Expression levels of TAO kinase 2 (TAOK2), a serine/threonine kinase gene, had the most significant association with smaller brain volumes in ADHD (Pearson’s r = −0.86, uncorrected p = 2.8 × 10−7, Bonferroni p = 3.8 × 10−4). The gene discussed is MARK2; the disease is attention deficit-hyperactivity disorder.