1. Laboratory of Molecular Pharmacology, National Cancer Institute, Developmental Therapeutics Program, National Institutes of Health, Bethesda, Maryland 20892, Agouron Pharmaceuticals, Inc., 3565 General Atomics Court, San Diego, California 92121, and Macromolecular Structure Laboratory, National Cancer Institute-Frederick Cancer Research and Development Center, ABL-Basic Research Program, Frederick, Maryland 21702