1. Department of Chemistry and Immunology and The Skaggs Institute for Chemical Biology and Worm Institute for Research and Medicine (WIRM), The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037, and Laboratory of Viral Immunology, the Lindsey F. Kimball Research Institute, The New York Blood Center, 310 East 67th Street, New York, New York 10021