The immunohistochemical analysis of TH content in the substantia nigra region of the four groups of mouse showed a reduction in TH protein levels in the PD and PD with T2DM groups, indicating loss of dopaminergic neurons, with the PD with T2DM group showing a more severe damage to dopaminergic neurons than the PD group (Figure 1E–F). This evidence concerns the gene TH and Parkinson disease.