PSEN1 and Alzheimer disease: It has been revealed that there is a link between FAD mutations and increased ratio of Aβ42/40 [16,17], which indicates that increased Aβ42 levels (as compared to Aβ40) play a crucial role in AD pathogenesis, possibly via providing the core for Aβ assembly into amyloidogenic plaques, fibrils, and oligomers [18,19].