1. Laboratory of Mathematical Biology, National Cancer Institute, Bethesda, Maryland 20205
2. Laboratory of Chemical Pharmacology, National Cancer Institute, Bethesda, Maryland 20205
3. Department of Nuclear Medicine, National Institutes of Health, Bethesda, Maryland 20205
4. Immunology Branch, National Cancer Institute, Bethesda, Maryland 20205
5. Laboratory of Viral Diseases, National Institute of Allergy and Infectious Diseases, Bethesda, Maryland 20205