KTN1 and attention deficit-hyperactivity disorder: In contrast, if an allele has a smaller regulatory effect on putamen GMVs than its regulatory effect on ADHD, the overall risk effect would be positive, as in previous reports that nine other KTN1 alleles reliably increased risk for ADHD [23] and allele C of rs945270 increased risk for alcohol and nicotine co-dependence [24], comorbidity of ADHD.