2006). We therefore utilized our Norwegian ADHD sample to test for association between ARC SNPs and ADHD, but found no significant associations. Since present ADHD diagnostic categories are rather heterogeneous, more precise phenotypic categories should be analyzed. More importantly, a more thorough association analysis between ARC variants and other neuropsychiatric disorders deserves consideration. As a tightly controlled gene central to brain plasticity and cognition, variation in ARC may still prove to confer beneficial and/or deleterious effects on human cognitive abilities. This evidence concerns the gene ARC and attention deficit-hyperactivity disorder.