APOE, a lipoprotein of which the ε4 allele (APOE‐ε4) is a major risk factor for AD and also implicated in other neurodegenerative diseases (Yamazaki et al., 2019]), is upregulated in disease‐associated microglia (DAM) in mice and humans, but downregulated in astrocyte and oligodendrocyte subpopulations (Grubman et al., 2019; Hammond et al., 2019; Keren‐Shaul et al., 2017; Mathys et al., 2019). Here, APOE is linked to Alzheimer disease.