There have been a number of examples of neurotrophic factors being successfully tested in animal models for neurodegenerative diseases, but failing to bring any benefit over placebo when tested in human clinical trials, such as glial cell-derived neurotrophic factor (GDNF) for Parkinson disease [53] or ciliary neurotrophic factor (CNTF) for amyotrophic lateral sclerosis [54]. Here, GDNF is linked to amyotrophic lateral sclerosis.