CD4 and infection: The panel includes four SHIVs encoding HIV-1 Env variants adapted through passage in cell culture and/or macaques (AD8-EO, AD8-OG, SF162P3, and 1157ipd3N4) and four SHIVs encoding HIV-1 Env variants isolated directly from infected individuals without culture or macaque adaption, three of which represent variants from early in infection (QF495AE, Q23AE, MG505GV, and BG505AE) and all of which encode a single amino acid substitution (A204E or G312V) to allow macaque CD4 use.