CXCR4 and infection: We were unable to detect a productive infection in human monocyte-derived macrophages (MDMs) or CD4+ T cells with TA1, whereas the R3A virus can use CCR5 or CXCR4 to enter MDMs, and uses CXCR4 as the preferred coreceptor for entry into CD4+ T cells (Yanji Yi, personal communication).