SLC6A4 and attention deficit-hyperactivity disorder: Polymorphisms in the dopamine transporter gene (DAT1, SLC6A3) and the dopamine 4 (D4) receptor gene (DRD4) have been most often associated with ADHD; other candidate genes with significant associations in meta-analyses include the dopamine D5 receptor (DRD5), serotonin transporter (5HTT, SLC6A4), serotonin receptor 1B (5HT1B, HTR1B), and synaptosomal-associated protein 25 (SNAP25) (Gizer et al. 2009; Smith et al. 2009).