APOE and Alzheimer disease: APOEε4 variant has been identified as the most common genetic risk factor for AD and proven to be a ligand for TREM2, suggesting that the interplay between APOE-TREM2 might play a role in the regulation of cellular functions (Jendresen et al. 2017; Shi & Holtzman 2018; Wolfe et al. 2018).