It should be however borne in mind that, unlike PD and synucleinopathies (De Guilhem De Lataillade et al., 2020), all existing studies suggest that pathological tau species are not observed in the gut of subjects with either AD or PSP (Shankle et al., 1993; Lionnet et al., 2018; Dugger et al., 2019). Here, MAPT is linked to Alzheimer disease.