TREM2 and Alzheimer disease: AD is a predominantly sporadic disorder with several risk factors the best defined of which are aging as well as mutations and variations of a large number of susceptibility genes, among which the most studied and accepted are expression of the ApoE4 (apolipoprotein E4) isoform and mutations in TREM2 (triggering receptor expressed on myeloid cells-2), a protein expressed within the brain by microglia [1,2,3,4].