PSEN2 and familial Alzheimer disease: An autosomal dominant mutation of PS‐1/PS‐2 leads to the early onset of familial Alzheimer's disease (FAD), and the PS1 complex displays greater γ‐secretase activity than the PS2 complex (Lai et al., 2003), which indicates that PS1 might be a critical catalytic component of γ‐secretase.