PSEN1 and Alzheimer disease: For example, fibroblasts with the presenilin 1 M146L mutation—a mutation responsible for early‐onset Alzheimer's disease—produce more β‐amyloid1‐42 than the same fibroblasts that had the presenilin 1 gene corrected to the wildtype allele by a CRISPR‐Cas9 gene editing system (Konstantinidis et al. 2022).