JAK2 and essential thrombocythemia: In 2005, with the discovery of the Janus kinase 2 (JAK2) V617F mutation, a major advance has been made in understanding the pathogenesis of increased signaling by the JAK/STAT pathway in MPNs.1–4 The JAK2V617F mutation is present in 95%, 50%, and 60% of PV, ET, and PMF patients, respectively.5 Two other mutations (JAK2 exon 12 and mutation in the thrombopoietin receptor gene, myeloproliferative leukemia, MPL) directly affecting this pathway were then described.