1. Synchrotron Radiation Research Section, Macromolecular Crystallography Laboratory, National Cancer Institute, Argonne, Illinois 60439, Basic Research Program, SAIC-Frederick, 9700 South Cass Avenue, Argonne, Illinois 60439, Department of Anatomy and Structural Biology, and Gruss-Lipper Biophotonics Center, Albert Einstein College of Medicine, 1300 Morris Park Avenue, Bronx, New York 10461, and Protein Structure Section, National Cancer Institute at Frederick, Building 536, Frederick, Maryland 21702