The following reagents were obtained through the National Institutes of Health AIDS Research and Reference Reagent Program, Division of AIDS, NIAID, NIH: pcDNA-Vphu from Stephan Bour and Klaus Strebel; pNL4-3 from Malcolm Martin; MAGI-CCR5 from Julie Overbaugh; HeLa CD4 (HT4-6C) from Bruce Chesebro; HIV-1NL4-3 Vpu antiserum from Klaus Strebel; CD4 monoclonal (specific antibody) from James Hildreth. This evidence concerns the gene CD4 and AIDS.