TH and Parkinson disease: Because there are axonal transport deficits in Parkinson’s disease (Chu et al., 2012) and there is a comprehensive loss of TH expression in nigrostriatal fibres by the time subjects were enrolled in the study, it was considered essential to deliver NTRN directly to nigral neurons to try to upregulate repair genes and induce a clinical benefit (Bartus, et al. 2013; Olanow et al., 2015b).