APOE and Cognitive impairment: A systematic review (Ali et al., 2018) (n = 36 articles) showed that the frequency of APOE ε4 was significantly lower in healthy controls than in groups with objective cognitive impairment (OCI), including MCI and AD dementia, but showed no difference in the SCD group, suggesting that APOE ε4 may not be directly related to the development of SCD.