1. Departments of Cellular and Molecular Pharmacology and Pharmaceutical Chemistry, University of California, San Francisco, 600 16th Street, San Francisco, California 94158, Center for Pharmaceutical Biotechnology, m/c 870, University of Illinois, 900 South Ashland Avenue, Chicago, Illinois 60607, and Institut für Klinische Immunologie and Transfusionsmedizin, Justus-Liebig-Universität Giessen, Langhansstrasse 7, 35385 Giessen, Germany