CD4 and HIV-1 infection: These findings support a model where CXCR6 use is a characteristic feature of natural monkey SIV infections, including the lineage from which SIVcpz/HIV-1 emerged, and that the loss of this coreceptor usage is associated with altered CD4+ T cell targeting to subpopulations that may be less able to tolerate SIVcpz/HIV-1 infection.