DRD4 and attention deficit-hyperactivity disorder: Kahn, Khoury, Nichols, and Lanphear (2003) found the combination of maternal prenatal smoking with a DAT1 genotype leading to ADHD, while in another study, maternal stress during pregnancy together with dopamine receptor 4 gene (DRD4) were associated with severity of ADHD symptoms (Grizenko et al., 2012).