MAPT and Alzheimer disease: At the molecular level, AD is characterized by overproduction of the 40 and 42 amino acid isoforms of β-amyloid (Aβ1–40 and Aβ1–42), which form insoluble, extracellular aggregates of Aβ called amyloid plaques, and by the hyperphosphorylation and aggregation of the microtubule-associated protein tau within neurons.