The mean duration of symptoms in PSEN1-related AD families is significantly shorter (ranging from 5.8 to 6.8 years) than in PSEN2 (ranging from 4.4 to 10.8 years) and APP mutations carriers (ranging from 9.0 to 16 years), indicating that the severity of disease is associated with specific PSEN1 mutations. This evidence concerns the gene PSEN1 and Alzheimer disease.