Interestingly, the ε4 population distribution provides a genetic basis for variance in age of AD onset, as evidenced by an analysis of AD age of onset as a function of APOE genotype: the mean age of onset varies from less than 70 years for ε4/ε4 to over 90 years for ε3/ε2 (Roses, 1996). Here, APOE is linked to Alzheimer disease.