APOE and Alzheimer disease: In experimental models, transcriptomics identified two main gene clusters associated with microglia, these being a homeostatic cluster given the term ‘M0,’ defined by P2ry12, Tmem119, Cx3Cr1, and an inflammatory cluster termed ‘MGnD’ defined by Ccl2, Csf1, and Apoe. The same study also reported that TREM2 induces the APOE pathway which then causes a switch between M0 and MGnD microglial phenotypes, and thus targeting this pathway could reinstate the homeostatic nature of microglia in AD (Krasemann et al., 2017).