PSEN1 and Alzheimer disease: In the AD brain, there is an imbalance between Aβ production and clearance, which can be caused by mutations in the APP (e.g., KM670/671NL) [59], PSEN1 (e.g., I83T, M84T) [60,61], and/or PSEN2 (e.g., A23A, G34S) [62,63] genes.