 In both mice and humans, a mutation was located on the eighth exon of the MAO-A gene, which created a dysfunctional MAO-A gene.The regular function of MAO-A, breaking down monoamines, is disrupted, and monoamines build up within the brain.Mice that lacked a functional MAO-A gene displayed higher levels of aggression, in comparison to mice with a functional MAO-A gene.