PSEN1 and Alzheimer disease: A small portion (<5%) of AD cases are early-onset variants caused by mutations in the APP, PSEN1, or PSEN2 genes.106 However, the majority of AD patients is affected by late-onset forms with the disease affecting 1 in 9 individuals over the age of 65 and 1 in 3 individuals over the age of 85, making age the leading risk factor of AD.107 The most significant genetic risk factor for late-onset AD is the polymorphism in the apolipoprotein E (ApoE) gene.