Suppression of siRNA-mediated RNAi-silencing by P19 or P19 mutants in HEK293T cells. HEK293T cells were transfected with expression plasmids for firefly (Fluc) and Renilla luciferase (Rluc) together with a siRNA that targets Fluc (si-Fluc) or a control irrelevant siRNA (scrambled siRNA, lane 1). Increasing doses of the indicated expression plasmids for FLAG-P19 or FLAG-P19 mutants were also transfected into HEK293T cells. Luciferase activities were quantified at 20 h post transfection. Fluc/Rluc ratios are graphed based on the averages from four independent experiments. Please note that in the top and bottom graphs the identical values from wild type P19 are presented twice simply for the purpose of easier comparison with the values from the respectively graphed P19 mutants.