DDC and attention deficit-hyperactivity disorder: Genes encoding for enzymes involved in the catecholaminergic system like catechol-O-methyl transferase (COMT) [19, 20], dopamine decarboxylase (DDC) [21–23], dopamine beta hydroxylase [5, 15], and monoamine oxidase [15, 18, 22, 23] have also gained a lot of importance in exploring the etiological basis of ADHD.