Erythropoietin hormone complex. Computer model showing the secondary structure of a molecule of the human hormone Erythropoietin (EPO), complexed with an erythropoetin receptor molecule. Erythropoietin regulates blood oxygen levels in the body. When levels are low (hypoxia) it is released by the kidneys and travels to the bone marrow, where it stimulates red blood cell precursor cells to mature into red blood cells (erythrocytes). This allows the blood to carry more oxygen. Because of this role it is sometimes used illegally as a performance-enhancing drug by athletes.