MAPT and Alzheimer disease: Some biomarker measurements directly reflect the pathology of AD, including positron emission tomography (PET) amyloid imaging and cerebrospinal fluid (CSF) beta-amyloid (Aβ) 42, while other biomarkers reflect downstream neuronal injury associated with AD, including CSF tau (both total tau and phosphorylated tau), fluorodeoxyglucose (FDG)-PET imaging and MRI.