APP and Alzheimer disease: However, immunostaining showed augmented Aβ deposition, and elevated levels of amyloid precursor protein (APP) (the source of the Aβ peptide) and phosphorylated Tau—the key components of senile plaques and neurofibrillary tangles, the two histopathological hallmarks of AD [15], in the hippocampus of ALP TSs (Figures 1F–H and S1A,B).