1. Structural Biology Department, Hauptman−Woodward Medical Research Institute, 700 Ellicott Street, Buffalo, New York 14203, Structural Biology Department, School of Medicine and Biological Sciences, State University of New York at Buffalo, Buffalo, New York 14260, Department of Pharmacology and Toxicology, Indiana University School of Medicine, Indianapolis, Indiana 46202, and Dana-Farber Cancer Institute, Harvard Medical School, Boston, Massachusetts 02115