CXCR4 and HIV-1 infection: It should also be noted in this study that CXCR4, as an another co-receptor of HIV-1, can function as the co-receptor for R5-tropic HIV-1 infection at late stage or some HIV-1 variants can enter cells via CXCR4 directly but not CCR5, just as the results of our study with additional experiments about CCR5 and CXCR4 editing simultaneously in TZM.bl resisting R5-tropic and X4-tropic HIV-1 infection, which makes the specific CCR5 disruption only may not be sufficient.