NR4A2 and Parkinson disease: Overexpressed HPRT1, in the presence of lentiviral oe-HPRT1, impeded the dopaminergic neuron loss and neuron apoptosis in 6-OHDA-induced PD mice, accompanied by increased mRNA expression of Nurr-1, Pitx-3, Ngn-2 and NeuroD1.