Activation of SEK1/MKK4 by hematopoietic growth factors. MC/9 cells were incubated at 37°C in RPMI 1640 for 1 hour before simulation. Cells were then left untreated (CON), or (A) treated with maximal doses of IL-4 (IL-4) or SLF (SLF ) or (B) treated with maximal doses of GM-CSF (GM), SLF (SLF ), or 0.2 mol/L NaCl (NaCl) for the indicated times. After lysis, cell extracts were analyzed by SDS-PAGE, and were immunoblotted with an antibody that specifically recognizes the activated form of SEK1/MKK4.