In 1993, a non-sense mutation of the monoamine oxidase A gene (MAOA), which results in MAOA deficiency, was associated with a syndrome characterized by mild retardation, extreme reactive aggression, and violent and impulsive behaviour among males in a single Dutch kindred, the Brunner syndrome (Brunner et al. 1993). This evidence concerns the gene MAOA and Monoamine oxidase A deficiency.