APP and Alzheimer disease: Genetic studies have shown that four genes confer susceptibility to AD: amyloid-β precursor protein (APP) on chromosome 21 [1-6], Presenilin 1 (PS1) on chromosome 14 [7-10], Presenilin 2 (PS2) on chromosome 1 [11-13] and apolipoprotein E (ApoE) on chromosome 19 [14,15].