MAPT and Alzheimer disease: Concentrations of Tau N368 in these CSF samples are both correlated tightly with Tau in PET images from AD brains and related to the extent of disease deterioration, indicating that δ-secretase is proteolytically activated in AD patient brains and mediates Tau pathologies16, fitting with the observations that abnormal Tau hyperphosphorylation and deposition are better related to disease severity and cognitive disorders17.