As we will discuss shortly, “prionoid,” or prion-like propagation of alpha synuclein triggered by some unknown pathogen or toxin in the GI tract is another potential pathogenic mechanism for PD (Braak et al., 2003b; Kodower et al., 2008; Luk et al., 2012; Recasens et al., 2014; Thomzig et al., 2021). Here, SNCA is linked to Parkinson disease.