In the two PD monkeys subjected to BDA injection in PM, one animal (Mk-LL, −67% of TH+ neuronal loss) had numbers of labeled axonal boutons in STN that were comparable to intact animals (Figure 4A, p > 0.0125), whereas the second monkey (Mk-MY, −72% of TH+ neuronal loss) showed a dramatically higher number of axonal boutons in STN (Figure 4B, p < 0.0125). Here, TH is linked to Parkinson disease.