1. Protein Chemistry Laboratory and Structural Biochemistry Program, SAIC Frederick, NCI-FCRDC, P.O. Box B, Frederick, Maryland 21702, Department of Molecular Genetics, SmithKline Beecham Pharmaceuticals, 709 Swedeland Road, King of Prussia, Pennsylvania 19406, and Abteilung Virologie, Institute Medizinische Mikrobiologie und Hygiene, Haus 47, Universitätskliniken des Saarlandes, 66421 Homburg/Saar, Germany