The OPRD1 gene encodes the delta-opioid receptor (DOR). DOR is not the primary target of any commonly abused opioids; however, the receptor is involved in reward pathways [19], and evidence suggests it regulates factors with clear connections to substance use, such as mood and contextual learning [20, 21]. Levran et al. performed a candidate gene analysis in heroin-dependent subjects of European descent (case n = 412, control n = 184) [7], and nominally significant associations were observed for three variants in OPRD1: rs2236861, rs2236857, and rs3766951. In a larger Australian cohort (case n = 1459, control n = 1495), rs2236857 and rs3766951 were significantly associated with opioid dependence [22], while rs2236861 remained nominally significant. A European study (case n = 142, control n = 142) did find a significant association between rs2236861 and opioid dependence [23]. In contrast, Randesi et al. found rs2236861 to be significant associated only with non-dependent opioid use (case n = 163, control n = 153) but not opioid dependence (case n = 281) in a Dutch population [24], with no effect observed for either rs2236857
