JAK2V617F is detected in approximately 95% of polycythemia vera (PV) and 50–60% of essential thrombocythemia (ET) and primary myelofibrosis (PMF), whereas JAK2 exon 12 mutations are found exclusively in 2–3% of PV patients lacking the more common JAK2V617F. This evidence concerns the gene JAK2 and essential thrombocythemia.