Absolute erythrocytosis can be rarely primary (polycythemia vera or other erythroid neoplasia), but rather secondary due to an increase in erythropoietin concentration related to hypoxia (chronic cardiopulmonary disease, right-to-left cardiovascular shunts), or due to different neoplasms or renal disorders that produce erythropoietin [68]. The gene discussed is EPO; the disease is acquired polycythemia vera.