Absence of MAPK induction by the EPOR in HT-2 cells. HCD57 cells (lanes 1 and 2) or HT-2.EPOR.3 cells (lanes 3 through 6) were starved and stimulated with no cytokine (U), 50 U/mL EPO (E), 10 ng/mL murine IL-4 (4), or 10 nmol/L IL-2 (2). Total cell lysates were prepared and immunoprecipitated with anti-phosphoMAPK antibody. Immunoprecipitates were subjected to in vitro kinase assays with an Elk1-glutathione S-transferase fusion protein, separated by 10% SDS-PAGE, transferred to nitrocellulose, and probed with anti–phospho-Elk1 antibodies. Arrow indicates phosphorylated Elk1.