SLC6A1 and attention deficit-hyperactivity disorder: Accordingly, a familial duplication of a cluster of four GABAA receptor subunit genes segregates with multiple NDDs [38], polymorphisms in GAD1 are associated in particular with the hyperactive/impulsive domain in children with ADHD [39], variants of the GABA-transporter gene GAT1 (SLC6A1) correlates with the risk for ADHD in a case–control study [40] and multiple genetic variants with known direct relation to GABAergic or glutamatergic signalling are associated to symptom severity in ADHD [41].