DRD4 and attention deficit-hyperactivity disorder: As rare variants inside the VNTR are hypothesized as being a possible confounder for usual DRD4 VNTR and ADHD association studies, all 2R(n=31) and 7R(n=175) and many randomly selected 4R(n=521) homozygous had the VNTR region sequenced, comprising a sample size of 727 individuals.