DRD4 and attention deficit-hyperactivity disorder: Other examples of gene-environment interaction include the s-allele of the serotonin transporter gene (5-HTTLPR), which predisposes to depression if accompanied by stressful life-events [4], and the seven-repeat variant of the dopamine receptor D4 gene (DRD4), which increases the vulnerability for externalizing problems and ADHD in children whose mothers are insensitive to their children's needs [5].