Frigerio et al. (2019) showed that a microglial population termed ‘activated response microglia’ (ARMs) occur naturally in aging mice and in human brain, but the conversion to this state is accelerated in response to Aβ plaques (Frigerio et al., 2019). A number of AD risk genes including ApoE were found to be upregulated in ARMs, conversely depletion of ApoE blocked the recruitment of microglia to Aβ plaques (Frigerio et al., 2019). Given the association of the ApoE4 allele with AD, future studies should investigate whether this allele influences the production of ARMs (Lambert et al., 2013). This evidence concerns the gene APOE and Alzheimer disease.