TREM2 and Alzheimer disease: Many of AD risk genes, such as triggering receptor expressed in myeloid cells 2 (TREM2), complement receptor 1 (CR1), cluster of differentiation 33 (CD33) and inositol polyphosphate-5-phosphatase (INPP5D), are preferentially expressed in microglia (Shi and Holtzman, 2018; Verheijen and Sleegers, 2018; Tejera and Heneka, 2019; Hashemiaghdam and Mroczek, 2020; Leng and Edison, 2021; Streit et al., 2021).