Finally, tumor microenvironment conditions, the cross-talk between cancer progression and NPY/PYY/PP release from nerve fibers, the interaction between YRs and other receptors (e.g., TrkB), signaling pathways mediated by YRs in tumor proliferation, how the expression of NPY/PYY/PP and their receptors are regulated, and the antitumor strategy combination using NPY/PYY/PP/YR antagonists and chemotherapy must be studied in-depth and elucidated. This evidence concerns the gene PPY and cancer.