Of the 13 genes evaluated, only four of them showed variants that significantly alter the effects of Hg exposure on neuropsychological domains in children: the gene encoding the heme pathway enzyme, CPOX4, the MTs with their two isoforms MT1M and MT2A and the cathecol-O-methyltransferase gene (COMT) that has been linked to diverse neuropsychiatric conditions such as schizophrenia and ADHD. The gene discussed is COMT; the disease is attention deficit-hyperactivity disorder.