Activation of p38 but not MEK is required for stress-induced erythroid differentiation. SKT6 cells mixed with various concentrations of SB203580 (left panel) or PD98059 (right panel) were treated by (A) osmotic or (B) heat shock for 1 hour, and the percentage of hemoglobinized cells (%) was counted after 4.5 days. The percentage of hemoglobinized cells after stress treatment without inhibitor is defined as 100%. Values shown are the mean of five experiments.