TREM2 and Alzheimer disease: Several of the causal and risk factor genes for AD—amyloid precursor protein (APP), presenilin-1, presenilin-2, ApoE, clusterin (CLU), phosphatidylinositol-binding clathrin assembly protein (PICALM), triggering receptor expressed on myeloid cells 2 (TREM2)—are expressed not only by neurons but also, if not predominantly, by astrocytes [102], corroborating the idea that astrocytes are important players in AD pathogenesis.