Erythropoietin also known as EPO, hematopoietin, or hemopoietin, is a glycoprotein hormone that controls erythropoiesis, or red blood cell production. It is a cytokine (protein signaling molecule) for erythrocyte (red blood cell) precursors in the bone marrow. Human EPO has a molecular weight of 34 kDa. Erythropoietin is produced by interstitial fibroblasts in the kidney in close association with peritubular capillary and proximal convoluted tubule. It is also produced in perisinusoidal cells in the liver. While liver production predominates in the fetal and perinatal period, renal production is predominant during adulthood. Erythropoietin in neuroprotection is the use of the glycoprotein erythropoietin (Epo) for neuroprotection. Epo controls erythropoiesis, or red blood cell production. Erythropoietin and its receptor are both present in the central nervous system with erythropoietin alpha capable of crossing the blood brain barrier via active transport. The presence of Epo within the spinal fluid of infants and the expression of Epo-R in the spinal cord, suggesting a role by Epo within the CNS. Epo and Epo-R is expressed in the mammalian retina, and therefore Epo represents a potential therapy to protect photoreceptors damaged from hypoxic pretreatment. Erythropoietin has been shown to protect nerve cells from hypoxia-induced glutamate toxicity. Acute hypoxia inducement in the adult mouse retina stimulates expression of Epo in addition to other growth factors. Epo response is stimulated by hypoxia and is capable of protecting against apoptosis of erythroid progenitors via a mechanism that is described in the Mechanism of Action section. Epo-R is present in cultured hippocampal and cerebral cortical neurons isolated from rat embryos. Epo was capable of protecting the cultured neurons from glutamate neurotoxicity after only a short exposure. It was concluded that Epo-mediated increase in intracellular calcium concentration is indicative of Epo's neuroprotective role after CNS-related hypoxia or ischemia.