Brain studies of patients with ADHD have shown abnormalities in frontal-striatal and cerebellar circuitry.6, 7 Although dopaminergic and noradrenergic medications are clinically effective,8 animal models provide support for the involvement of additional molecular pathways.9 Twin studies estimate a heritability of up to 90%,10 indicating a substantial genetic contribution. However, genetic association studies thus far have not identified any genes definitively conferring major risk.10, 11 ADHD comorbidity occurs in many neuropsychiatric disorders, including autism, Tourette syndrome and in children with schizophrenic parents.12, 13
