CCR5 and infection: We selected SHIVSF162P3 for two reasons: 1) the parental Env, CCR5-tropic HIV-1SF162, is particularly resistant to CCR5-targeting agents [20] and 2) SHIVSF162P3 can switch tropism to use non-CCR5 coreceptors in macaques infection, which can help assess risk of viral escape [21].