Erythropoietin (Epo) plays a critical role in the regulation of red cell mass (RCM) according to tissue oxygen levels.Hypoxia inducible factor-2 (HIF-2) is known to be a master regulator for Epo expression.The liver is the major source of Epo in mice during embryonic and early postnatal development.In adults,the kidneys become the major source of Epo.