Furthermore, C-terminal fragment (CTF) β (C99) is the earliest β-APP catabolite and a main contributor to intracellular β-APP-related immunoreactivity [26], and has been reported to be neurotoxic due to its ability to form large insoluble aggregates, and thus, C99 is considered to play a key role in the pathogenesis of AD [27]. This evidence concerns the gene PPIB and Alzheimer disease.