STAT activation in 32D cell lines. 32D cell lines were stimulated with diluent (D), 200 pmol/L mGM-CSF (M), or 400 pmol/L hGM-CSF (H), and lysates were prepared and analyzed by EMSA using a³²P-labeled DR probe. Specificity of binding was determined by competition with 200× cold DR probe and 200× cold GRE probe. The DR probe binds STAT5 and STAT1 with high affinity, whereas the GRE probe binds only the tetrameric form of STAT1 with high affinity.