PSEN1 and Alzheimer disease: It is hypothesized that this substantial variation in the ADAOO is the result of interactions between PSEN1 and other key genes to modify ADAOO, and that this modification results in some members of this pedigree developing signs and symptoms of AD at an earlier or later age than other members (that is, these gene interactions with PSEN1 either accelerate or decelerate ADAOO).