CD33 and Alzheimer disease: Many genetic risk factors associated with late-onset AD are highly or specifically expressed in microglia.64 Many AD risk genes (i.e., APOE, CLU, ABCA7, GRN, TREM2, and CD33) have been linked to microglia phagocytosis.65 Phagocytosis of toxic aggregates by microglia decreases the progression of neurodegenerative disorders, including AD,64 while defective phagocytosis could contribute to the risk of these diseases.66