γ-Secretase, an enzyme linked to overproduction of insoluble Aβ, is encoded by presenilin 1 and presenilin 2.52 In sporadic AD, polymorphism in multiple genes has been identified.53 Among them is polymorphism in the ε4 and ε2 variants of the apolipoprotein E (APOE) gene.54 So, APOE is considered one of the most fundamental risk factors of sporadic AD.55 Some studies revealed that the genetic factor accounts for about 80% of AD.54 This evidence concerns the gene APOE and Alzheimer disease.