STAT5 is transcriptionally active in K562 cells. (A) K562 cells were transfected with different reporter plasmids (2 μg) by electroporation. Two days posttransfection, cells were harvested and CAT activity was determined. The two reporters containing STAT5 binding sites (β-casein-CAT and GAS-CAT) were more active compared with the empty reporter and the reporters containing STAT1 and STAT3 binding sites (IRE-CAT and SIE-CAT). (B) K562 cells were transfected with as described in (A) with 2 μg β-casein CAT and increasing amounts of dominant negative (DN) STAT5 (δ750) or STAT3 (STAT3β). Only DN STAT5 is able to (partially) block the activity of the β-casein-CAT reporter in K562 cells.