Because plaques of aggregated β-amyloid (Aβ) peptide and tangles of hyperphosphorylated tau protein are prominent anatomical hallmarks in the brains of AD patients (Braak and Braak, 1991, 1995), the discovery of mutations in genes encoding these peptides in familial AD patients promised to be a major breakthrough for the understanding of the disease. The gene discussed is MAPT; the disease is Alzheimer disease.