Amyloid Precursor Protein (APP) is an integral membrane protein with a large ectodomain, a transmembrane domain, and a short cytoplasmic tail. APP is proteolytically cleaved by b-secretase (BACE) to generate the APP N-terminal fragment (sbAPP) with Mr 100kD and a C-terminal fragment (C99) with Mr 12kD. The smaller 12kD fragment can be further cleaved by g-secretase, an enzyme activity recently attributed to presenilin-2. This second cleavage produces the insoluble b-amyloid (Ab), a peptide of ~4kD. The accumulation of b-amyloid in intracellular neurofibrillary tangles and extracellular plaques is observed in the brains of Alzheimer