TREM2 and Alzheimer disease: In particular, the discovery that a heterozygous missense mutation in the TREM2-encoding gene is a risk factor for development of AD, to a similar extent as that observed for apolipoprotein ε4, highlights the role TREM2, and hence microglia, play in AD neuropathology (Guerreiro et al., 2013; Jonsson et al., 2013; Slattery et al., 2014; Rosenthal et al., 2015).