Monoamine oxidases A and B [MAOA and MAOB; amine:oxygen oxidoreductase (deaminating) (flavin-containing), EC 1.4.3.4] play important roles in the metabolism of neuroactive, vasoactive amines and the Parkinsonism-producing neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Human MAOA and MAOB genes isolated from X chromosome-specific libraries span at least 60 kilobases, consist of 15 exons, and exhibit identical exon-intron organization. Exon 12 codes for the covalent FAD-binding-site and is the most conserved exon; the MAOA and MAOB exon 12 products share 93.9% peptide identity. These results suggest that MAOA and MAOB are derived from duplication of a common ancestral gene and provide insight on the structural/functional relationship of the enzyme products.