Upregulation of the c-fos promoter activity by extracellular Tat. Twenty-four hour serum–starved (1% FCS) Jurkat cells were transfected with FC3 then cultured with recombinant Tat or recombinant p24. Alternatively cells were incubated with recombinant Tat pretreated with a MoAb to Tat or p24. CAT activity is expressed as folds of activation with respect to cell lines transfected with FC3 alone. Data are reported as means ± SDs of three independent transfection experiments performed in duplicate.