TREM2 and Alzheimer disease: AD genetic association studies interrogating whole genome, whole exome, and targeted datasets generated from familial and unrelated cohorts have identified several rare variants in protein-coding genes, such as ABCA7, ABI3, PLCG2, SORL1 and TREM2, all of which are specifically or highly expressed in myeloid cells as compared to other brain cell types and play critical roles in the innate immune response, cholesterol metabolism, and endocytosis/phagocytosis [23–27].