Reduced APOE plasma levels are associated with smaller hippocampal size in AD patients, especially in APOE4 carriers (Teng et al., 2015), and it has been hypothesized that the use of APOE mimetics (Vitek et al., 2015) or increasing levels of APOE (Poirier et al., 2014) could be therapeutic for AD. Here, APOE is linked to Alzheimer disease.