In early AD, TREM2 predominantly mediates the conversion of microglia to a synaptoprotective state and mediates the scavenging of Aβ by microglia, however, in late AD, TREM2 predominantly mediates the overactivation of microglia and the scavenging of synapses (Krasemann et al., 2017). Here, TREM2 is linked to Alzheimer disease.