SNCA and Parkinson disease: In fact, the concept of “brain-first” vs “gut-first” subtypes of PD has been put forward, with those with so-called “gut-first” PD proposed to have pathological alpha-synuclein aggregates that originate in the enteric nervous system and are transmitted to the central nervous system via retrograde vagal transport (for review, see Borghammer & Van Den Berge, 2019 [173]).