1. Departments of Biochemistry and Chemistry and Division of Biomedical Sciences, University of California, Riverside, California 92521, Hoffmann-La Roche Inc., Nutley, New Jersey 07110, Division of Hematology/Oncology, Cedars−Sinai Medical Center, UCLA School of Medicine, 8700 Beverly Boulevard, B-208, Los Angeles, California 90048, and Department of Medical Specialties, The University of Texas M. D. Anderson Cancer Center, Houston, Texas 77030