JAK2 and myelofibrosis: Polycythemia vera (PV) is a myeloproliferative neoplasm characterized by trilineage expansion, including an abnormal increase in red cell mass and elevations in platelet and white blood cell (WBC) counts; and it is commonly associated with mutations in the Janus kinase 2 (JAK2) gene (V617F or exon 12) leading to hyperactivation of the JAK-STAT signaling pathway.1,2 Patients with PV have a considerable symptom burden and are at an increased risk of thrombotic events and progression to myelofibrosis (MF) and blast phase.3