MPL and essential thrombocythemia: The three major entities, essential thrombocythemia (ET), polycythemia vera (PV), and primary myelofibrosis (PMF), are driven by mutations in just three genes: JAK2, CALR, and MPL. These mutations originate at the level of hematopoietic stem cells (HSCs) but, depending on the intrinsic and extrinsic factors, can lead to differential skewing of hematopoiesis predominantly into one of the myeloid lineages presenting clinically with one of the three phenotypes (2).