MAPT and Alzheimer disease: In addition to established AD biomarkers, such as monomeric Aβ1–42, the ratio of Aβ1–42 to Aβ1–40, phosphorylated Tau, and total Tau in CSF, and magnetic resonance imaging and positron emission tomography (PET), i.e., Amyloid-PET or Tau-PET [2,27,28], oligomeric forms of Aβ represent a promising biomarker candidate for early AD diagnosis [5,6,29].