Monoamine oxidases (MAO-A and MAO-B) have a key role in the degradation of amine 
neurotransmitters, such as dopamine, norepinephrine and serotonin. We identified 
an inherited 240 kb deletion on Xp11.3-p11.4, which encompasses both monoamine 
oxidase genes but, unlike other published reports, does not affect the adjacent 
Norrie disease gene (NDP). The brothers who inherited the deletion, and thus 
have no monoamine oxidase function, presented with severe developmental delay, 
intermittent hypotonia and stereotypical hand movements. The clinical features 
accord with published reports of larger microdeletions and selective MAO-A and 
MAO-B deficiencies in humans and mouse models and suggest considerable 
functional compensation between MAO-A and MAO-B under normal conditions.