After surgical removal of the primary tumor, mice were treated with a classic DOR agonist [D-Ala2,D-Leu5]-Enkephalin (hereby indicated as “opioid”) for 10 d (Fig. 2A), and distant metastases were examined in the lungs of the mice by immunohistochemistry (Fig. 2B and C). The gene discussed is OPRD1; the disease is neoplasm.