1. Vaccine Research Center, National Institute of Allergy and Infectious Diseases (NIAID), National Institutes of Health (NIH), 40 Convent Drive, Bethesda, MD 20892, USA.
2. Division of Viral Pathogenesis, Department of Medicine, Beth Israel Deaconess Medical Center, Harvard Medical School, RE113, P. O. Box 15732, Boston, MA 02115, USA.
3. HIV-Specific Immunity Section, Laboratory of Immunoregulation, NIAID, NIH, Bethesda, MD 20892, USA.
4. Biostatistics Research Branch, NIAID, NIH, Bethesda, MD 20892, USA.