TREM2 and Alzheimer disease: Genetic evidence directly implicates microglia function in AD pathogenesis as genome-wide association studies have uncovered rare variants of Triggering Receptor Expressed on Myeloid Cells 2 (TREM2), which was originally cloned in neutrophils and monocytes6 and whose expression in the central nervous system is restricted to microglia7, that increase the risk of developing AD8.