SLC6A3 and attention deficit-hyperactivity disorder: The proposal that dysfunction of the dopamine (DA) system is responsible for some of the symptoms of ADHD is supported by reports of alterations in the dopamine transporter (DAT) (Dougherty et al., 1999; Krause, Dresel, Krause, Kung, & Tatsch, 2000) and abnormalities in DOPA decarboxylase activity (an enzyme involved in presynaptic storage of DA) within the PFC (Ernst, Zametkin, Matochik, Jons, & Cohen, 1998) in ADHD patients.