In AD, which is marked by amyloid-beta (Aβ) plaques and tau tangles, key genetic contributors such as amyloid beta precursor protein (APP), presenilin (PSEN1), and presenilin 2 (PSEN2) play a significant role in early-onset familial AD due to their influence on Aβ accumulation. The gene discussed is PSEN2; the disease is Alzheimer disease.