GDNF and Parkinson disease: Many previous studies have demonstrated that expression-level changes in specific NTFs, such as BDNF, CNTF, and glial cell line-derived neurotrophic factor (GDNF), are associated with the pathogenesis of neurodegenerative diseases, such as AD [19,60,61], PD [62,63,64], HD [65,66], and ALS [67,68] (Table 2).