BDNF, mesencephalic astrocyte-derived neurotrophic factor (MANF), glial cell line-derived neurotrophic factor (GDNF), and cerebral dopamine neurotrophic factor (CDNF) have been shown to be neuroprotective and neurorestorative toward damaged dopaminergic neurons in cell cultures and in various PD animal models (Allen et al., 2013; Voutilainen et al., 2015; Lindahl et al., 2017). Here, BDNF is linked to Parkinson disease.