Phosphopeptide inhibition of Stat3/β binding to hSIE. Whole-cell extracts (20 μg) of an acute leukemia cell line, EM2,28 stimulated with G-CSF (100 ng/mL) for 30 minutes were incubated with the indicated tyrosine phosphorylated peptides (+P) or nonphosphorylated peptides (−P) at 0, 30, 100, 300, and 400 μmol/L for 60 minutes at 37°C before addition of radiolabeled duplex hSIE and EMSA. The gels were dried and exposed to Kodak XAR film for before developing. The results shown are representative of three experiments.