The dopamine D4 receptor (DRD4) has been associated with smoking behavior (Laucht et al. 2008), schizophrenia (Shi et al. 2008), novelty seeking (Munafò et al. 2008) and impulsivity (Munafò et al. 2008), as well as responses to stimulant drugs. The exon III VNTR polymorphism of DRD4 is the most commonly studied. This polymorphism has 8 alleles, varying from 2 to 8 and 10 repeats (Lichter et al. 1993). The 7-repeat allele has been associated with ADHD in some studies (Brookes et al. 2006; Faraone et al. 2001; Li et al. 2006), but not in all (Johansson et al. 2008). Because of its association with ADHD, the 7-repeat allele has been the focus of most studies involving stimulant drug responses.
