This is the first investigation of the differential association between three SNPs, which affect brain functions, 5-HTTLPR (serotonin), MAOA (monoamine oxidase) and COMT (dopamine), and a wide range of clinically important behavioural characteristics in individuals with FXS, a well-defined genetically mediated syndrome associated with a heightened risk of displaying such behaviours. This evidence concerns the gene MAOA and fragile X syndrome.