Amyloid precursor protein (APP) is an integral membrane protein expressed in many tissues and concentrated in the synapses of neurons. Its primary function is not known, though it has been implicated as a regulator of synapse formation and neural plasticity. APP is best known and most commonly studied as the precursor molecule whose proteolysis generates amyloid beta (Aβ), a 39- to 42-amino acid peptide whose amyloid fibrillar form is the primary component of amyloid plaques found in the brains of Alzheimer's disease patients.Several alternative splicing isoforms of APP have been observed in humans, ranging in length from 365 to 770 amino acids, with certain isoforms preferentially expressed in neurons; changes in the neuronal ratio of these isoforms have been associated with Alzheimer's disease.