1. Tri-Institutional Training Program in Chemical Biology, Tri-Institutional Research Program and Molecular Pharmacology & Chemistry Program, Memorial Sloan-Kettering Cancer Center, 1275 York Avenue, Box 422, New York, New York 10021, and Department of Microbiology & Immunology and Molecular Biology Program, Weill Medical College of Cornell University, 1300 York Avenue, Box 62, New York, New York 10021