Erythropoietin (Epo) is a 30-kDa, heavily glycosylated protein produced primarily by the kidney. It is the principal factor that regulates erythropoiesis. The production of Epo by kidney cells is increased in response to hypoxia or anemia. The cDNAs for Epo have been cloned from many species. The mature proteins from the various species are highly conserved, exhibiting greater than 80% sequence identity at the amino acid level.