Motor symptoms in PD manifest because of the degeneration of DA neurons in the substantia nigra pars compacta (SNpc) when DA loss reaches a threshold of 40%–50%.24 We performed stereological quantitation of SNpc DA neurons, which are immunoreactive to tyrosine hydroxylase (TH), in WT and auxilin KO mice to understand the cellular basis for the motor symptoms we observed. Here, TH is linked to Parkinson disease.