MAOA is an X-linked gene encoding monoamine oxidase A, a mitochondrial enzyme that metabolizes monoamine neurotransmitters including norepinephrine, dopamine, and serotonin. MAOA knockout mice have higher levels of serotonin, norepinephrine, and to lesser extent dopamine, and manifest increased aggressive/impulsive behaviour and stress reactivity.121 In 1993, Brunner and colleagues122 reported a Dutch pedigree with eight males affected by borderline mental retardation and impulsive behaviors such as aggression, arson, attempted rape, fighting, and exhibitionism (Fig. 11A). Affected individuals were hemizygous for a stop-codon in the eighth exon of MAOA leading to a complete and selective deficiency of MAOA activity. Consistently with an X-linked recessive pattern of transmission, heterozygous women were unaffected. This stop codon variant has not been found in other populations. More recently, a common MAOA polymorphism influencing MAOA transcription was discovered.123 This locus, termed the MAOA-linked polymorphic region (MAOA-LPR), is a VNTR located approximately 1.2 kb upstream from the MAOA start codon and within the gene's transcriptional control region123 (see Fig. 11B). Alleles at this VNTR have a different number of tandem copies of a 30-bp sequence, with the three-
