For example, PD patients with primary substantia nigra (SN) neuron mitochondrial defects presented with POLG mutations,174 and substantial loss in the number of SN neurons was observed in patients with POLG mutations, suggesting a correlation between mtDNA mutation accumulation and brain pathology.174 Proteome analysis has revealed the important roles of mitochondria in brain tissue. Here, POLG is linked to Parkinson disease.