MEK phosphorylation studies. Western analyses for phosphorylated MEK1/2 (pMEK; upper left), total MEK1/2 (Total MEK; upper right), phosphorylated SMAD2 (pSMAD2; lower left), or total SMAD2 (Total SMAD2; lower right) are shown. Total protein (25 μg/well) was extracted from culture day-7 cells harvested at 0 (top row), 10 (middle row), and 30 (bottom row) minutes after the addition of SCF or TGF-B (denoted by + at the bottom of the figure).