After adjusting significant G × E interactions for comorbid ADHD symptom scores, we found that the G × Es of rs4714329-GG and rs9471290-AA with smoking during pregnancy (P = 0.0014 and P = 0.0010, respectively, males), as well as the G × E between MAOA-LPR and maltreatment remained significant (P = 0.0013, females). Here, MAOA is linked to attention deficit-hyperactivity disorder.