CXCR4 and infection: There are three common combinations of co-receptor usage and cellular tropism: (i) CCR5-utilizing T-cell-tropic (T-tropic) HIV, which accounts for transmitted/founder viruses and most circulating viruses, (ii) CXCR4-utilizing T-tropic HIV, arising in approximately 50% of late-stage HIV patients, and (iii) macrophage-tropic (M-tropic) HIV that principally uses CCR5 co-receptors, and arises in some individuals late in infection and in T-cell-depleted environments such as in advanced HIV-1 disease [92,93].