TREM2 and Alzheimer disease: TREM2 is expressed highly and exclusively in microglia in the brain.162 GWAS showed that the R47H variant of TREM2 was associated with a 2- to 4-fold increased risk for the development of AD.163,164 Several other TREM2 variants that affect the expression of TREM2 also increased the risk of AD, including R62H, T66M, H157Y, and D87N.165–168 As a result of the genetic association of TREM2 variants with AD, how TREM2 impacts the microglial response to AD pathology has been studied.