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. This evidence concerns the gene MPL and essential thrombocythemia.