CASP6 and Cognitive impairment: Tau fragments cleaved by caspase-6 at amino acids 421 (tauΔD421), as well as active caspase-6, are present in pre- and mature NFTs composed mainly of tau filaments in the patients’ brains with mild cognitive impairment (MCI) or early stage of AD, while absent in the non-AD brains [150, 152].