In a study of 111 children, mean age 9.25 years, with attention deficit hyperactivity disorder (ADHD) who were treated with methylphenidate, dopamine transporter (DAT1), and dopamine receptor D4 (DRD4), promoter methylation was assessed in blood samples collected pre-treatment to see if this predicted treatment response [32]. This evidence concerns the gene DRD4 and attention deficit-hyperactivity disorder.