Constitutive STAT5 signaling has been identified in the pathogenesis of various hematological malignancies, including BCR-ABL-induced chronic myeloid leukemia (CML), acute myeloid leukemia (AML), acute lymphoid leukemia (ALL) and myeloproliferative disorders (MPDs) such as chronic myelomonocytic leukemia (CMML) and polycythemia vera (PV) [4]. The gene discussed is ABL1; the disease is acquired polycythemia vera.