Withdrawal of EPO produces activation of JNKs and p38 MAPK and inactivation of ERKs in human primary erythroid colony-forming cells. Purifed day-8 human erythroid colony-forming cells, which are mainly CFU-E, were EPO-starved for the indicated periods of time. Cell extractions and Western blot analyses were performed as described in Figs 1 and 2.