CD4 and infection: We used the CD4+ T-cell line SupT1-R5 for this study, since these cells are highly susceptible to infection by X4- and R5-tropic strains of HIV-1, express high levels of CD4, similar to activated primary T cells, and show little unspecific virus attachment, with HIV-1 binding being largely CD4 dependent and occurring on CD4 clusters (38, 39).