To better understand the underlying Env determinants of C-HIV tropism for CD4+ T-cell subsets, we quantified the susceptibility of CD4+ T-cells isolated from peripheral blood of four HIV-seronegative donors to infection by GFP reporter viruses pseudotyped with X4 1109-F-30 Env that was mutated to express the V3 region of the R5 1109-E-10 Env (referred to as 1109-F-30-E10V3) (Additional file 1: Table S3). Here, CD4 is linked to infection.