(FAD) is very rare; less than 1% of AD patients have FAD. FAD Patients with FAD become sick at a relatively young age, typically, in their 40s or 50s, and the disease is more aggressive than sporadic AD. FAD can be caused by mutations in three different genes. The most common ones are in presenilin 1, which is involved in the production of amyloid β-protein (Aβ). Mutations in a counterpart gene called presenilin 2 have a similar impact. Both cause an increase in production of more toxic forms of Aβ. The third gene is the one producing the amyloid β-protein precursor itself, from which Aβ is produced. These mutations cause elevated production of Aβ or of Aβ forms that are more toxic and more prone to aggregation.