Erythropoietin molecule bound to receptors. Computer model of a molecule of erythropoietin (EPO) (orange) bound to two extracellular EPO receptors (pink and purple). EPO is a glycoprotein that is produced by the kidneys in response to low blood oxygen levels. It stimulates the erythrocyte (red blood cell) precursor cells in the bone marrow to mature into erythrocytes. It is used therapeutically to treat anaemia caused by renal failure or chemotherapy. It is also abused by athletes who use it to enhance their performance by increasing blood oxygen levels. However, this use also causes thickening of the blood which can lead to death.