MAPT and Alzheimer disease: Thus, total Tau (T-tau), hyperphosphorylated Tau (P-Tau), and the oligomeric species have been used as key biomarkers for AD [27] even though they are found at low levels, making their detection difficult in biological fluids such as cerebrospinal fluid (CSF) (low 195 pg/mL or 4.7 pM) [28].