Myeloperoxidase (MPO) is a peroxidase enzyme most abundantly expressed in neutrophil granulocytes, and produces hypohalous acids to carry out their antimicrobial activity. MPO oxidation of apoA-I reduces HDL-mediated inhibition of apoptosis and inflammation. MPO mediates protein nitrosylation and the formation of 3-chlorotyrosine and dityrosine crosslinks. MPO deficiency is a hereditary deficiency of the enzyme, which predisposes to immune deficiency. Studies have reported an association between elevated MPO levels and the severity of coronary artery disease. It has also been suggested that MPO plays a significant role in the development of the atherosclerotic lesion and rendering plaques unstable.