Threonine 231 is one of the phosphorylation sites of human tau (phospho-tau 231, p-tau 231). Tau is a microtubule-stabilizing protein primarily localized in neurons of the central nervous system but also expressed at low levels in astrocytes and oligodendrocytes. Tau consists of six isoforms in the human brain with molecular weights of 48 KDa to 67 KDa, depending on isoform. Tau elevation is observed in the cerebrospinal fluid (CSF) of patients with neurodegenerative disease and severe head injuries, suggesting its extracellular release during neuronal damage and a role as a biomarker with specificity for brain injury. In Alzheimer’s disease and related neurodegenerative diseases, including chronic traumatic encephalopathy, tau is abnormally phosphorylated and aggregated into bundles of filaments. Phosphorylated tau has believed to be a more relevant biomarker for Alzheimer’s disease. Tau phosphorylated at threonine 231 has been shown to differentiate Alzheimer’s disease from healthy controls.