The observation that infection with CCR5 tropic HIV is rapidly extinguished in the absence of vif while CXCR4 tropic HIV lacking vif can sustain replication suggests that vif-independent HIV replication is occurring in a location with a paucity of cells expressing CCR5, such as the thymus where <5% of cells express CCR5 while a far greater number of cells (30–40%) express CXCR4 [29]–[31]. This evidence concerns the gene CCR5 and infection.