1. Laboratory of Theoretical Biology, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20014
2. Laboratory of Vision Research, National Eye Institute, National Institutes of Health, Bethesda, Maryland 20014
3. Immunology Branch, National Cancer Institute, Bethesda, Maryland 20014
4. Laboratory of Chemical Physics, National Institute ofArthritis, Metabolism, and Digestive Diseases, National Institutes of Health, Bethesda, Maryland 20014