TREM2 and Alzheimer disease: Heterozygous TREM2 variants confer similar risk for AD as one copy of APOE4. Significantly, the AD-associated TREM2 variants are largely coding variants, in contrast to most of the single nucleotide polymorphisms (SNPs) identified in GWAS [7], making it more straightforward to translate into in vitro and in vivo models and perhaps also into therapeutics [8].