JAK2 mutation, specifically V617F, is prevalent in classic (BCR-ABL negative) myeloproliferative neoplasms (MPNs) and is reported in up to 95% of polycythemia vera, 50–60% of essential thrombocythemia and 40–50% of primary myelofibrosis cases [136,137,138,139]. This evidence concerns the gene JAK2 and acquired polycythemia vera.