exon III of the DRD4 gene ranges from 2 to 11 repeats. Previous studies have indicated that the longer the repeat, the more dampened the response to dopamine. The DAT1 transporter gene regulates re-uptake of dopamine into presynaptic terminals, terminating dopaminergic neurotransmission, and maintaining dopamine homeostasis. DAT1 contains a polymorphic 40-base pair VNTR which has been previously associated with lower risk of early smoking onset and current smoking (10). The gene cluster TTC12-ANKK1-DRD2 plays a central role in modulating dopamine reward system, by mediating the reinforcing effect of all known addictive substances (35).
