MPL and essential thrombocythemia: MPNs including polycythemia vera (PV), essential thrombocythemia (ET), and primary myelofibrosis (MF) are characterized by chronic proliferation of mature myeloid cells8, and the myeloproliferative phenotype is driven by somatic mutations in JAK2, CALR, and MPL. Among MPNs, JAK2V617F, an activating somatic mutation in JAK2, is the most frequently observed driver mutation; it has been observed in over 95% of PV patients as well as 50–60% of ET and primary MF patients9–11.