JAK2 and acquired polycythemia vera: The diagnosis of PV is made by meeting three major criteria of the World Health Organization (WHO): (1) hemoglobin of more than 16.5 g/dL in men or more than 16 g/dL in women or hematocrit of more than 49% in men or more than 48% in women or increased red blood cell mass, (2) bone marrow trilineage proliferation with pleomorphic mature megakaryocytes, and (3) presence of a JAK2 mutation (JAK2V617F or a mutation at exon 12 of JAK2).