Chronic administration of rotenone, a potent well-studied toxicant of PD, triggered tyrosine hydroxylase-positive neuronal loss in the substantia nigra in both conventionally raised animals (Kanthasamy et al., 2019; Bhattarai et al., 2021) and germ-free animals (Bhattarai et al., 2021). Here, TH is linked to Parkinson disease.