Testing patients with ADHD and controls separately revealed that patients carrying at least one copy of the DAT1 9-6 haplotype had larger striatum volume (7.4 %; 1.37 ml; β = 1.37; 95 % CI 0.80–1.94; p = .00001), while this effect was not significant in the control group (3.0 %; 0.57 ml; β = 0.57; 95 % CI −0.25 to 1.39; p = .17) (Table 3 and supplementary Table 5). Here, SLC6A3 is linked to attention deficit-hyperactivity disorder.