At the genetic level, gene products of the HSV-1 life cycle have been shown to interact with AD susceptibility genes, such as presenilin 1 and 2 (PSEN1 and PSEN2), apolipoprotein E allele 4 (ApoE4) and clusterin genes, to promote both viral infectivity and risk of AD. This evidence concerns the gene PSEN1 and Alzheimer disease.