The monoclonal antibody 266-6K1 recognizes human myeloperoxidase (MPO), an ~135 glycoprotein expressed in all cells of the myeloid linage. MPO functions as an α2β2 heteromultimer consisting of two heavy (α) and two light (β) chains of 55 and 15 kDa respectively. MPO is abundantly present in azurophilic granules of polymorphonuclear neutrophils (PMNs). It is an important enzyme used during phagocytic lysis of engulfed foreign particles which takes part in the defense of the organism through production of hypochlorous acid (HOCl), a potent oxidant. In the stimulated PMN, MPO catalyzes the production of hypohalous acids, primarily hypochlorous acid in physiologic situations, and other toxic intermediates that greatly enhance PMN microbicidal activity. Upon activation of neutrophils, MPO can be 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 to MPO (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 attracted more clinical attention.