MAPT and Alzheimer disease: The most extensively studied blood biomarkers in AD are those that reflect brain beta-amyloid (Aβ42 and Aβ40) deposition, accumulation of phosphorylated tau (p-tau), neurodegeneration (neurofilament-light chain; NfL) and astrocyte reactivity (glial fibrillary acidic protein; GFAP) (8–10).