Genome-wide association studies have identified risk variants for AD in several proteins expressed by microglia (such as TREM2, TREM1, APOE, PLCG2, ABI3, CR1, and CD33), further linking microglial function to AD and suggesting a role in disease pathogenesis (Sims et al., 2017; Hansen et al., 2018; Henstridge et al., 2019; McQuade and Blurton-Jones, 2019). Here, CD33 is linked to Alzheimer disease.