JAK2 mutations, specifically V617F, are common in myelofibrosis (35–57%), ubiquitous in polycythemia vera, occurring in 96–99% of patients, and are also the most commonly mutated gene in essential thrombocythemia (50–60%) [11,12,13,14,15,16,17]. The gene discussed is JAK2; the disease is acquired polycythemia vera.