The 48-bp VNTR in DRD4 that we examined is a polymorphism in exon 3 that codes for a 16 amino acid insert in the dopamine D4 receptor. The VNTR contains 2 to 11 repeats, with the 4-repeat and 7-repeat alleles being most common. The 7-repeat allele functions in a way that yields a protein structure that produces less reactive D4 receptors in both in vitro and in vivo tests of responsiveness, resulting in weaker transmission of intracellular signals for those with the 7-repeat allele versus the 4-repeat allele (e.g., Levitan et al., 2006). It has been difficult to characterize fully the functional significance of the less common repeat alleles. Accordingly, in the current report we focus on contrasts between individuals with at least one 7-repeat allele versus those with two 4-repeat alleles.
