For instance, in POLG-related diseases, some adult patients develop parkinsonism with abnormal dopamine transporter scans, and the neuropathological studies have revealed high heteroplasmy levels of mtDNA deletions and depletion in substantia nigra neurons of the midbrain and severe nigrostriatal neurodegeneration [59,60,61,62]. The gene discussed is SLC6A3; the disease is Parkinson disease.