DRD4 and attention deficit-hyperactivity disorder: After adjusting for age, gender, years of parents’ education, yearly income, subtype of ADHD, and IQ, we found a significant interaction of the MEHP concentrations with the DRD4 genotype in relation to omission errors (F = 6.57, p = 0.012) and commission errors (F = 5.80, p = 0.018).