1. Molecular Aspects of Drug Design Section, Structural Biophysics Laboratory, National Cancer InstituteFrederick, Frederick, Maryland 21702, Human Genetics Section, Laboratory of Genomic Diversity, National Cancer InstituteFrederick, Frederick, Maryland 21702, and Biophysics Resource, Structural Biophysics Laboratory, National Cancer InstituteFrederick, P.O. Box B, Frederick, Maryland 21702