The most frequent MPN-associated JAK2 mutation is the exon 14 JAK2V617F, which is responsible for almost all the JAK2 mutations in ET and PMF, and 97% of those seen in PV; the remainder 3% of JAK2 mutations in PV are spread across exons 12, 13, and 143,4. This evidence concerns the gene JAK2 and essential thrombocythemia.