MAPT and Alzheimer disease: Among several factors being studied as possible biomarkers for AD are the amyloid-β in the brain as shown in positron emission tomography (PET) imaging and levels of certain proteins in fluid (for example, levels of Aβ and phosphorylated tau, a major microtubule associated protein of a mature neuron) in the cerebrospinal fluid and levels of particular groups of proteins in blood) [14].