While R5 viruses, including T/F viruses, predominate in the early, acute, and asymptomatic phases of infection, dual-tropic R5/X4 and X4 variants can then emerge at a later stage of infection in some patients, when the proportion of CCR5-positive, memory CD4+ T cells drops in the peripheral blood, but resting naïve CD4+ T cells expressing mainly CXCR4 are still maintained [91,148]. This evidence concerns the gene CXCR4 and infection.