EPO, Erythropoietin, is an acidic glycoprotein hormone with a molecular mass of 34 kD. It is a cytokine for erythrocyte (red blood cell) precursors in the bone marrow. This gene is mapped to 7q22.1. It is produced by interstitial fibroblasts in the kidney in close association with peritubular capillary and tubular epithelial cells. It is also produced in perisinusoidal Ito cells in the liver. While liver production predominates in the fetal and perinatal period, renal production is predominant during adulthood. As the prime regulator of red cell production, its major functions are to promote erythroid differentiation and to initiate hemoglobin synthesis. It also has other known biological functions. For example, erythropoietin plays an important role in the brain's response to neuronal injury. EPO is also involved in the wound healing process.