1. Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, Massachusetts 022151;
2. Merck Research Laboratories, West Point, Pennsylvania 194862;
3. Laboratory of Viral Diseases3 and
4. National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, Maryland 20852; and Southern Research Institute, Frederick, Maryland 217014
5. Laboratory of Molecular Microbiology,5