Erythropoietin hormone, molecular model. Erythropoietin (EPO) is a hormone produced by the kidneys that stimulates the production of oxygen- carrying red blood cells (erythrocytes). It regulates blood oxygen levels in the body. When levels are low (hypoxia) it is released and travels to the bone marrow, where it stimulates red blood cell precursor cells to mature into red blood cells. This allows the blood to carry more oxygen, which in turn decreases the production of EPO. Because of this role it is sometimes used illegally as a performance-enhancing drug by athletes, for whom high blood oxygen levels are advantageous. The atomic structure of the molecule is faintly seen behind the helices of the secondary structure.