Amine oxidase (flavin-containing) A (MAOA) catalyses the oxidative deamination of biogenic and dietary amines, the regulation of which is critical for mental state homeostasis. MAOA, located on the mitochondrial outer membrane and requiring FAD as cofactor (Weyler 1989), preferentially oxidises biogenic amines such as 5-hydroxytryptamine (5HT), dopamine, noradrenaline and adrenaline. Defects in MAOA can cause Brunner syndrome (BRUNS; MIM:300615), a form of X-linked non-dysmorphic mild mental retardation. Male patients are affected by mild mental retardation and exhibit abnormal behaviour, including impulsive aggression (Brunner et al. 1993, Shih et al. 1999, Shih 2004).