A precursor to the AMYLOID-BETA PROTEIN (beta/A4). Alterations in the expression of the amyloid beta-protein precursor (ABPP) gene, located on chromosome 21, plays a role in the development of the neuropathology common to both ALZHEIMER DISEASE and DOWN SYNDROME. ABPP is associated with the extensive extracellular matrix secreted by neuronal cells. Upon cleavage, this precursor produces three proteins of varying amino acid lengths: 695, 751, and 770. The beta/A4 (695 amino acids) or beta-amyloid protein is the principal component of the extracellular amyloid in senile plaques found in ALZHEIMER DISEASE; DOWN SYNDROME and, to a limited extent, in normal aging.