Common Philadelphia chromosome-negative MPNs, including polycythemia vera, essential thrombocythemia, and primary myelofibrosis (MF), frequently harbor the JAK2 V617F mutation, detected in ~95% of polycythemia vera and 50-60% of essential thrombocythemia and primary MF cases [2-5]. The gene discussed is JAK2; the disease is essential thrombocythemia.