CD4 and AIDS: We also thank Drs. W.C. Greene, S.L. Grice, D. Gabuzda, A.S. Bour, K. Strebel, and M. Emerman for providing anti-A3G, anti-RT, anti-Vif antibodies, pcDNA-hVif, and HeLa-CD4/CCR5-β-Gal cells, which were obtained through the AIDS Research Reference Reagent Program, Division of AIDS, NIAID, NIH.