It is well documented that reactive microglia is associated with AD and several genes involved in the immune response have been identified as risk genes, among them TREM2, CR1, SHIP1, BIN1, CD33, PICALM, CLU, PCLG2, Clec7A, and MS4A (Gale et al., 2014; Karch and Goate, 2015). Here, TREM2 is linked to Alzheimer disease.