1. R. Wyatt is in the Department of Cancer Immunology/AIDS, Dana-Farber Cancer Institute, Department of Pathology, Harvard Medical School, Boston, MA 02115, USA. J. Sodroski is in the Department of Cancer Immunology/AIDS, Dana-Farber Cancer Institute, Department of Pathology, Harvard Medical School, and Department of Immunology and Infectious Diseases, Harvard School of Public Health, Boston, MA 02115, USA.