1. Howard Hughes Medical Institute, Departments of Physiology and Microbiology & Molecular Genetics, Molecular Biology Institute, University of California, Los Angeles, Los Angeles, California 90095-1662, Molecular Immunology Department, Mail Code UE 0548, SmithKline Beecham, 709 Swedeland Road, King of Prussia, Pennsylvania 19406, and Department of Molecular Pharmacology, Albert Einstein College of Medicine, 1300 Morris Park Avenue, Bronx, New York 10461