Numerous studies have sought to correlate polymorphic DRD4 variants to psychiatric disorders, including association of the D4.7 variant with increased risk for ADHD (see Pharmacological Properties and Involvement in Psychiatric Disorders), and with the personality trait of novelty seeking (Ebstein et al., 1996), although some failed to replicate the original findings (for example, Hawi et al., 2000). Here, DRD4 is linked to attention deficit-hyperactivity disorder.