1. Laboratory of Viral Diseases, National Institute of Allergy and Infectious Diseases, Bethesda, Maryland 20014
2. Hematology Branch, National Cancer Institute, Bethesda, Maryland 20014
3. Virus Research Unit, Division of Infectious Diseases, Children's Hospital Medical Center, Harvard Medical School, Boston, Massachusetts 02115
4. Division of Hematology-Oncology, Department of Medicine, University of Missouri Medical Center, Columbia, Missouri 65201