MAPT and Alzheimer disease: In accordance with the β-amyloid (Aβ) theory, AD starts with the accumulation in the brain of extracellular Aβ deposits that subsequently leads to tau protein hyperphosphorylation, resulting in tau neurofibril aggregation or neurofibrillary tangles (NFTs) inside neurons and finally in neurodegeneration and both neuronal and synaptic loss [1].