The familial form of AD (early onset) accounts for about 5% of cases of AD and is associated with mutations in the genes encoding the precursor protein for Aß and presenilins 1 and 2, which leads to increased aggregation of Aß, but a small part of mutations in the gene encoding presenilin 1 is not familial and occurs de novo [13]. The gene discussed is PSEN1; the disease is Alzheimer disease.