1. Department of Microbiology and Immunology and Kimmel Cancer Institute, Thomas Jefferson University, Philadelphia, Pennsylvania 19107, Howard Hughes Medical Institute and Department of Microbiology and Immunology, Stanford University School of Medicine, Stanford, California 94305, Abbott Diagnostics Division, Abbott Laboratories, Abbott Park, Illinois 60064, Department of Biochemistry and Molecular Biophysics, Columbia University, New York, New York 10032, and AIDS Research Center, Massachusetts General...