CXCR4 and infection: Interestingly, we found that the dual-tropic 89.6 virus caused higher levels of infection in the X4- and X4R5-DRSCs than in our R5-DRSCs, potentially indicating that the 89.6 virus infection was more efficient in utilizing the CXCR4 coreceptor relative to CCR5 (Fig. 5D).