Presence of a mutation in JAK2, CALR, and MPL is among the major criteria for the diagnosis of myelofibrosis (MF) or essential thrombocythemia (ET), while presence of JAK2 V617 or exon 12 mutations is among the major criteria for the diagnosis of polycythemia vera (PV) [1, 53, 54]. The gene discussed is JAK2; the disease is myelofibrosis.