CXCR4 and infection: In addition, we found no difference in subset tropism between CAP88 T1 Envs that infected NP2/CD4/CXCR4 cells (R5/X4 using n = 3) and those that did not (R5 using n = 3), suggesting that the level of CXCR4 usage by these viruses was too low to detect a difference in infection of CD4+ T cell subsets.