Monoamine oxidase (MAO) catalyzes the oxidative deamination of a number of biogenic amines in the brain and peripheral tissues by the production of hydrogen peroxide(H₂O₂). The cloning of MAO A and B genes has demonstrated that the enzymes are made of different polypeptides. MAO A and B genes are located on the X-chromosome (Xp11.23) and consist of 15 exons with identical intron-exon organization, which suggests that they are derived from the same ancestral gene. MAO A and B exhibit distinct differences in the substrate selectivity and inhibitor sensitivity and play different role in the neurotrasmitter metabolism and behavior.