TH and Parkinson disease: Although large-scale overt loss of DA neurons is not associated with normal aging in nonhuman primates, aging-related declines in staining for TH is readily observed,25–28 and the loss of TH phenotype is an early stage of the degenerative process in PD.22 Our observations confirmed that no significant overt loss of SN DA neurons occurred in aged nonhuman primates, but an aging-related decline in staining for TH in midbrain DA neurons was present and significantly exaggerated (−63%) in the vtSN neurons most vulnerable to degeneration in PD29 (Fig. 1a–d).