OPRM1 and head and neck squamous cell carcinoma: Other genes with potential diagnostic utility in HNSCC include methylenetetrahydrofolate dehydrogenase 1 (MTHFD1L), the opioid receptor genes OPRL1 (opioid-related nociceptin receptor 1) and OPRM1 (opioid receptor mu 1), and the xenotropic and multimodal retrovirus receptor 1 (XPR1).