STAT5A and chronic myelogenous leukemia, BCR-ABL1 positive: In order to evaluate whether the phosphate group of IST5 is critical for IST5 activity in cell-based assays, we compared the potency of IST5 (Figure 4A) to that of IST5-M (IST5 without the phosphate group) (Figure 4B) in inhibiting phosphorylation of Stat5 in PC and CML cells.