1. Laboratory of Virology, Division of Intramural Research, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Hamilton, MT 59840, U.S.A.
2. Present address: Department of Microbiology, Graduate School of Medicine, Hokkaido University, Sapporo 060-8638, Japan
3. Rocky Mountain Veterinary Branch, Division of Intramural Research, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Hamilton, MT 59840, U.S.A.