1. The Clayton Foundation Laboratories for Peptide Biology, The Salk Institute for Biological Studies, 10010 North Torrey Pines Road, La Jolla, California 92037, Division of Cell Biology and Experimental Cancer Research, Institute of Pathology, University of Berne, CH-3012 Berne, Switzerland, and Department of Medicinal Chemistry and Institute of Pharmaceutical Chemistry, Albert Szent-Györgyi Medical University, Szeged 6720, Hungary