TREM2 and Alzheimer disease: Triggering receptor expressed on myeloid cells 2 (TREM2), which is expressed in myeloid cells including microglia in the brain, is an AD risk gene and the R47H variant shows a robust odds ratio (Guerreiro et al., 2013; Jonsson et al., 2013) and is implicated in the development of late-onset AD (Ulland and Colonna, 2018).