Some studies have found that the DRD4 7-repeat (7R) allele was associated with persistent ADHD (Biederman et al., 2009; Langley et al., 2009), while other studies found that the patients with persistent ADHD symptoms were less likely have DRD4 7R allele (Shaw et al., 2007). Here, DRD4 is linked to attention deficit-hyperactivity disorder.