1. Division of Viral Pathogenesis, Department of Medicine, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, Massachusetts 022151;
2. Department of Pathology, Bernhard Nocht Institute for Tropical Medicine, Hamburg, Germany2;
3. Primedica-Mason Laboratories, Worchester, Massachusetts 016083; and
4. Merck Research Laboratories, West Point, Pennsylvania 194864