In cancer, STAT5A has been reported to exert context-dependent effects—oncogenic in some hematologic malignancies but tumor-suppressive in breast and other epithelial cancers [5]—whereas STAT5B has been implicated in both oncogenic JAK–STAT signaling in leukemias and potential tumor-suppressive functions in solid tumors [14,15,16,17,18,19]. Here, STAT5B is linked to leukemia.