shall
constitute at least 20% of the total shares of Conversion Shares originally
issued if such holder or holders shall request the registration of less than all
shares of Conversion Shares then held by such holder or holders (or any lesser
percentage if the reasonably anticipated aggregate price to the public of such
public offering would exceed $10,000,000). For purposes of this Section 4 and
Sections 5 and 6, the term "Conversion Shares" shall be deemed to include the
number of Conversion Shares which would be issuable to a holder of Preferred
Shares upon the conversion of all Preferred Shares held by such holder at such
time, and for purposes of Section 13(a), the term "Restricted Stock" shall be
deemed to include the number of shares of Conversion Shares which would be
issuable to a holder of Preferred Shares upon conversion of all Preferred Shares
held by such holder at such time, PROVIDED, HOWEVER, that the only securities
which the Company shall be required to register pursuant to Sections 4, 5 or 6
shall be shares of Common Stock, and PROVIDED, FURTHER, HOWEVER, that, in any
underwritten public offering contemplated by this Section 4 or Sections 5 and 6,
the holders of Preferred Shares shall be entitled to sell such Preferred Shares
to the underwriters for conversion and sale of the shares of Common Stock issued
upon conversion thereof. Notwithstanding anything to the contrary contained
herein, no request may be made under this Section 4 within 120 days after the
effective date of a registration statement filed by the Company covering a firm
commitment underwritten public offering in which the holders of Conversion
Shares shall have been entitled to join pursuant to Sections 5 or 6 and in which
there shall have been effectively registered all shares of Restricted Stock as
to which registration shall have been requested.

               (b) Following receipt of any notice under this Section 4, the
Company shall immediately notify all holders of Conversion Shares from whom
notice has not been received and shall use its best efforts to register within
30 days of such notice (unless the Company is

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required to conduct an audit of its financial statements, in which case the
Company shall use its best efforts to effect such registration as promptly as is
practical after such 30-day period) under the Securities Act, for public sale in
accordance with the method of disposition specified in such notice from
requesting holders, the number of shares of Conversion Shares specified in such
notice (and in all notices received by the Company from other holders within 30
days after the giving of such notice by the Company). If such method of
disposition shall be an underwritten public offering, the holders of a majority
of Conversion Shares to be sold in such offering may designate the managing
underwriter of such offering, subject to the approval of the Company, which
approval shall not be unreasonably withheld or delayed. The Company shall be
obligated to register Conversion Shares pursuant to this Section 4 on four
occasions only, PROVIDED, HOWEVER, that such obligation shall be deemed
satisfied only when a registration statement covering all Conversion Shares
specified in notices received as aforesaid, for sale in accordance with the
method of disposition specified by the requesting holders, shall have become
effective and, if such method of disposition is a firm commitment underwritten
public offering, all such shares shall have been sold pursuant thereto. If a
registration pursuant to this Section 4 is an underwritten public offering and
the managing underwriter advises the Company in writing that in its opinion the
inclusion of the number of Conversion Shares creates a substantial risk that the
price per share of Common Stock will be reduced, the number of shares of
Conversion Shares to be included in such an underwriting may be reduced (pro
rata among the requesting holders based upon the number of shares requested to
be included therein).

               (c) The Company shall be entitled to include in any registration
statement referred to in this Section 4, for sale in accordance with the method
of disposition specified by the requesting holders, shares of Common Stock to be
sold by the Company for its own account, except as and to the extent that, in
the opinion of the managing underwriter (if such method of disposition shall be
an underwritten public offering), such inclusion would adversely affect the
marketing of the Restricted Stock to be sold. Except for registration statements
on Forms S-4, S-8 or any successor thereto, the Company will not file with the
Commission any other registration statement with respect to its Common Stock,
whether for its own account or that of other stockholders, from the date of
receipt of a notice from requesting holders pursuant to this Section 4 until the
completion of the period of distribution of the registration contemplated
thereby.

               5. INCIDENTAL REGISTRATION. If the Company at any time (other
than pursuant to Section 4 or Section 6) 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 another form not
available for registering the Restricted Stock for sale to the public), each
such time it will give written notice to all holders of outstanding Restricted
Stock of its intention so to do. Upon the written request of any such holder,
received by the Company within 30 days after the giving of any such notice by
the Company, to register any of its Restricted Stock, the Company will use its
best efforts to cause the Restricted Stock 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 of such
Restricted Stock so registered. In the event that any registration pursuant to
this Section 5 shall be,

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in whole or in part, an underwritten public offering of Common Stock, the number
of shares of Restricted Stock to be included in such an underwriting may be
reduced (pro rata among the requesting holders based upon the number of shares
of Restricted Stock owned by such holders) if and to the extent that the
managing underwriter shall be of the opinion that such inclusion would adversely
affect the marketing of the securities to be sold by the Company therein,
PROVIDED, HOWEVER, that such number of shares of Restricted Stock shall not be
reduced if any shares are to be included in such underwriting for the account of
any person other than the Company or requesting holders of Restricted Stock, and
PROVIDED, FURTHER, HOWEVER, that in no event may less than one-third of the
total number of shares of Common Stock to be included in such underwriting be
made available for shares of Restricted Stock. Notwithstanding the foregoing
provisions, the Company may withdraw any registration statement referred to in
this Section 5 without thereby incurring any liability to the holders of
Restricted Stock.

               6. REGISTRATION ON FORM S-3. If at any time (i) a holder or
holders of Preferred Shares or Restricted Stock request that the Company file a
registration statement on Form S-3 or any successor thereto for a public
offering of all or any portion of the shares of Restricted Stock held by such
requesting holder or holders, the reasonably anticipated aggregate price to the
public of which would exceed $5,000,000, and (ii) the Company is a registrant
entitled to use Form S-3 or any successor thereto to register such shares, then
the Company shall use its best efforts to register within 30 days of such notice
(unless the Company is required to conduct an audit of its financial statements,
in which case the Company shall use its best efforts to effect such registration
as promptly as is practical after such 30-day period) under the Securities Act
on Form S-3 or any successor thereto, for public sale in accordance with the
method of disposition specified in such notice, the number of shares of
Restricted Stock specified in such notice. Whenever the Company is required by
this Section 6 to use its best efforts to effect the registration of Restricted
Stock, each of the procedures and requirements of Section 4 (including but not
limited to the requirement that the Company notify all holders of Restricted
Stock from whom notice has not been received and provide them with the
opportunity to participate in the offering) shall apply to such registration,
PROVIDED, HOWEVER, that there shall be no limitation on the number of
registrations on Form S-3 which may be requested and obtained under this Section
6, and PROVIDED, FURTHER, HOWEVER, that the requirements contained in the first
sentence of Section 4(a) shall not apply to any registration on Form S-3 which
may be requested and obtained under this Section 6.

               7. REGISTRATION PROCEDURES. If and whenever the Company is
required by the provisions of Sections 4, 5 or 6 to use its best efforts to
effect the registration of any shares of Restricted Stock under the Securities
Act, the Company will, as expeditiously as possible:

               (a) prepare and file with the Commission a registration statement
(which, in the case of an underwritten public offering pursuant to Section 4,
shall be on Form S-1 or other form of general applicability satisfactory to the
managing underwriter selected as therein provided) with respect to such
securities and use its best efforts to cause such registration statement to
become and remain effective for the period of the distribution contemplated
thereby (determined as hereinafter provided);

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               (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 in paragraph (a) above and comply with the provisions of
the Securities Act with respect to the disposition of all Restricted Stock
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 Restricted Stock and to each
underwriter such number of copies of the registration statement 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 Restricted Stock covered by such registration statement;

               (d) use its best efforts to register or qualify the Restricted
Stock covered by such registration statement under the securities or "blue sky"
laws of such jurisdictions as the sellers of Restricted Stock 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;

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

               (f) immediately notify each seller of Restricted Stock 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;

               (g) if the offering is underwritten and at the request of any
seller of Restricted Stock, use its best efforts to furnish on the date that
Restricted Stock is 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 and to such
seller, stating that such registration statement has become effective under the
Securities Act and that (A) to the best knowledge of such counsel, no stop order
suspending the effectiveness thereof has been issued and no proceedings for that
purpose have been instituted or are pending or contemplated under the Securities
Act, (B) the registration statement, the related prospectus and each amendment
or supplement thereof comply as to form in all material respects with the
requirements of the Securities Act (except that such counsel need not express
any opinion as to financial statements contained therein) and (C) to such other
effects as reasonably may be requested by counsel for the underwriters or by
such seller or its counsel and (ii) a letter dated such date from the
independent public accountants retained by the Company, addressed to

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the underwriters and to such seller, 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 business days prior to the date of such letter)
with respect to such registration as such underwriters reasonably may request;
and

               (h) make available for inspection by each seller of Restricted
Stock, any underwriter participating in any distribution pursuant to such
registration statement, and any attorney, accountant or other agent retained by
such seller or underwriter, all financial and other records, pertinent corporate
documents and properties of the Company, 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.

               For purposes of Section 7(a) and 7(b) and of Section 4(c), the
period of distribution of Restricted Stock 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 Restricted Stock in any other registration shall be deemed to
extend until the earlier of the sale of all Restricted Stock covered thereby and
120 days after the effective date thereof.

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

               In connection with each registration pursuant to Sections 4, 5 or
6 covering an underwritten public offering, the Company and each seller agree to
enter into a written agreement with the managing underwriter selected in the
manner herein provided in such form and containing such provisions as are
customary in the securities business for such an arrangement between such
underwriter and companies of the Company's size and investment stature.

               8. EXPENSES. All expenses incurred by the Company in complying
with Sections 4, 5 and 6, 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 insurance and
fees and disbursements of one counsel for the sellers of Restricted Stock
selected by the holders of a majority of the Restricted Stock to be sold in any
related offering, but excluding any Selling Expenses, are called "Registration
Expenses". All underwriting discounts and selling commissions applicable to the
sale of Restricted Stock are called "Selling Expenses".

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               The Company will pay all Registration Expenses in connection with
each registration statement under Sections 4, 5 or 6. All Selling Expenses in
connection with each registration statement under Sections 4, 5 or 6 shall be
borne by the participating sellers in proportion to the number of shares sold 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.

               9. INDEMNIFICATION AND CONTRIBUTION. (a) In the event of a
registration of any of the Restricted Stock under the Securities Act pursuant to
Sections 4, 5 or 6, the Company will indemnify and hold harmless each seller of
such Restricted Stock thereunder, each underwriter of such Restricted Stock
thereunder and each other person, if any, who controls such seller or
underwriter within the meaning of the Securities Act, against any losses,
claims, damages or liabilities, joint or several, to which such 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 any untrue statement or alleged
untrue statement of any material fact contained in any registration statement
under which such Restricted Stock was registered under the Securities Act
pursuant to Sections 4, 5 or 6, any preliminary prospectus or final prospectus
contained therein, or any amendment or supplement thereof, or arise out of or
are based upon the omission or alleged omission to state therein a material fact
required to be stated therein or necessary to make the statements therein not
misleading, and will reimburse each such seller, 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, 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 information
furnished by any such seller, 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 Restricted Stock
under the Securities Act pursuant to Sections 4, 5 or 6, each seller of such
Restricted Stock 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 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, 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 any untrue statement or alleged untrue statement of any material
fact contained in the registration statement under which such Restricted Stock
was registered under the Securities Act pursuant to Sections 4, 5 or 6, any
preliminary prospectus or final prospectus contained therein, or any amendment
or supplement thereof, or arise out of or are based upon the omission or alleged
omission to state therein a material fact required to be stated therein or
necessary to make the statements therein not misleading, and will reimburse the
Company and each such officer, director, underwriter and controlling person for
any legal or other expenses reasonably incurred by them in connection with
investigating or defending any

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such loss, claim, damage, liability or action, PROVIDED, HOWEVER, that such
seller will be liable hereunder in any such case if and only 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
made in reliance upon and in conformity with information pertaining to such
seller, as such, furnished in writing to the Company by such seller specifically
for use in such registration statement or prospectus, and PROVIDED, FURTHER,
HOWEVER, that the liability of each seller hereunder shall be limited to the
proportion of any such loss, claim, damage, liability or expense which is equal
to the proportion that the public offering price of the shares sold by such
seller under such registration statement bears to the total public offering
price of all securities sold thereunder, but not in any event to exceed the
proceeds received by such seller from the sale of Restricted Stock covered by
such registration statement and (ii) that the Seller will not be liable in any
such case to the extent that any such loss, claim, damage, liability or action
arises out of or is based upon an untrue or alleged untrue statement or omission
or an alleged omission made in any preliminary prospectus of final prospectus if
(1) the Company, underwriter, or controlling a person failed to send or deliver
a copy of the final prospectus or prospectus supplement with or prior to the
delivery of written confirmation of the sale of the Restricted Stock, and (2)
the final prospectus or prospectus supplement would have corrected such untrue
statement or omission.

               (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 9 and shall only relieve
it from any liability which it may have to such indemnified party under this
Section 9 if and to the extent the indemnifying party is 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 9 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 if
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.

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               (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 Restricted Stock exercising rights under this Agreement, or any
controlling person of any such holder, makes a claim for indemnification
pursuant to this Section 9 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 9 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 circumstances for which indemnification is
provided under this Section 9; 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
so that such holder is responsible for the portion represented by the percentage
that the public offering price of its Restricted Stock offered by the
registration statement bears to the public offering price of all securities
offered by such registration statement, and the Company is responsible for the
remaining portion; PROVIDED, HOWEVER, that, in any such case, (A) no such holder
will be required to contribute any amount in excess of the public offering price
of all such Restricted Stock 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.

               10. CHANGES IN COMMON STOCK OR PREFERRED SHARES. If, and as often
as, there is any change in the Common Stock or the Preferred Shares 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 or the Preferred Shares as so changed.

               11. 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 Restricted Stock to the public without registration,
at all times after 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;

               (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 Restricted Stock forthwith upon
request a written statement by the Company as to its compliance with the
reporting requirements of such Rule 144 and of the Securities Act and the
Exchange Act, a copy of the most recent annual or quarterly report of the
Company, and 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 Restricted Stock without
registration.

                                      -10-
<PAGE>

               12. Form S-3 Qualification. The Company will use its best efforts
to comply with the reporting and filing provisions that are prerequisites to is
se of a Form S-3 under the Securities Act.

               13. REPRESENTATIONS AND WARRANTIES OF THE COMPANY. The Company
represents and warrants to you as follows:

               (a) The execution, delivery and performance of this Agreement by
the Company have been duly authorized by all requisite corporate action and will
not violate any provision of law, any order of any court or other agency of
government, the charter or by-laws of the Company or any provision of any
indenture, agreement or other instrument to which it or any or its properties or
assets is bound, conflict with, result in a breach of or constitute (with due
notice or lapse of time or both) a default under any such indenture, agreement
or other instrument or result in the creation or imposition of any lien, charge
or encumbrance of any nature whatsoever upon any of the properties or assets of
the Company.

               (b) This Agreement has been duly executed and delivered by the
Company and constitutes the legal, valid and binding obligation of the Company,
enforceable in accordance with its terms.

               14. MISCELLANEOUS.

               (a) All covenants and agreements contained in this Agreement by
or on behalf of any of the parties hereto shall bind and inure to the benefit of
the respective successors and assigns of the parties hereto (including without
limitation transferees of any Preferred Shares or Restricted Stock), whether so
expressed or not, PROVIDED, HOWEVER, that registration rights conferred herein
on the holders of Preferred Shares or Restricted Stock shall only inure to the
benefit of a transferee of Preferred Shares or Restricted Stock if (i) there is
transferred to such transferee by the transferor at least 20% of the total
shares of Founders Shares, Series A Preferred Stock, Series B Preferred Stock,
Series C Preferred Stock or Series D Preferred Stock originally issued to such
Investor(pursuant to a subscription agreement or pursuant to the Series B
Convertible Preferred Stock Purchase Agreement, the Series C Preferred Stock
Purchase Agreement or the Series D Convertible Stock Purchase Agreement) (ii)
such transferee is a partner, shareholder, affiliate or an entity which is
controlled by an affiliate of a party hereto.

               (b) All notices, requests, consents and other communications
hereunder shall be in writing and shall be delivered in person, mailed by
certified or registered mail, return receipt requested, or sent by telecopier or
telex, addressed as follows:

         if to the Company or any other party hereto, at the address of such
       party set forth in the applicable Purchase Agreement;

         if to any subsequent holder of Preferred Shares or Restricted Stock, to
      it at such address as may have been furnished to the Company in writing by
      such holder;

                                      -11-
<PAGE>

or, in any case, at such other address or addresses as shall have been furnished
in writing to the Company (in the case of a holder of Preferred Shares or
Restricted Stock) or to the holders of Preferred Shares or Restricted Stock (in
the case of the Company) in accordance with the provisions of this paragraph.

               (c) This Agreement shall be governed by and construed in
accordance with the laws of the State of Delaware.

               (d) This Agreement may not be amended or modified, and no
provision hereof may be waived, without the written consent of the Company and
the holders of at least two-thirds of the number of shares of Common Stock
issued or issuable upon conversion of all Preferred Shares at the time of such
amendment, modification or waiver.

               (e) This Agreement may be executed in two or more counterparts,
each of which shall be deemed an original, but all of which together shall
constitute one and the same instrument.

               (f) The obligations of the Company to register shares of
Restricted Stock under Sections 4, 5 or 6 shall terminate on the fifteenth
anniversary of the date of this Agreement.

               (g) If requested in writing by the underwriters for the initial
underwritten public offering of securities of the Company, each holder of
Restricted Stock who is a party to this Agreement shall agree not to sell
publicly any shares of Restricted Stock or any other shares of Common Stock
(other than shares of Restricted Stock or other shares of Common Stock being
registered in such offering), without the consent of such underwriters, for a
period of not more than 180 days following the effective date of the
registration statement relating to such offering; PROVIDED, HOWEVER, that all
persons entitled to registration rights with respect to shares of Common Stock
who are not parties to this Agreement, all other persons selling shares of
Common Stock in such offering, all persons holding in excess of 1% of the
capital stock of the Company on a fully diluted basis and all executive officers
and directors of the Company shall also have agreed not to sell publicly their
Common Stock under the circumstances and pursuant to the terms set forth in this
Section 13(g).

               (h) Notwithstanding the provisions of Section 7(a), the Company's
obligation to file a registration statement, or cause such registration
statement to become and remain effective, shall be suspended for a period not to
exceed 90 days in any 24-month period (or until the material nonpublic
information is made public) if there exists at the time material non-public
information relating to the Company which the Board of Directors of the Company
determines in good faith is not in the best interests of the Company to be
disclose (the "Blackout Period"). The Company shall promptly notify (but in no
event later than two business days following the determination of the Board of
Directors) those holders of Restricted Stock who have requested registration
pursuant to this Agreement of any Blackout Period and shall include in such
Blackout Notice an undertaking by the Company to promptly notify such holders of
Restricted

                                      -12-
<PAGE>

Stock as soon as a registration statement may be effected or sales
may resume; provided, however, that the Company may only suspend its obligation
to file a registration statement pursuant to this Section 14(h) within thirty
(30) days of the delivery of the notice requesting a registration statement. In
making any such determination to suspend or terminate a Blackout Period, the
Company shall not be required to consult with or obtain the consent of any
holder of Restricted Stock, and any such determination shall be made solely by
the Board of Directors of the Company. Each holder of Restricted Stock shall
treat all notices received from the Company pursuant to this Section 14(h) in
the strictest confidence and shall not disseminate such information.

               (i) The Company shall not grant to any third party any
registration rights more favorable than or inconsistent with any of those
contained herein, so long as any of the registration rights under this Agreement
remains in effect.

               (j) If any provision of this Agreement shall be held to be
illegal, invalid or unenforceable, such illegality, invalidity or
unenforceability shall attach only to such provision and shall not in any manner
affect or render illegal, invalid or unenforceable any other provision of this
Agreement, and this Agreement shall be carried out as if any such illegal,
invalid or unenforceable provision were not contained herein.

               (k) In the event shares of Preferred Stock or Restricted Stock
are transferred pursuant to Section 14(a) hereof, such transferee shall become
entitled to the benefits conferred by this Agreement. Additional transfers shall
become parties to this Agreement by executing and delivering a Signature Page,
and at such time the Company shall amend Schedule I attached hereto to include
such Additional Investor and send notice of the amendment to all other
Investors.

                                      -13-
<PAGE>

         IN WITNESS WHEREOF, this Agreement has been executed as of the date and
year first above written.

                                                  COMPANY:

                                                  EXACT LABORATORIES, INC.

                                                  By: /s/ Stanley N. Lapidus
                                                      ------------------------
                                                      Stanley N. Lapidus
                                                      President

                                      -14-
<PAGE>

                           SIXTH AMENDED AND RESTATED
                          REGISTRATION RIGHTS AGREEMENT
                             INVESTOR SIGNATURE PAGE

         By executing this counterpart signature page, the undersigned agrees to
and accepts the Sixth Amended and Restated Registration Rights Agreement between
the undersigned and Exact Laboratories, Inc. and hereby authorizes this
counterpart signature page to be attached to such Agreement.

                                        FOUNDER:

                                        /s/ Stanley N. Lapidus
                                        --------------------------
                                        Stanley N. Lapidus

                                      -15-
<PAGE>

                                   SCHEDULE I

                      INVESTORS IN SERIES A PREFERRED STOCK

                                INVESTORS                              SHARES

             One Liberty Fund III, L.P.                                 403,821
             Stanley N. Lapidus                                         126,295
             Gilde International BV                                       4,079
             Greylock Equity Limited Partnership                        279,858
             Frederick R. Blume                                           8,620
             David Shaw                                                  43,103
             Noubar Afeyan, PhD.                                         21,552
             Erwin Workman                                               15,086
             ==================================================================

             TOTAL                                                      902,414

                      INVESTORS IN SERIES B PREFERRED STOCK

                                   INVESTORS                           SHARES

             One Liberty Fund III, L.P.                                 278,481
             Greylock Equity Limited Partnership                        278,481
             Highland Capital Partners III Limited Partnership          267,341
             Highland Entrepreneurs' Fund Limited Partnership            11,139
             RS & Co., Limited Partners                                  86,075
             Bayview Investors, Ltd.                                     15,189
             Stanley N. Lapidus                                           5,063
             Charles Schwab & Co., Inc.                                   7,594
             FBO:  Stanley N. Lapidus
             David Shaw                                                   2,531
             Frederick R. Blume                                           2,531
             Edmund R. Pitcher                                            5,063
             Thomas F. Farb                                               5,063
             Theodore H. Ashford                                          6,329
             Stephen F. Wiggins                                          25,316
             ==================================================================

             TOTAL                                                      996,196

                                      -58-
<PAGE>

                      INVESTORS IN SERIES C PREFERRED STOCK

                                   INVESTORS                           SHARES

             Highland Capital Partners III Limited Partnership          256,000
             Highland Entrepeneurs' Fund III Limited Partnership         10,666
             One Liberty Fund III, L.P.                                  95,238
             One Liberty Fund IV, L.P.                                   95,238
             Greylock Equity Limited Partnership                        190,476
             CB Healthcare Fund, L.P.                                   119,047
             Mayo Foundation for Medical Education and Research          80,952
             B.U.N.P.                                                    38,095
             HLM Partners VII, LP                                        71,428
             RS & Co., Limited Partners                                  36,428
             Bayview Investors, Ltd.                                      6,429
             Patrick J. Sullivan                                            952
             William R. Liebke                                              952
             James Klingenstein                                             952
             Stanley N. Lapidus                                           2,381
             Terral Jordan                                                1,000
             Edmund R. Pitcher                                              952
             ==================================================================

             TOTAL                                                    1,007,186

                                      -59-
<PAGE>

                           SIXTH AMENDED AND RESTATED
                          REGISTRATION RIGHTS AGREEMENT
                       ADDITIONAL INVESTOR SIGNATURE PAGE

         By executing this counterpart signature page, the undersigned agrees to
and accepts the Sixth Amended and Restated Registration Rights Agreement between
the undersigned and Exact Laboratories, Inc. and the persons listed as Investors
on SCHEDULE I thereto, and hereby authorizes this counterpart signature page to
be attached to such Agreement.

                                            INVESTOR:

                                                     -----------------

                                      -61-<PAGE>

                                                                 Exhibit 10.5

                            EXACT LABORATORIES, INC.

                       Restricted Stock Purchase Agreement

         AGREEMENT, dated as of this 11th day of February, 1998 by and between
Stanley N. Lapidus (the "Purchaser") and Exact Laboratories, Inc. (the
"Company"), a Delaware corporation.

         1. SALE. The Company hereby sells and issues to the Purchaser 35,000
shares (the "Shares") of Common Stock, $.01 par value, of the Company, for a
purchase price of $0.40 per share (the "Purchase Price"), to be paid in cash or
by check payable to the order of the Company. This sale is made pursuant and
subject to the following terms and conditions.

         2. RECEIPT OF PURCHASE PRICE AND SHARES. The Company hereby
acknowledges receipt of $14,000.00, the aggregate purchase price for the Shares,
in accordance with Section 1. The Purchaser hereby acknowledges receipt of a
stock certificate representing the Shares.

         3. COMPANY PURCHASE OPTION.

                  3.1 OPTION TO THE COMPANY TO PURCHASE THE SHARES. The
Purchaser hereby grants to the Company an irrevocable right and option (the
"Purchase Option") to purchase all or any portion of the Shares on the terms and
conditions set forth in this Agreement.

         At any time within (90) ninety days after the Purchaser ceases to be an
employee of the Company or a subsidiary of the Company, subject to the
provisions of Section 3.2 in the event of a Termination Without Cause (as
defined in Section 3.2.2) or a Business Event (as defined in Section
3.2.1(iii)), the Company shall have the right to exercise the Purchase Option
and to purchase from the Purchaser (or her estate, it being understood that
Shares released from the Purchase Option under this Agreement shall pass upon
the Purchaser's death to the Purchaser's estate), for an amount per Share equal
to the Purchase Price (the "Option Price"), up to the number of Shares specified
in the following table:

<TABLE>
<CAPTION>

         IF CESSATION OF EMPLOYMENT OCCURS                    % OF SHARES SUBJECT TO PURCHASE OPTION
         <S>                                                  <C>
         Before January 6, 1999                                                   100%

         On January 6, 1999                                                       80%

         On the 6th day of each calendar month
         beginning February 6, 1999 an
         additional 583.33 Shares shall be
         released from the Purchase Option.

         On or after January 6, 2003.                                             None
</TABLE>

<PAGE>

                                      -2-

                  3.2.1    EARLY TERMINATION OF PURCHASE OPTION.

                           Notwithstanding the foregoing, in the event the
Purchaser continues to be employed by the Company on the earlier to occur of the
following events prior to January 6, 2003, the Purchase Option shall immediately
expire and the Company shall have no further right and option to repurchase all
or any portion of the Shares:

         (i) the Termination Without Cause (as defined below) of the Purchaser;

         (ii) the Purchaser suffers a substantial diminution in job
responsibility following a Business Event;

         (iii) the Company moves the Purchaser's place of employment more than
60 miles from the Company's office location (immediately prior to the Business
Event) following a Business Event. For purposes of this Agreement, each of the
following shall be referred to as a "Business Event": the closing of (A) the
sale by the Company of all or substantially all of its assets, (B) the sale or
exchange in a single transaction, or a series of related transactions, of shares
of capital stock of the Company representing at least a majority of the issued
and outstanding shares of capital stock of the Company (measured on the basis of
voting power), or (C) the merger or consolidation of the Company with or into
another entity in a transaction where the shares of the Company's capital stock
outstanding immediately prior to the closing of such merger or consolidation
represent or are converted into or exchanged for shares that represent less than
a majority of the shares of capital stock of the resulting or surviving entity
outstanding immediately after the closing of such merger or consolidation.

                  3.2.2 For purposes of this Agreement, "Termination Without
Cause" shall mean any termination of the Purchaser's employment by the Company
without cause or for cause other than (A) the Employee's gross negligence in the
performance of her duties as an employee and officer of the Company (as
determined by a majority of the directors of the Company) or (B) criminal
misconduct by the Purchaser in connection with the performance of her duties as
an employee and officer of the Company.

                  3.3 EXERCISE OF OPTION. The Purchase Option shall be exercised
by written notice delivered or mailed by the Company to the Purchaser (or her
estate), accompanied by payment by check for the Option Price for the Shares
being repurchased.

                  3.4 DELIVERY OF REPURCHASED SHARES. The Purchaser agrees that
she (or her executors or administrators or other legal representative, as the
case may be) shall promptly deliver to the Company, at the Company's principal
office, certificates for any Shares which are repurchased by the Company
pursuant to this Section 3, and that such certificates shall be duly endorsed in
blank for transfer to the Company; provided, however, that if the Purchaser
fails to deliver such certificates to the Company on or before the fifth day
following delivery to the Purchaser by the Company of the notice and payment
required by subsection 3.3, the Shares repurchased shall be deemed to be
transferred to the Company on such date, and all rights of the Purchaser with
respect to such Shares shall thereupon cease.

<PAGE>

                                      -3-

                  3.5 SHARES NOT REPURCHASED. In the event the Company declines
in writing to repurchase any or all of the Shares or does not exercise its
option for repurchase as provided herein, the Shares shall remain the property
of the Purchaser.

         4. ADDITIONAL SHARES. If, from time to time during the term of the
Purchase Option, there is any stock dividend or liquidating dividend of cash
and/or property, stock split, subdivision, or other change in the character or
amount of any of the outstanding Common Stock of the Company, then in such event
any and all new, substituted or additional securities or other property to which
the Purchaser is entitled by reason of her ownership of the Shares shall be
immediately subject to the Purchase Option and be included in the term "Shares"
for all purposes of the Purchase Option with the same force and effect as the
Shares presently subject thereto. After each such event, the Option Price per
share upon exercise of the Purchase Option shall be appropriately adjusted and
the number of shares which are released periodically from the Purchase Option
shall be appropriately adjusted.

         5. INVESTMENT REPRESENTATIONS. The Purchaser represents, warrants and
covenants that:

                           (i) The Purchaser is repurchasing the Shares for her
                  own account for investment only, and not with a view to, or
                  for sale in connection with, any distribution of the Shares in
                  violation of the Securities Act of 1933 (the "Securities
                  Act"), or any rule or regulation under the Securities Act.

                           (ii) The Purchaser is aware of the Company's business
                  affairs and financial condition, and has had such opportunity
                  as she has deemed adequate to obtain from representatives of
                  the Company such information as is necessary to permit the
                  Purchaser to evaluate the merits and risks of her investment
                  in the Company.

                           (iii) The Purchaser can afford a complete loss of the
                  value of the Shares and is able to bear the economic risk of
                  holding the Shares for an indefinite period.

                           (iv) The Purchaser has sufficient experience in
                  business, financial and investment matters to be able to
                  evaluate the risks involved in the purchase of the Shares and
                  to make an informed investment decision with respect to such
                  purchase.

                           (v) The Purchaser understands that: (A) the Shares
                  will not be registered under the Securities Act and are
                  "restricted securities" within the meaning of Rule 144 and/or
                  Rule 701 under the Securities Act; (B) the Shares cannot be
                  sold, transferred or otherwise disposed of unless they are
                  subsequently registered under

<PAGE>

                                      -4-

                  the Securities Act or an exemption from registration is then
                  available; (C) in any event, the exemption from registration
                  under Rule 144 or otherwise will not be available for at least
                  two years and even then may not be available unless a public
                  market then exists for the Common Stock, adequate information
                  concerning the Company is available to the public, and other
                  terms and conditions of Rule 144 are complied with, and (D)
                  there is now no registration statement on file with the
                  Securities and Exchange Commission with respect to any stock
                  of the Company and the Company has no obligation or current
                  intention to register the Shares under the Securities Act.

         6. LEGENDS.

                  6.1 RESTRICTED STOCK AGREEMENT LEGEND. All certificates
representing shares which are subject to the Purchase Option shall have affixed
thereto a legend substantially in the following form:

                  "The securities represented by this certificate are subject to
                  certain restrictions on transfer and to certain rights of the
                  Company to purchase such securities as set forth in a
                  Restricted Stock Purchase Agreement between the corporation
                  and the registered holder (or his/her predecessor in
                  interest), a copy of which is available for inspection without
                  charge at the principal office of the corporation."

         7. RESTRICTIONS ON TRANSFER. Except as otherwise provided in this
Section 7, the Purchaser shall not sell, assign, transfer, pledge, hypothecate
or otherwise dispose of, by operation of law or otherwise (collectively
"transfer") any of the Shares, or any interest therein, unless and until such
Shares are no longer subject to the Company's Purchase Option. Notwithstanding
the foregoing the Purchaser may transfer Shares by gift, provided that such
Shares shall remain subject to this Agreement (including without limitation the
Purchase Option and the restrictions on transfer set forth herein), and such
permitted transferee shall, as a condition to such transfer, deliver to the
company a written instrument confirming that such transferee shall be bounded by
all of the terms and conditions of this Agreement.

         8. EFFECT OF PROHIBITED TRANSFER. The Company shall not be required (a)
to transfer on its books any of the Shares which shall have been sold or
transferred in violation of any of the provisions set forth in this Agreement,
or (b) to treat as owner of such Shares or to pay dividends to any transferee to
whom any such Shares shall have been so transferred.

         9. RIGHT OF STOCKHOLDER. Subject to the provision of Sections 7 and 8
above, the Purchaser shall, during the term of this Agreement, exercise all
rights and privileges of a stockholder of the Company with respect to the
Shares.

         10. LOCK-UP AGREEMENT. The Purchaser agrees that in connection with any
underwritten public offering of Common Stock, commencing upon notice from the
Company or the principal underwriter managing such public offering, the Shares
may not be sold, offered for sale or otherwise disposed of without the prior
written consent of the Company or such

<PAGE>

                                      -5-

underwriter, as the case may be, for at least 270 days after the effectiveness
of the Registration Statement filed in connection with such offering or such
longer period of time as the Board of Directors may determine if all of the
Company's directors and officers agree to be similarly bound. The obligation
contained in this paragraph 10 shall survive the termination of this Agreement
and shall remain in full force and effect for such time as the Board of
Directors, in their discretion, may determine.

         11. REFUSAL RIGHTS.

                  11.1 EXERCISE OF RIGHT. If the Purchaser desires to sell all
or any part of the shares acquired under this Agreement (including any
securities received in respect thereof pursuant to recapitalizations and the
like), and an offeror (the "Offeror") has made an offer therefor, which offer
the Purchaser desires to accept, the Purchaser shall: (i) obtain in writing an
irrevocable and unconditional bona fide offer (the "Bona Fide Offer") for the
purchase thereof from the Offeror; and (ii) give written notice (the "Option
Notice") to the Company setting forth his/her desire to sell such shares, which
Option Notice shall be accompanied by a photocopy of the original executed Bona
Fide Offer and shall set forth at least the name and address of the Offeror and
the price and terms of the Bona Fide Offer. Upon receipt of the Option Notice,
the Company shall have an option to purchase any or all of such shares specified
in the Option Notice, such option to be exercisable by giving, within 30 days
after receipt of the Option Notice, a written counter-notice to the Purchaser.
If the Company elects to purchase any or all of such shares, it shall be
obligated to purchase, and the Purchaser shall be obligated to sell to the
Company, such shares at the price and terms indicated in the Bona Fide Offer
within 60 days from the date of receipt by the Company of the Option Notice.

                  11.2 SALE OF OPTION SHARES TO OFFEROR. The Purchaser may sell,
pursuant to the terms of the Bona Fide Offer, any or all of such shares not
purchased or agreed to be purchased by the Company for 60 days after the
expiration of the 30-day period during which the Company may give the aforesaid
counter-notice; PROVIDED, HOWEVER, that the Purchaser shall not sell such shares
to the Offeror if the Offeror is a competitor of the Company and the Company
gives written notice to the Purchaser, within 30 days of its receipt of the
Option Notice, stating that the Purchaser shall not sell his shares to the
Offeror; and provided, further, that prior to the sale of such shares to the
Offeror, the Offeror shall execute an agreement with the Company pursuant to
which the Offeror agrees not to become a competitor of the Company and further
agrees to be subject to the restriction set forth in this Section 11. If any or
all of such shares are not sold pursuant to a Bona Fide Offer within the time
permitted above, the unsold shares shall remain subject to the terms of this
Section 11.

                  11.3 ADJUSTMENTS FOR CHANGES IN CAPITAL STRUCTURE: If there
shall be any change in the Common Stock of the Company through merger,
consolidation, reorganization, recapitalization, stock dividend, split-up,
combination or exchange of shares, or the like, the restrictions contained in
this Section 11 shall apply with equal force to additional and/or substitute
securities, if any, received by the Purchaser in exchange for, or by virtue of
his or her ownership of, Shares.

<PAGE>

                                      -6-

                  11.4 FAILURE TO DELIVER OPTION SHARES: In the event the
Purchaser fails or refuses to deliver on a timely basis duly endorsed
certificates representing Option Shares to be sold to the Company pursuant to
this Section 11, the Company shall have the right to deposit the purchase price
for the Option Shares in a special account with any bank or trust company in the
Commonwealth of Massachusetts, giving notice of such deposit to the Purchaser,
whereupon such Option Shares shall be deemed to have been purchased by the
Company. All such monies shall be held by the bank or trust company for the
benefit of the Purchaser. All monies deposited with the bank or trust company
but remaining unclaimed for two (2) years after the date of deposit shall be
repaid by the bank or trust company to the Company on demand, and the Purchaser
shall thereafter look only to the Company for payment. The Company may place a
legend on any stock certificate delivered to the Purchaser reflecting the
restrictions on transfer provided in this Section 11.

                  11.5 EXPIRATION OF COMPANY'S RIGHT OF FIRST REFUSAL: The
refusal rights of the Company set forth above shall remain in effect until such
time, if ever, as a distribution to the public is made of shares of the
Company's Common Stock for an aggregate public offering price of at least $5
million or more pursuant to a registration statement filed under the Securities
Act of 1933, as amended, or a successor statute, at which time the refusal
rights set forth herein will automatically expire.

         12. FURTHER INSTRUMENTS AND ACTIONS. The parties agree to execute such
further instruments and to take such further action as may reasonably be
necessary to carry out the intent of this Agreement.

         13. NOTICES. Any notice required or permitted hereunder shall be given
in writing and shall be deemed effectively given upon personal delivery or upon
deposit with the United States Post Office, by registered or certified mail,
addressed to each other party hereto at his/her address hereinafter shown below
her signature or at such other address as such party may designate by ten (10)
days advance written notice to all other parties hereto.

         14. EMPLOYMENT STATUS. Nothing contained in this Agreement shall be
construed as giving the Purchaser any right to continue employment with the
Company.

         15. WITHHOLDING TAXES. The Purchaser acknowledges and agrees that any
tax consequences of the purchase, exercise and sale of Shares will be borne by
Purchaser and not by the Company and that the Company has the right to deduct
from payments of any kind otherwise due to the Purchaser any federal, state or
local taxes of any kind required by law to be withheld with respect to the
Shares.

         16. MISCELLANEOUS

                  16.1 ENTIRE AGREEMENT. This Agreement constitutes the entire
contract between the parties hereto with regard to the subject matter hereof.

<PAGE>

                                      -7-

                  16.2 GOVERNING LAW. This Agreement shall be governed by and
construed under the laws of the State of Delaware as such laws are applied to
contracts entered into and performed in such state.

                  16.3 SUCCESSOR AND ASSIGNS. Except as otherwise expressly
provided in this Agreement, the provisions hereto shall inure to the benefit of,
and be binding upon, the successors and assigns of the parties hereto.

                  16.4 AMENDMENTS. The terms and provisions of this Agreement
may not be modified or amended except in a writing executed by the parties
hereto.

<PAGE>

                                      -8-

         IN WITNESS WHEREOF, the parties have executed this Agreement as of the
date first above written.

EXACT LABORATORIES, INC.                    PURCHASER

By: /s/ Stanley N. Lapidus                  By: /s/ Stanley N. Lapidus
    ----------------------------                ------------------------------
    Stanley N. Lapidus                          Stanley N. Lapidus
    President

Address:  63 Great Road                     Address:  12 Old Evergreen Road
          Maynard, MA 01754                           Beford, NH 03110

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