As described by Glatt et al. in their review of 29 different studies evaluating at least one other characteristic of healthy aging in addition to longevity [26], APOE, the known risk factor for Alzheimer's disease, has been implicated in successful aging in several studies, however this association has not been consistently identified, and further study is therefore needed. This evidence concerns the gene APOE and early-onset autosomal dominant Alzheimer disease.