Tau is a microtubule-stabilizing protein primarily expressed in central nervous system (CNS) neurons, but also at low levels in astrocytes and oligodendrocytes. Tau consists of six isoforms in the human brain with molecular weights ranging from 48-67 kDa. Under physiological conditions, tau is primarily localized within neurons. However, elevated tau in cerebrospinal fluid (CSF) is a hallmark of neurodegenerative diseases and severe brain injuries, indicating its extracellular release during neuronal damage. In Alzheimer’s disease (AD) and related conditions, tau becomes abnormally phosphorylated, forming filamentous bundles. Among its phosphorylation sites, aberrant phosphorylation of tau at threonine 181 (p-Tau 181) leads to the formation of neurofibrillary tangles, a hallmark pathological feature of AD. Elevated levels of p-Tau 181 in CSF and blood have been consistently associated with cognitive decline and neurodegeneration in AD patients, making p-Tau 181 a valuable diagnostic and prognostic marker. Download our fact sheet to learn more.