For example, a recent single-cell RNA sequencing study on human post-mortem Alzheimer’s disease (AD) brains showed that activated microglia (relevant to the disease state) express high levels of APOE unlike naïve microglia (relevant to healthy/homeostatic state) in the prefrontal cortex, indicating that APOE expression is associated with immune function (Mathyset al., 2019). This evidence concerns the gene APOE and Alzheimer disease.