APP and Alzheimer disease: Moreover, both enhanced Aβ protein precursor (AβPP) accumulation in the gut and Firmicutes/Bacteroidetes ratio were found in 5xFAD mice (a transgenetic model of AD; 5xFAD mice express human APP and PSEN1 transgenes with a total of five AD-linked mutations: APP KM670/671NL, APP I716V, APP V7171, and PSEN1M146L and PSEN1L286V) following the change of the gut microbiota composition in these mice since the earliest phase of the diseases (Brandscheid et al., 2017).