SLC6A3 and attention deficit-hyperactivity disorder: 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].