Microglial activation precedes neuronal loss in patients with AD, and recent genome-wide association studies have revealed that microglial genes such as CD33, triggering receptor expressed on myeloid cells 2 (TREM2) and human leukocyte antigen-DR isotype (HLA-DR) are associated with susceptibility to late-onset AD [443]. This evidence concerns the gene TREM2 and Alzheimer disease.