OPRM1 and cancer: The findings of this review have highlighted the role of genetic variants in drug-metabolizing enzymes (CYP2D6, CYP3A5, UGT2B7), neurotransmitter-metabolizing enzymes (COMT, ABAT), transporters (ABCB1) and drug receptor (OPRM1) and ion channels (P2RX7) in contributing to inter-individual variability in opioid treatment responses in Asian patients with cancer pain.