Myeloperoxidase (MPO) is a type of peroxidase that is encoded in humans by the MPO gene located on chromosome 17. It is a lysosomal protein present in the cyanophil granules of neutrophils and released into the extracellular space during degranulation. MPO is most richly expressed in neutrophils and can produce halogenic acids with antimicrobial activity. MPO has heme pigments that cause the secretions in neutrophils to appear green, like pus. In addition, MPO mediates protein nitrosylation, 3-chlorotyrosine formation and dityrosine crosslinking. MPO deficiency is a genetic disease that causes the body to become immune deficient. Elevated MPO levels significantly double the risk of cardiovascular death. It has also been suggested that MPO plays an important role in the development of atherosclerotic lesions, resulting in plaque instability.