Myeloperoxidase (MPO), a heme protein, is a major component of azurophilic granules of neutrophil granulocytes (NGs). Optimal oxygen-dependent microbicidal activity depends on MPO as the critical enzyme for the generation of hypochlorous acid and other toxic oxygen products, which are proposed to contribute to tissue damage during inflammation. MPO is a marker for myeloid cells. It may also be weakly expressed in cells of monocytic origin. It is useful for differentiating acute myelogenous leukemia from acute lymphoblastic leukemia. In addition, MPO is thought to be involved in the pathology Alzheimer's disease.