1. Department of Pharmacology and University of Pittsburgh Cancer Institute, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania 15213, Department of Pharmacology and Toxicology, University of Arkansas for Medical Sciences, and Central Arkansas Veterans Healthcare System, Little Rock, Arkansas 72205, Institute for Cancer Prevention, Valhalla, New York 10595, and Macromolecular Crystallography Laboratory, National Cancer Institute, Frederick, Maryland 21702