Document:

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                 SUPPLEMENTAL AGREEMENT TO AGREEMENT CONCERNING
            CONVERSION OF CONVERTIBLE NOTE AND CONNECTICUT PRESENCE

         SUPPLEMENTAL AGREEMENT, dated as of November 23, 1999, between
Genaissance Pharmaceuticals, Inc. (the "Company") and Connecticut Innovations,
Incorporated (the "Investor").

         WHEREAS, the Company and the Investor are parties to a certain
Agreement Concerning Conversion of Convertible Note and Connecticut Presence
(the "Existing Agreement") (capitalized terms used and not otherwise defined
herein shall have the meanings assigned to such terms in the Existing
Agreement); and

         WHEREAS, as of the date hereof, the Company has issued a certain
Mandatory Convertible Promissory Note (the "Note") to the Investor in the amount
of $500,000; and

         WHEREAS, as of the date hereof, the Company has issued a certain Common
Stock Purchase Warrant (the "Warrant") to the Investor; and

         WHEREAS, the Note is convertible, upon the terms set forth therein,
into certain shares (the "Preferred Shares") of the Series B Preferred Stock of
the Company; and

         WHEREAS, the Warrant is exercisable, upon the terms set forth therein,
for certain shares (the "Warrant Shares") of the Common Stock of the Company;
and

         WHEREAS, the parties hereto desire to provide the Investor with certain
put and other rights with respect to the Preferred Shares and the Warrant Shares
substantially similar to the put and other rights that are set forth in Section
5 of the Existing Agreement.

         NOW THEREFORE, in consideration of the foregoing, and for good and
valuable consideration, the receipt and sufficiency of which are hereby
acknowledged, the parties hereto, intending to be legally bound, do hereby agree
as follows:

         1. The Preferred Shares and the Warrant Shares shall be treated as if
they are Preferred Shares and Common Shares, respectively, under the Existing
Agreement and, as such, shall be subject to the Put provisions of Section 5 of
the Existing Agreement; PROVIDED, HOWEVER, that

         (a)      Section 5(c)(x)(ii), as it applies to the Preferred Shares,
 shall read as follows:

                  an amount equal to the Series B Preferred Stock Issue Price
                  (as defined in the Note) of such Share plus an amount
                  calculated to yield to the Investor an aggregate twenty-five
                  percent (25%) annually compounded rate of return on such share
                  of Series B Preferred Stock for the period from the
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                  Series B Preferred Stock Issue Date (as defined in the
                  Note) through and including the date when the Put Price is
                  paid to the Investor, less the amount of any dividends
                  previously paid to the Investor with respect to such Share.

         (b)      Section 5(c)(y)(ii), as it applies to the Warrant Shares,
shall read as follows:

                  an amount equal to the Adjusted Exercise Price of such Share
                  (at the time of exercise) plus an amount calculated to yield
                  to the Investor an aggregate twenty-five percent (25%)
                  annually compounded rate of return on the Adjusted Exercise
                  Price of such Share for the period from the date of exercise
                  through and including the date when the Put Price is paid to
                  the Investor, less the amount of any dividends previously paid
                  to the Investor with respect to such Share.

         (c)      for purposes of Section 5(c), the Put Price as to each share
of Common Stock underlying each unexercised Warrant shall be the excess, if any,
of the Current Market Price (as defined in the Existing Agreement) of one share
of Common Stock purchasable upon exercise of such Warrant over the Adjusted
Exercise Price (as defined and determined under the Warrant) of such Warrant as
of the date of determination.

         2.       The terms and conditions of this Agreement shall inure to the
benefit of and be binding upon the respective successors and assigns of the
parties (including transferees of the Preferred Shares and the Warrant Shares);
PROVIDED, HOWEVER, that the provisions of Section 1 hereof shall not succeed to
or be assigned to any non-affiliate of the Investor.

         3.       Notices given under this Agreement shall be given in the same
fashion as under the Existing Agreement.

         4.       This Agreement shall be governed by and construed under the
laws of the State of Connecticut, without regard to principles of conflicts of
laws and rules of such state.

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

                      [THE NEXT PAGE IS THE SIGNATURE PAGE]
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         IN WITNESS WHEREOF, the undersigned have hereunto set their hands as of
the day and year first above written.

                                GENAISSANCE PHARMACEUTICALS, INC.

                                By:  /s/ Kevin Rakin
                                     ----------------------------------
                                     Name: Kevin Rakin
                                     Title:   Executive Vice President

                                CONNECTICUT INNOVATIONS, INCORPORATED

                                By:  /s/ Victor R. Budnick
                                     ----------------------------------
                                     Name:  Victor R. Budnick
                                     Title:  Executive Director<PAGE>

                        GENAISSANCE PHARMACEUTICALS, INC.
                        5 SCIENCE PARK, BOX 6, SUITE 2104
                              NEW HAVEN, CT 06511

February 17, 2000

Connecticut Innovations, Incorporated
999 West Street
Rocky Hill, CT 06067

Re:  INVESTMENT

Gentlemen:

         Genaissance Pharmaceuticals, Inc. (the "COMPANY"), Connecticut
Innovations, Incorporated ("CII") and certain other parties have entered into a
Preferred Stock Purchase Agreement dated as of the date hereof (the "PURCHASE
AGREEMENT") pursuant to which CII has agreed to purchase and the Company has
agreed to sell to CII shares of the Company's Series B Redeemable Convertible
Preferred Stock (the "SERIES B SHARES"). As an inducement for CII to purchase
the Series B Shares and as a condition thereto, the Company agrees as follows:

         1.       All of the Series B Shares and all shares of the Company's
capital stock (including shares of capital stock underlying any warrants) held
by CII as of the date hereof shall be subject to and included in the definition
of "Preferred Shares" in a certain Supplemental Agreement Concerning Conversion
of Convertible Note and Connecticut Presence, dated November 23, 1999, by and
between the Company and CII.

         2.       The following agreements by and between CII and the Company
are hereby amended as follows:

                  (a) Sections 7.18, 7.19 and 8.3(ii) of that certain Purchase
         Agreement dated as of March 10, 1994 (the "FIRST AGREEMENT") are hereby
         deleted without substitution.

                  (b) Sections 7.10, 7.19, 7.20 and 9.3(a)(ii) of that certain
         Financing Agreement dated as of November 16, 1994 (the "SECOND
         AGREEMENT") are hereby deleted without substitution.

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Connecticut Innovations, Incorporated
February ___, 2000
Page 2 of 3

                  (c) Sections 7.9, 7.17, 7.18 and 9.3(a)(ii) and (c) of that
         certain Financing Agreement dated as of September 10, 1996 (the "THIRD
         AGREEMENT") are hereby deleted without substitution.

                  (d) Sections 7.9, 7.18 and 7.19 of that certain Stock Purchase
         Agreement dated as of December 24, 1997 are hereby deleted in their
         entirety without substitution, and Section 10 of such Stock Purchase
         Agreement is also deleted in its entirety without substitution, except
         that the numbering of such Section 10 shall be retained (without any
         content) in order to preserve the numbering scheme for such Agreement
         as a whole.

         3.       Sections 11.2 and 11.3 of that certain Stock Subscription
Warrant No. 1, dated March 10, 1994, issued by the Company (when its name was
BIOS Laboratories, Inc.) to CII, are hereby deleted without substitution.

         4.       Section 9 of the First Agreement, Section 12 of the Second
Agreement and Section 11 of the Third Agreement each contain securities
registration rights as to shares of the Company's common stock granted to CII by
the Company (collectively, with any other registration rights as to shares of
the Company's common stock that may have been granted to CII by the Company,
"CII'S COMMON STOCK REGISTRATION RIGHTS"). Notwithstanding anything to the
contrary in the First Agreement, the Second Agreement or the Third Agreement,
the shares of the Company's common stock (including shares of common stock
underlying any warrants) held by CII as of the date hereof shall be included in
the definition of and treated as "Other Shares" under a certain Amended and
Restated Registration Rights Agreement dated as of the date hereof among the
Company, CII and certain other parties (the "SERIES A REGISTRATION RIGHTS
AGREEMENT") and under a certain Registration Rights Agreement dated as of the
date hereof among the Company, CII and certain other parties (the "SERIES B
REGISTRATION RIGHTS AGREEMENT") and CII's Common Stock Registration Rights shall
be governed by the provisions of the Series A Registration Rights Agreement and
the Series B Registration Rights Agreement to the extent, and only to the
extent, that any of CII's Common Stock Registration Rights conflict with the
provisions of the Series A Registration Rights Agreement and/or the Series B
Registration Rights Agreement. However, if the number of shares which CII
desires to register upon any exercise of CII's Common Stock Registration Rights
is reduced by more than twenty-five percent (25%) as a result of the application
of the provisions of the Series A Registration Rights Agreement and/or the
Series B Registration Rights Agreement, then such exercise shall not count
against any limit on the number of times that CII may exercise CII's Common
Stock Registration Rights under the applicable agreements. In no event shall the
provisions of this Section 4 apply or extend to any shares of common stock
issuable upon conversion of the Series B Shares or upon conversion of the shares
of the Company's Series A Redeemable Convertible Preferred Stock held by CII.

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Connecticut Innovations, Incorporated
February ___, 2000
Page 3 of 3

         If the terms set forth above are satisfactory to you, please indicate
your acceptance by signing the enclosed copy of this letter in the place below
provided for that purpose and returning it to the undersigned, whereupon it
shall become a binding agreement between us.

                                            Very truly yours,

                                            GENAISSANCE PHARMACEUTICALS, INC.

                                            By: /s/ Kevin Rakin
                                                ------------------------------
                                                Kevin Rakin
                                                Its Executive Vice President

Accepted and agreed to:

CONNECTICUT INNOVATIONS,
INCORPORATED

By: /s/ Victor R. Budnick
    --------------------------------
    Victor R. Budnick
    Its President and Executive Director

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