MAPT and Alzheimer disease: The classification uses three types of biomarkers to determine the extent of pathology typical of AD: A (amyloid, represented either by cerebrospinal fluid (CSF) levels of amyloid-β 42 (Aβ42) or amyloid plaque deposition in brain as seen with amyloid-PET); T (tau, measured as the level of CSF hyperphosphorylated tau (p-tau) or tangle-formation as seen by tau-PET); and N (neurodegeneration as shown by structural MRI, CSF levels of total tau (t-tau), or brain metabolism as measured with fluorodeoxyglucose (FDG-PET)).