This situation is even further aggravated by the appearance of early-onset familial AD (FAD) caused by mutations in one or more of at least three genes known as amyloid-beta precursor protein (APP), presenilin 1 (PSEN 1), and presenilin 2 (PSEN2) (https://www.alzforum.org/mutations, accessed on 7 May 2023). Here, APP is linked to familial Alzheimer disease.