Role of STAT5 in BCR/ABL-dependent expression of MCL-1. (A) Detection of active STAT5 in normal bone marrow cells (left) and primary leukemic cells in a patient with CML by electromobility shift assay (EMSA; right). Normal bone marrow cells were incubated with control medium (control), rhIL-3 (100 ng/mL), or rhGM-CSF (100 ng/mL) for 30 minutes at 37°C before being analyzed. Whole-cell extracts were subjected to EMSA using the STAT5 β-casein response element. Two different antisera for supershift analysis were used (α-S5N, N-terminal STAT5 epitope; α-S5C, C-terminal STAT5 epitope). Normal bone marrow cells displayed active STAT5 only in the presence but not in the absence of IL-3, whereas primary CML cells expressed active STAT5 in a constitutive manner. (B) Expression of a dnSTAT5 mutant in BCR/ABL-expressing C7.28 cells resulted in a substantial decrease in mcl-1 promoter activity. Results represent the mean ± SD from 3 independent experiments; ^*P < .05.