For instance, R5 HIV-1 viruses are transmitted and propagated preferentially during the early and asymptomatic stages of infection while viruses showing CXCR4 tropism (X4 HIV-1 and, mainly, dual tropic X4R5 HIV-1) emerge progressively and become detectable in roughly 40–50% of infected people at later stages of the infection or during the AIDS phase. This evidence concerns the gene CXCR4 and infection.