1. Department of Microbiology, University of Pennsylvania, Philadelphia, Pennsylvania 19104.
2. Departments of Virology and Computational Biology, GlaxoSmithKline, Research Triangle Park, North Carolina 27709.
3. Current address: Kathryn M. Kitrinos, Gilead Sciences Inc., Durham, North Carolina 27707.
4. Current address: Celia C. LaBranche, Global Vaccines, Inc., Research Triangle Park, North Carolina 27709.