SLC6A4 and attention deficit-hyperactivity disorder: Fowler et al. (2009) found that a variable number tandem repeat (VNTR) polymorphism in the gene encoding monoamine oxidase A (responsible for catabolism of serotonin and other catecholamine neurotransmitters), and an insertion/deletion polymorphism in the SLC6A4 promoter (5-HTTLPR) were associated with what the authors term the “emotional dysfunction” aspect of psychopathy (a construct that corresponds with CU traits) in a sample of adolescents with attention deficit hyperactivity disorder (ADHD) [18].