In 2005, several groups independently reported the discovery of a somatic mutation of the gene encoding JAK2 in a high proportion of patients with myeloproliferative neoplasms (MPNs) : >95% for polycythemia vera (PV), and 50% for essential thrombocythemia (ET) and primary myelofibrosis (PMF) [4–8]. The gene discussed is JAK2; the disease is essential thrombocythemia.