1. C.-C. Tsai, K. E. Follis, A. Sabo, T. W. Beck, R. F. Grant, University of Washington Regional Primate Research Center, Seattle, WA 98195, USA.
2. N. Bischofberger, Gilead Sciences, Foster City, CA 94404, USA.
3. R. E. Benveniste, National Cancer Institute, Frederick, MD 21701, USA.
4. R. Black, Division of AIDS, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, MD 20852, USA.