SLC6A3 and attention deficit-hyperactivity disorder: The dopamine transporter knockout mouse, an animal model of the disorder, has altered expression of BDNF in the frontal cortex (Fumagalli et al., 2003); conversely animal models in which BDNF gene expression is altered show several phenotypes characteristic of ADHD, including hyperactivity (Kernie et al., 2000; Rios et al., 2001).