The key words used were ADHD and genetic, inherited, gene-environment interaction, gene-environment interplay, aetiology, linkage, genome-wide association study, copy number variant, twin studies, adoption studies, COMT, conduct disorder, candidate gene, environment, toxins, lead, polychlorinated biphenyls, PCB, pesticides, diet, prenatal, perinatal, pregnancy, smoking, tobacco, substance, stress, birth weight and prematurity. The gene discussed is COMT; the disease is attention deficit-hyperactivity disorder.