Macrophage-tropic (mac-tropic) R5 strains can still replicate despite low levels of cell surface CD4 on macrophages for infection, while non-macrophage-tropic (non-mac-tropic) R5 viruses require high amounts of CD4, presumably limiting them to CD4+ T-cells that express significantly more CD4 compared to macrophages [2,3,11]. This evidence concerns the gene CD4 and infection.