Characterization of the GAS-binding STAT protein complexes in GM-CSF–stimulated BM-derived macrophages from STAT5A null, WT, and HZ mice. BM-derived macrophages from null (A), WT (B), and HZ (C) mice were incubated with either medium (lane 1) or 10 ng/mL of GM-CSF (lanes 2-7) for 15 minutes at 37°C, and whole-cell extracts (5 μg of protein per sample) were analyzed by EMSA using the ³²P-labeled β-casein probe as described in Materials and Methods. Extracts were preincubated with NRS (which resulted in a nonspecifically displaced band near the top of the gel) or the indicated anti-STAT antibodies before the addition of the probe. Arrows labeled a, b, and c indicate the uppermost, middle, and lowermost bands containing GAS-binding complexes, respectively. α5A, anti-STAT5A; α5B, anti-STAT5B; αp91, anti-STAT1.