The current meta-analysis, could confirm previous findings showing weak association between the 10-repeat allele of the DAT1 3′-UTR VNTR gene and child and adolescent ADHD, that reached significance only in the European population; however, this was accompanied by high heterogeneity that was in some cases due to literature bias but in other cases due to heterogeneity in clinical phenotyping, age or ethnicity (Li et al. 2006; Gizer et al. 2009). This evidence concerns the gene SLC6A3 and attention deficit-hyperactivity disorder.