PSEN1 and Alzheimer disease: For example, mutations in several genes (e.g., amyloid precursor protein, presenilin 1, and presenilin 2) [2] are known causes of early onset AD, while some genes and gene variants such as Apolipoprotein E (APOE) are associated with an increasing risk for developing late-onset AD [3,4,5,6,7,8,9].