APP and early-onset autosomal dominant Alzheimer disease: The β-amyloid peptide (Aβ) is a primary component of senile plaques found in the brains of Alzheimer's disease (AD) patients, and the existence of mutations in the gene encoding Aβ (Amyloid Precursor Protein, APP) in a subset of familial AD cases argues for a causal role for this peptide in this disease.