Erythropoietin. Computer model of a molecule of the hormone erythropoietin. This is a glycoprotein that consists of a single-chain polypeptide of 165 amino acids. Protein structural units are seen as the purple cylinders (alpha helices) and yellow arrows (beta sheets). The molecule has a molecular weight of around 30,000. Also known as EPO, this hormone is responsible for the regulation of red blood cell production. It is primarily synthesised in the kidneys. It is abused by athletes who use it as a drug to increase their red blood cell levels. This can improve performance in a sporting event, but the thickening of the blood can be dangerous and lead to death.