During the evaluation of erythrocytosis, PV should always be considered and the diagnosis excluded by the absence of a JAK2 mutation (V617F exon 14 and exon 12) [72, 73]; also helpful in this regard is serum EPO especially in cases of JAK2V617F-negative but exon 12 mutated PV, where serum EPO levels are often subnormal [74]. This evidence concerns the gene JAK2 and acquired polycythemia vera.