OPRM1 and lung cancer: Administration of morphine, an opioid peptide which binds to μ-opioid receptors (MOR), has been shown to promote tumor growth, tumor apoptosis, angiogenesis, migration, and invasiveness [21–24], while a MOR antagonist, methylnaltrexone, has been shown to possess antiproliferative and antimetastatic properties in lung cancer [25].