The normal, full-length form of CD33 inhibits microglial phagocytosis of Aβ, while the short form of CD33, lacking exon 2 encoding the sialic acid ligand-binding domain, does not inhibit phagocytosis of Aβ, and may thereby reduce AD risk (Bradshaw et al., 2013; Griciuc et al., 2013; Malik et al., 2013; Raj et al., 2014; Siddiqui et al., 2017; Estus et al., 2019). This evidence concerns the gene CD33 and Alzheimer disease.