In familiar forms of AD, autosomal dominant gene mutations of the presenilin 2 genes on chromosome 1, the amyloid precursor protein gene on chromosome 21, and the presenilin 1 gene on chromosome 14 can lead to the early onset of AD in the atypical type, which is about 1% to 2% of AD cases [6]. Here, PSEN2 is linked to Alzheimer disease.