TH and Parkinson disease: In Parkinson’s, αSyn aggregation occurs throughout the brain, yet dopaminergic neurons are particularly susceptible to degeneration.42 In familial PD caused by the αSyn G51D mutation, degeneration of the substantia nigra was extensive.10,11 However, quantification of TH+ neurons in the substantia nigra of Snca+/+, SncaG51D/+ and SncaG51D/G51D animals at either 6 months or 12 months indicated there was no difference between the number of TH+ neurons (Supplementary Fig. 6A to H).