TREM2 and Alzheimer disease: TREM2, CD33, and CR1 are expressed in microglia, the innate immune cells of the central nervous system (CNS) and have been found to be associated with a higher risk of AD (Polvikoski et al., 1995; Bradshaw et al., 2013; Crehan et al., 2013; Griciuc et al., 2013; Guerreiro et al., 2013; Farfel et al., 2016).