MAPT and Alzheimer disease: Elevated levels of tau protein in the cerebrospinal fluid (CSF) are markers of active neuronal degeneration [11], while levels of abnormally phosphorylated tau (P-tau) appear to correlate with the quantity of neurofibrillary tangles in the brain, suggesting that CSF P-tau may serve as an in vivo biomarker of the neurofibrillary pathology of AD [12].