JAK/STAT activating mutations are found in 50–95% of patients with myeloproliferative neoplasms, polycythemia vera (PV), essential thrombocytosis (ET), and primary myelofibrosis (PMF), particularly in JAK2 (valine to phenylalanine change, V617F), and now play an increasing evident role in other hematologic malignancies as well. The gene discussed is JAK2; the disease is hematologic disorder.