The hormone erythropoietin (Epo) is the main humoral regulator of erythropoiesis. It binds to specific receptors belonging to the cytokine receptor superfamily. Epo stimulates proliferation and differentiation of erythroid precursor cells, but may also bind to and exert some additional effects in nonhemopoietic tissues. It is mainly produced in the kidneys and to minor extents also in the liver and in the brain. The plasma concentration of erthyropoietin is inversely related to the oxygen content of the blood. The secretion of Epo into the circulation and hence its plasma concentrations are mainly determined by the transcription rate of the Epo gene, which itself is essentially under control of the cellular oxygen concentration. Sinks of the oxygen concentrations increase the activity of the hypoxia-inducible transcription factor (HIF), which in turn triggers Epo gene transcription. Disorders of kidney function lead to inappropriate Epo production, what may result in anemia or polycythemia.