The V617F mutation in JAK2 is strongly associated with myeloproliferative neoplasms (MPNs), including myelofibrosis, polycythemia vera and essential thrombocythemia, occurring in nearly 100% of patients with polycythemia vera and over 75% of patients with essential thrombocythemia [21]. The gene discussed is JAK2; the disease is myelofibrosis.