EXHIBIT 10.1 [LEXICON GENETICS LETTERHEAD] March 28, 2005 VIA FEDERAL EXPRESS Dr. C. Thomas Caskey Five Post Oak Park 4400 Post Oak Parkway, Suite 1400 Houston, Texas 77027 Dear Tom: We are pleased to invite you to become a consultant to Lexicon...Consulting Agreement • April 1st, 2005 • Lexicon Genetics Inc/Tx • In vitro & in vivo diagnostic substances • Texas
