1. Department of Microbiology, Mount Sinai School of Medicine, New York, New York 10029
2. Department of Microbiology, University of Texas Southwestern Medical Center, Dallas, Texas
3. Special Pathogens Branch, Division of Viral and Rickettsial Diseases, National Center for Infectious Diseases, Centers for Disease Control and Prevention, 1600 Clifton Road, MS G-14, Atlanta, Georgia 30329
4. Department of Immunology, The Scripps Research Institute, 10550 North Torrey Pines Rd., La Jolla, California 92037