1. Department of Medical Biophysics, University of Toronto,1 and
2. Amgen Research Institute, Toronto, Ontario, Canada M5G 2C12;
3. Wyeth-Lederle Vaccines and Pediatrics, Pearl River, New York 109653;
4. Division of Animal Resources and Department of Pathology, Emory University, Atlanta, Georgia 303224; and
5. Division of Viral and Rickettsial Diseases, Centers for Disease Control and Prevention, Atlanta, Georgia 303335
6. Ontario Cancer Institute,6 Toronto, Ontario, Canada M5G 2M9;