Roles of the MAP kinase family in Epo-induced erythroid differentiation and proliferation. In differentiation-inducible SKT6 cells (right panel), JNKs and p38 function in erythroid differentiation (solid lines), but they have a role in apoptosis when the cells are exposed to environmental stresses (broken lines). However, in Epo-dependent FD-EPO cells (left panel), ERKs, JNKs, and p38 are all involved in cell proliferation (solid lines), whereas JNKs and p38 function in apoptotic signals when the cells are exposed to cellular stresses (broken lines).