JAK2V617F has been reported in more than 70% of MPN, namely, 95% of PV, 50% of ET, and 60% of PMF, and paved the way for the subsequent discovery of JAK2 exon 12 mutation in 2–3% of PV [25] and, in 2006, of activating mutations in exon 10 of the thrombopoietin receptor gene MPL, occurring at the W515 residue [26,27]. Here, JAK2 is linked to acquired polycythemia vera.