Nevertheless, in the midbrain of PD, the low oxygen tension, through upregulating hypoxia inducible factor-1 (HIF-1), accelerates NSCs proliferation, stimulates NSCs that differentiate into neonatal dopaminergic neurons which have been verified to secrete dopamine, and promotes erythropoietin (EPO) and vascular endothelial growth factor (VEGF) expression to further exert neuroprotective effects on midbrain dopaminergic neurons [32]. Here, EPO is linked to Parkinson disease.