Although the COMT Val158Met polymorphism is widely studied in relation to ADHD (Thapar et al., 2005; Caspi et al., 2008), future studies aiming to probe the effect of gene-by-early adversity on ADHD symptoms should expand the focus on other candidate genes or polygenic risk scores, including other DA system genes, serotonin system genes, adrenergic genes, and cholinergic genes. This evidence concerns the gene COMT and attention deficit-hyperactivity disorder.