SCD5 and AIDS: The following reagents were obtained through the NIH AIDS Research and Reference Reagent Program, Division of AIDS, NIAID, NIH: pNL4.3.LUC.R-E- from Dr. Nathaniel Landau; TZM-bl cells from Dr. John C. Kappes, Dr. Xiaoyun Wu, and Tranzyme Inc.; VRC03 from Dr. John Mascola; 10E8 from Dr. Mark Connors; sCD4 from Progenics.