DRD4 and attention deficit-hyperactivity disorder: Importantly, DRD4 polymorphisms have been associated with personality traits that constitute endophenotypes for impulse control-related neuropsychiatric disorders [1], with the most consistent associations found between the gene encoding D4.7R and attention-deficit hyperactivity disorder (ADHD) [8, 10–12] and substance use disorders (SUDs) [13].