SLC6A4 and Oppositional defiant disorder: Compared with those carrying TT genotype, the risk association with ADHD-ODD was increased significantly in individuals carrying the SLC6A4 rs6354 GT or GG genotype (adjusted OR = 2.18, 95% CI = 1.04–4.56, P = 0.034), but no association was observed in other ADHD symptom domains.