Indeed, in the two hemispheres from the subject treated with both putamenal and nigral NRTN gene delivery, focal areas of the putamen with NRTN staining demonstrated intense and dense TH-positive fibre expression similar to that seen in age-matched controls, whereas putamenal areas with absent NRTN staining exhibited much lower TH-positive fibres densities similar to levels found in untreated Parkinson’s disease patients (Decressac et al., 2011; Bartus et al., 2015; Chu et al., 2018). This evidence concerns the gene TH and Parkinson disease.