CD4 and HIV-1 infection: HIV-1 infection of host cells is a multi-step process, the virus entry into cells begin with viral envelop protein gp120 binding to CD4 and one of the co-receptors CCR5 or CXCR4 on cell surface, the co-receptor CCR5 or CXCR4 is required for different HIV-1 strains, with CCR5 for CCR5(R5)-tropic strain and CXCR4 for CXCR4(X4)-tropic strains [4, 5].