APOE and Alzheimer disease: Heterozygous missense mutations in the gene encoding the triggering receptor expressed on myeloid cells 2 (TREM2) have been recently described to significantly increase the risk of late onset Alzheimer's disease (AD) with an odds ratio similar to that of carrying an apolipoprotein E (APOE) ε4 allele (Guerreiro et al, 2013a; Jonsson et al, 2013).