MPL and essential thrombocythemia: The Philadelphia-negative myeloproliferative neoplasms (MPNs) include essential thrombocythemia (ET), polycythemia vera (PV) and primary myelofibrosis (PMF), which in the large majority are acquired hematopoietic stem cell diseases, arising and driven by somatic stem cell mutations (JAK2V617F, CALR, MPL).