Selective blocking of the effect of Tat on the c-fos promoter activity. (A) Jurkat and (B) U937 cells were transiently cotransfected with tat/S, tat/AS and FC3. Controls are represented by cells cotransfected with FC3 together with tat/S or tat/AS. 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.