APOE and Alzheimer disease: Interestingly, APOE is a key regulator of the microglial transcriptional signature, as demonstrated in post-mortem human brain studies, human cellular models as well as in AD mouse models and cultured microglia in vitro (Keren-Shaul et al., 2017; Krasemann et al., 2017; Pimenova et al., 2017; Lin et al., 2018; Olah et al., 2018).