The monoclonal antibody 266-6K1 recognizes human myeloperoxidase (MPO), an ~135 glycoproteinexpressed in all cells of the myeloid linage. MPO functions as an α2β2 heteromultimer consisting oftwo heavy (α) and two light (β) chains of 55 and 15 kDa respectively. MPO is abundantly present inazurophilic granules of polymorphonuclear neutrophils (PMNs). It is an important enzyme used duringphagocytic lysis of engulfed foreign particles which takes part in the defense of the organism throughproduction of hypochlorous acid (HOCl), a potent oxidant. In the stimulated PMN, MPO catalyzes theproduction of hypohalous acids, primarily hypochlorous acid in physiologic situations, and other toxicintermediates that greatly enhance PMN microbicidal activity. Upon activation of neutrophils, MPO canbe rapidly released and as such useful in body fluids as marker for inflammatory status.Involvement of MPO has been described in numerous diseases such as atherosclerosis, lung cancer,Alzheimer"s disease, inflammatory bowel disease and multiple sclerosis. Autoimmune antibodies toMPO (so called ANCA) are involved in Wegener’s disease. Since the discovery of MPO deficiency,initially regarded as rare and restricted to patients suffering from severe infections, MPO has attractedmore clinical attention.