The D2 dopamine receptor is coded by the DRD2 gene located on chromosome 11q23 [62]. Polymorphisms in DRD2 have been associated with increased heroin use and vulnerability to addiction [47, 63]. A specific polymorphism at the TaqI RFLP (restriction fragment length polymorphism) site (rs1800497) involving a base pair substitution of C (A2) for its minor allele T (A1, minor allele frequency (MAF) = 0.01) has been investigated in subjects with opioid dependence [64]. This SNP was previously believed to be located on DRD2 but has since then been confirmed to be on the ANKK1 (ankyrin repeat and kinase domain containing 1) gene, just downstream of DRD2. The close relative distance can explain how rs1800497 was able to exert its effects on the DRD2 receptor gene [65].
