Erythropoietin (Epo) is a glycoprotein hormone produced in the kidney that acts on erythroid progenitor cells in the bone marrow. A negative feedback system, in which tissue oxygenation controls Epo production and Epo controls red blood cell (RBC) production, provides homeostasis in oxygen delivery to body tissues. The target cells for the action of Epo are committed erythroid progenitor cells, which have specific receptors for the hormone. The Epo receptor is a member of a larger family of hematopoietic growth factor receptors. No known second messenger system has been implicated in signal transduction from the Epo receptor. Although Epo may have some effect on mitosis in early erythroid progenitor cells, its control of RBC production appears to occur in later stages of erythroid cell development, where it prevents programmed cell death.