MAPT and Alzheimer disease: Pathologically, MCI brain is characterized similarly to AD brain, i.e., presence of senile plaques (composed mostly of aggregated fibrils of amyloid b-peptide, a 39–42 amino acid peptide that is neurotoxic, and dystrophic neurites), neurofibrillary tangles (composed of aggregates of hyperphosphorylated tau protein) and loss of synapses.