Constitutive STAT activation in factor-independent FDC-P1 cells expressing constitutive hβc mutants. Nuclear extracts prepared from factor-independent FDC-P1 cells expressing the indicated hβ_c mutants were subjected to EMSA using a β-casein promoter oligonucleotide probe. Also shown are FDC-P1 cells expressing wild-type hGMR that were either continuously cultured in 1 ng/mL hGM-CSF (lane 1) or starved of growth factor overnight and incubated without (lane 2) or with (lane 3) 50 ng/mL hGM-CSF for 10 minutes. The DNA-binding complexes are marked by an arrow.