CXCR6 and infection: This has raised key questions: (a) what entry pathways are used by the HIV-1 precursor SIVcpz, and its upstream guenon SIV forerunner that supplied its env gene; (b) was there a change in coreceptor use that occurred during the emergence of SIVcpz and HIV-1; and (c) what cells express CXCR6 in natural host species known to harbor CXCR6-using SIV, and are therefore targets for infection?