COMT and attention deficit-hyperactivity disorder: Such research indicates that: (a) the 5-HTTLPR S allele will be associated with ADHD characteristics and mood/interest and pleasure, whereas the L allele will be associated with repetitive behaviour, (b) the 4-repeat MAOA allele will be associated with ASD, (c) the AA COMT genotype will be associated with challenging behaviour, ADHD characteristics and mood/interest and pleasure whereas the GG genotype would be associated with repetitive behaviour.