A direct action on erythropoietin secretion would also provide an explanation for observations from animal studies that, while polycythemia induced by transfusion attenuates hypoxic erythropoietin production, polycythemia induced by previous exposure to hypoxia — which will tend to cause iron deficiency because of the iron demand for erythropoiesis (95) — has the opposite effect (98). This evidence concerns the gene EPO and polycythemia.