Indeed, transfection of CD4-positive CCR5-negative cells with mutant CCR5 unable to transduce signals made such cells fully susceptible to infection with R5 HIV-1 (R5 viruses infect cells expressing CD4 and CCR5, while X4 viruses infect cells with CD4 and CXCR4). This evidence concerns the gene CXCR4 and infection.