CD4 and HIV-1 infection: Although HIV-1 subtype C (C-HIV) strains account for >50% of HIV-1 infections worldwide and exhibit pathogenicity and tropism alterations that are distinct from other HIV-1 subtypes [12], at present there are few studies describing the susceptibility of CD4+ T-cell subsets to infection by clinical R5 C-HIV strains, and none concerning clinical X4 C-HIV isolates.