Although information about the roles of mucin-type O-glycans in the mammalian brain remains limited, recent studies have suggested that mucin-type O-glycosylation by several ppGalNAcTs is associated with human brain disorders, including Alzheimer’s disease (Akasaka-Manya et al., 2017; Liu et al., 2017) and a congenital disorder of glycosylation (CDG; Zilmer et al., 2020). Here, MUC5AC is linked to Alzheimer disease.