Myeloperoxidase (MPO) is a hemeprotein that belongs to the XPO subfamily of the heme peroxidase superfamily. Myeloperoxidase is synthesized as a preproprotein that undergoes proteolytic processing to generate a disulfide-linked heterodimer of the N-terminal beta-subunit (12 kDa) and C-terminal a subunit (60 kDa). Active myeloperoxidase is a tetramer of two beta-subunits and two alpha-subunits that are also disulfide-linked through the two alpha-subunits. Myeloperoxidase is stored in granules and is an abundant protein in neutrophils and monocytes. Myeloperoxidase is released upon activation to catalyze the formation of powerful oxidants such as hypochlorous acid, which kills microbes. Myeloperoxidase can also lead to tissue damage during chronic inflammation. Unprocessed pro- myeloperoxidase can also be released. By alternative splicing, three isoforms of myeloperoxidase have been described. Human and mouse myeloperoxidase share 87% amino acid sequence identity.