CXCR4 and infection: Fig. 2C shows that CCR5-independent infection by 5P12-RANTES selected CC1/85 virus was first detected at week 36 of round 4 of selection, and improved substantially during the ensuing six weeks. Surprisingly, a similar but delayed evolution of CCR5-independent infection occurred in virus from control cultures without any CCR5 inhibitor added (Fig. 2C) between weeks 44 and 55 of culture. We confirmed that infection was mediated by the CXCR4 coreceptor by inhibiting infection with the CXCR4-specific agent AMD3100 (Fig. 2D).