JAK2 and essential thrombocythemia: An acquired transversion in JAK2 exon 14 (c.1849G>T) that is confined to hematopoietic cells and results in p.Val617Phe (JAK2V617F) is observed in approximately 90% of patients with polycythemia vera (PV), 50% of essential thrombocythemia (ET) cases and 50% of primary myelofibrosis (PMF) cases [1], [3].