Confidential Materials Omitted and filed separately with the Securities and Exchange Commission

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                                                                EXHIBIT 10.56

          Confidential Materials Omitted and filed separately with the

         Securities and Exchange Commission. Asterisks denote omissions.

                          PROMETHEUS LABORATORIES INC.

                            5739 PACIFIC CENTER BLVD.

                            SAN DIEGO, CA 92121-4203

May 15, 2003

Genaissance Pharmaceuticals, Inc.

Five Science Park

New Haven, CT 06511

Attention Kevin L Rakin, President and Chief Executive Officer

Ladies and Gentlemen:

Reference is made to that certain Sublicense Agreement between DNA Sciences,

Inc. ("DNA SCIENCES") and Prometheus Laboratories Inc. ("PROMETHEUS") dated as

of August 16, 2001 (the "AGREEMENT"). Genaissance Pharmaceuticals, Inc.

("GENAISSANCE") has succeeded to the rights and obligations of DNA Sciences with

respect to the Agreement as a result of the assignment of the rights and

obligations of DNA Sciences to Genaissance pursuant to the Chapter 11 Bankruptcy

Case of DNA Sciences now pending in the United States Bankruptcy Court, Northern

District of California, Oakland Division (Case No. 03-41843 N11) (the

"BANKRUPTCY CASE"). Capitalized terms used but not defined herein shall have the

meanings assigned to them in the Agreement except to the extent inconsistent

with this Amendment.

This binding letter agreement (the "AMENDMENT") will confirm our understanding

regarding the amendment of the Agreement. Genaissance and Prometheus hereby

agree as follows:

1. For purposes of the Agreement, as amended by this Amendment, the term "MASTER

LICENSE" shall mean that certain License and Option Agreement between St. Jude

Children's Research Hospital, a Tennessee corporation, as licensor, and PPGx,

Inc., as licensee, made as of February 29