MAPT and Alzheimer disease: It should be noted that it is only in 4R tauopathies, like in CBD tau, that the core of the protofilament has the whole R2 region taking part in forming the β-sheet structure, and that this is absent in both the 3R tauopathy (PiD) and 3R + 4R tauopathy (AD and CTE), and therefore, the CBD tau folds are the longest in length, covering 107 residues, and the aggregates are characteristically observed in both neurons and astrocytes of the frontal cortex and basal ganglia [94].