Neurotrophic factors, including brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF), neurotrophin-3 and -4 (NT-3, NT-4), and glial cell line-derived neurotrophic factor (GDNF), are thought to be important mediators in the PNI disease process, as they have been shown to induce cancer cell growth towards nerves and promote axonal growth of neurons towards cancer. The gene discussed is GDNF; the disease is cancer.