Knowledge on the myeloproliferative neoplasms (MPNs) – polycythemia vera (PV), essential thrombocythemia (ET), primary myelofibrosis (PMF) – has accumulated since the discovery of the JAK/STAT-activating mutations associated with MPNs: JAK2V617F, observed in PV, ET and PMF; and the MPL and CALR mutations, found in ET and PMF. The gene discussed is MPL; the disease is essential thrombocythemia.