Protein O-linked fucosylation is a post-translational modification that begins in the endoplasmic reticulum with the attachment of a fucose via an α-glycosidic bond to the hydroxyl group of serine or threonine contained in epidermal growth factor-like (EGF-like) repeats and thrombospondin type 1 repeats (TSRs) of membrane and secreted proteins by Ofut1 and Ofut2, respectively. The O-linked fucose in EGF-like repeats can be further elongated in the Golgi apparatus with the addition of a N-acetyl-β-D-glucosamine (GlcNAc) by fng, which can be followed by the transfer of a glucuronic acid (GlcA) to either the GlcNAc or the fucose by an unknown glucuronyltransferase. O-linked fucosylation of EGF-like repeats is important for protein-protein interaction and stabilization of the repeat fold, while O-linked fucosylation of TSR repeats is important for protein secretion and stabilization of the repeat fold. (Adapted from PMID:30690220 and FBrf0244700.)