Given our previous published data indicating the mu opioid receptor (MOR) is increased in human lung cancer [12], that lung cancer cells do not form visible tumors in MOR knockout mice [12] and that MOR overexpression promotes NSCLC primary tumor growth and metastasis [4], [43], we hypothesized that MOR regulation of EMT might be a plausible explanation for the differences in recurrence rates observed in the epidemiologic studies. This evidence concerns the gene OPRM1 and lung carcinoma.