APOE and Alzheimer disease: Studies in mice have demonstrated that MG can be further divided into at least two subsets: i) homeostatic MG, and ii) neurodegenerative MG, which are induced by the triggering receptor expressed on myeloid cells 2 (TREM2) - apolipoprotein E (APOE) pathway; both of these MG subsets play a key role in detrimental neurodegenerative disorders such as Alzheimer’s disease (32, 33).