Specifically, brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF), vascular endothelial growth factor (VEGF), and glial cell line-derived neurotrophic factor (GDNF) levels have been reported to be decreased in post-mortem PD brains51–55 and CSF56, as well as in peripheral blood57–60. This evidence concerns the gene GDNF and Parkinson disease.