The gene encoding the mu-opioid receptor (OPRM1) to which opioids bind to produce their analgesic and rewarding effects is the most widely studied candidate gene for heroin and other opioid addictions.60 Functional OPRM1 polymorphisms identified in humans include the extensively-studied rs1799971 (A118G),61, 62, 63, 64 but a meta-analysis65 did not support its significant role in opioid addictions. Other aspects of the opioidergic system have also been queried. However, analyses involving prodynorphin (PDYN),66 proenkephalin (PENK),67 and the kappa (OPRK1)68, 69 and delta opioid receptors (OPRD1)69, 70, 71 have not produced consistent results.
