Forced activation of p38 induces erythroid differentiation without Epo stimulation. (A) SKT6 cells transfected with a gain-of-function MKK6 mutant MKK6(Glu) (lanes 3 and 4) or the vector (pCDNA3) alone (lanes 1 and 2) were cultured in the presence (+) or absence (−) of Epo, and the hemoglobinized cells were stained with DAF. The percentage of hemoglobinized cells of the mock-transfectants in the presence of Epo is defined as 100%. Each value represents the mean of six experiments. (B) MKK6(Glu) activates p38. The p38 activity in SKT6 cells (lanes 1 and 2), mock-transfected SKT6 cells (lane 3), or MKK6(Glu)-transfected SKT6 cells (lane 4) with (lane 2) or without (lanes 1, 3, and 4) Epo was measured in the immunoprecipitates with anti-p38–specific antibody and GST-ATF-2 as a substrate.