In addition to the OPRM1 gene, there have been reports that functional genetic differences in several dopamine system genes alter reward based brain mechanisms. The dopamine transporter (DAT1) is responsible for clearing dopamine from the synapse and, as such, it is important in DA synaptic control and neurotransmission. The DAT1 gene (SCL6A3) has a 40-bp variable number of tandem repeat (VNTR) sequences located in the 3′ untranslated region, the most common being nine and ten VNTR’s. The ten VNTR has been reported to show higher DAT expression than the nine or lesser VNTR (Fuke et al., 2001). Therefore, hypothetically, nine VNTR individuals (either homozygote or heterozygote) are likely to have less DAT and higher synaptic dopamine. It has been reported that 9/9 or 9/10 carriers compared to 10/10 carriers have a greater brain ventral striatal response to both anticipation and receipt of reward during fMRI paradigms (Dreher et al., 2009; Forbes et al., 2009), suggesting that greater synaptic DA availability might underlie reward salience. While nine VNTR carriers also were reported to have greater ventral striatal responses to smoking cues
