1. Laboratory of Infectious Diseases, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, Maryland 20205
2. Meloy Laboratories, Rockville, Maryland 20851
3. National Institute of Neurological and Communicative Disorders and Stroke, National Institutes of Health, Bethesda, Maryland 20205
4. Division of Virology, St. Jude Children's Hospital, Memphis, Tennessee 28101