Alzheimer's Disease is characterized by the presence of extracellular plaques and intracellular neurofibrillary tangles (NFTs) in the brain. The major component of these plaques is Ab peptide, a 40 to 43 amino acid peptide cleaved from amyloid precursor protein (APP). Increased release of the "longer forms" of Ab peptide, Ab42 or Ab43, which have a greater tendency to aggregate than Ab40, occurs in individuals expressing certain genetic mutations, expressing certain ApoE alleles, or may involve other, still undiscovered, factors. Many researchers theorize that this increased release of Ab42/Ab43 leads to the abnormal deposition of Ab and the associated neurotoxicity in the brains of affected individuals.