CD4 and HIV infectious disease: Since changes in HIV-1 cellular tropism may influence the size and composition of viral reservoirs, here we sought to quantify and compare the susceptibility of naïve and memory CD4+ T-cell subsets, including TSCM, to infection by clinical X4 and R5 C-HIV strains from patients who either maintained R5 viruses or experienced emergence of X4 strains during progression from chronic to advanced stages of C-HIV infection.