Meta-analysis in ADHD revealed a significant association between the 7-repeat allele of the DRD4 exon 3 VNTR (OR 1.33, 95 % CI 1.15–1.54) and the disorder (Gizer et al. 2009), while association of the DRD4 4-repeat allele and OCD was indicated by a case–control study (Camarena et al. 2007) and by a family-based study (Walitza et al. 2008), whereas an increased frequency of the DRD4 7-repeat allele was shown in a late-onset OCD group (Hemmings et al. 2004), and in a subgroup of OCD patients with comorbid tics (Cruz et al. 1997). Here, DRD4 is linked to attention deficit-hyperactivity disorder.