TREM2 and Alzheimer disease: Early onset AD which typically begins around 40 years of age (Seltzer and Sherwin, 1983) has been associated with mutations in APP, PSEN1 and PSEN2 (Bekris et al., 2010), and also with polymorphisms in several microglial genes including CD33 (Griciuc et al., 2013; Malik et al., 2013), ABI3 (Sims et al., 2017), PLCG2 (Sims et al., 2017), and TREM2 (Guerreiro et al., 2013; Jonsson et al., 2013; Suárez-Calvet et al., 2016; Sims et al., 2017).