Protein O-linked GlcNAcylation is a reversible post-translational modification characterized by the attachment of a N-acetyl-β-D-glucosamine (GlcNAc) to the hydroxyl group of serine or threonine residues via a β-glycosidic bond. The attachment and removal of O-linked GlcNAc from cytosolic and nuclear proteins is mediated by GlcNAc transferase sxc and GlcNAc glycosidase Oga, respectively, while in the endoplasmic reticulum, Eogt transfers GlcNAc to serine or threonine residues in epidermal growth factor-like (EGF-like) repeats of secreted and membrane proteins. In D. melanogaster, O-linked GlcNAc is not further elongated to form more complex glycan structures. O-GlcNAcylation is regulated by the level of glucose which is used to produce the donor sugar UDP-GlcNAc and thus plays a role in various biological processes including nutrient sensing. (Adapted from FBrf0235254 and FBrf0243014.)