COMT and attention deficit-hyperactivity disorder: Most studies to date have failed to identify a significant association between COMT polymorphisms and ADHD; two studies have suggested that sex may moderate an association between the most intensively studied Val/Met polymorphism and ADHD, whereby possessing the methionine (Met) allele conferred risk of ADHD in boys, whereas possessing the valine (Val) allele conferred risk of developing ADHD in girls (Biederman et al., 2008; Qian et al., 2003).