TREM2 and Alzheimer disease: The remaining cases of early-onset AD (without mutations in APP, PSEN1, and PSEN2) are caused by mutations in other genes, such as GRN (progranulin), MAPT (microtubule-associated protein tau), TREM2 (triggering receptor expressed on myeloid cells-2), NOTCH3 (neurogenic locus NOTCH3 homologous protein 3), and CLU (clusterin), or the causative mutation remains unidentified.