Polycythemia vera (PV), essential thrombocythemia (ET), and primary myelofibrosis (PMF) are chronic myeloproliferative neoplasms (MPNs) characterized by clonal expansion of an abnormal haematopoietic stem/progenitor cell1 that harbours somatic gene mutations: JAK2 mutations define PV, while mutations of calreticulin (CALR) and thrombopoietin receptor (MPL) are specific to the majority of JAK2 un-mutated ET and PMF2. Here, MPL is linked to primary myelofibrosis.