Low levels ofNGF and reduced trkA signaling play important roles in neurodegenerative disorders,constituting therapeutic targets in neurodegenerative disorders’ treatment [31, 81].Depletion of neurotrophic factors such as BDNF, GDNF, and NGF has been associatedwith Parkinson's, Alzheimer's, and Huntington's diseases [82]. The gene discussed is GDNF; the disease is Parkinson disease.