Epo rapidly and transiently activates both ERK1 and ERK2 during erythroid differentiation. SKT6 cells were stimulated with Epo for 0, 3, 5, 15, 30, 60, and 180 minutes (lanes 1 through 7) and for 1, 2, 3, and 4.5 days (lanes 8 through 11), and the activity of ERK1 (A) and ERK2 (B) was measured in the immunoprecipitates with each specific antibody and MBP as a substrate. Arrows indicate the phosphorylated MBP (molecular weight, 18 kD). The lower panels show the immunoblots of the immunoprecipitated ERK1 or ERK2 in each lane.