The Triggering Receptor Expressed on Myeloid Cell 2 (TREM2) regulates the change of microglia from a homeostatic state to a disease associated state (Keren‐Shaul et al, 2017; Krasemann et al, 2017) and is a well‐established in vivo proxy for microglial activation in AD (Suarez‐Calvet et al, 2016, 2019; Ewers et al, 2019, 2020; Franzmeier et al, 2020). Here, TREM2 is linked to Alzheimer disease.