Compared with untreated mice (4,920 ± 469 cells, n = 6), injection of MPTP led to a massive loss of mDA neurons: TH+ mDA neurons in the SN of the PD mice were reduced to 1,500 ± 164 cells (n = 20) 6 weeks post‐MPTP injection. This evidence concerns the gene TH and Parkinson disease.