TREM2 and Alzheimer disease: Many AD risk genes are reported to be primarily or exclusively expressed in microglia (e.g., Trem2, Cd33, Cr1, C1q) [74–76] or astrocytes(e.g., Apoe, Clu, Fermt2, Iqck, Agfg2, Scara3) [6, 77–79] which are the two most important resident immune cells in CNS.