PSEN2 and Alzheimer disease: We focus on three causal genes associated with early onset AD, which include PSEN1 (chr14:73603143–73690399), PSEN2 (chr1:227058273–227083804), and APP (chr21:27252861–27543138) as well as one causal gene APOE (chr19:45409039–45412650) associated with late onset AD.