PSEN1 and Alzheimer disease: Early-onset AD (EOAD) occurs in patients younger than 65 years, sometimes as young as 30 years, due to mutations in the gene encoding APP as well as the genes encoding for presenilin 1 (PSEN1) or PSEN2, both of which are key components of the γ-secretase complex and therefore result in an increase in Aβ42 if improperly expressed (reviewed in Hernandez-Sapiens et al., 2022).