CD4 and AIDS: We thank NIH AIDS Research Reagent and Reference Program, Division of AIDS , NIAID, NIH for making available the following reagents: 4E10 and 2F5 MAbs from Dr Herman Katinger, 17b from Dr James E. Robinson, CD4-IgG2 from Progenics Pharmaceuticals, TZM-bl cells from Drs John C Kappes, Xiaoyun Wu and Tranzym Inc. and TAK-779.