CD4 and infection: While R5 viruses, including T/F viruses, predominate in early, acute and asymptomatic phases of infection, dual-tropic R5/X4 and X4 variants can emerge at later stage of infection in some patients, when the proportion of CCR5-positive, memory CD4+ T cell drops in the peripheral blood, but resting naïve CD4+ T cells expressing mainly CXCR4 are still maintained (For recent reviews see references [16,97]).