COMT and attention deficit-hyperactivity disorder: Over the last two decades, molecular genetic studies have identified a few candidate genes with polymorphisms associated with ADHD symptoms and activation of DA within frontal cortex, with the one of most replicated and important being the Catechol-O-methyltransferase (COMT) Val158Met (rs4680) polymorphism (Sun et al., 2014; Mizuno et al., 2017; Jung et al., 2018; Taylor, 2018).