Constitutive tyrosine phosphorylation of ERK1/2 MAP kinases in factor-independent FDC-P1 cells expressing constitutive hβc mutants. Whole cell lysates from factor-independent FDC-P1 cells were analyzed by immunoblotting using (A) an anti-phospho-ERK1/2 antibody and (B) an anti-ERK1/2 antibody. Also shown are analyses of FDC-P1 cells expressing wild-type hGMR that were either starved of growth factor overnight and incubated with (lane 1) or without (lane 2) 50 ng/mL hGM-CSF for 10 minutes or continuously cultured in 1 ng/mL hGM-CSF (lane 3).