SNCA and Parkinson disease: Through a series of elegant neuropathological studies in postmortem PD patients, Braak and his colleagues proposed a hypothesis in which toxins or pathogens enter the host through the GI tract, causing inflammation and aggregation of alpha-synuclein protein in the enteric nervous system; this aggregated alpha-synuclein protein moves up to the central nervous system via the vagus nerve, resulting in the degeneration of dopaminergic neurons in the substantia nigra (Hawkes et al., 2007; Keshavarzian et al., 2020).