SLC6A3 and attention deficit-hyperactivity disorder: Genetic liability to ADHD is likely polygenic (Wood and Neale, 2010), but DA and 5-HT systems are believed to be involved in ADHD-like behavioral abnormality, as shown by phenotypic analysis of mice lacking DA transporter (DAT) (Gainetdinov et al., 1998; Zhuang et al., 2001), DA receptor 1 (Xu et al., 1994), or 5-HT receptor 1B (Brunner et al., 1999).