1. Department of Virology, Hoffmann-La Roche Inc., Nutley, NJ 07110, U.S.A.
2. Infectious Diseases Section, VA Medical Center, University of California, San Diego, CA 92161, U.S.A.
3. Molecular Virology Laboratory, St. Luke's/Roosevelt Hospital Center, Columbia University, New York, NY 10019, U.S.A.