Protein O-linked glycosylation is a post-translational modification involving the attachment of a monosaccharide to the hydroxyl group of serine, threonine or hydroxylysine residues within specific consensus sequences via a glycosidic bond. This monosaccharide can be further extended by the sequential addition of other sugars resulting in the formation of various types of branched and linear O-glycans with variable length and sugar composition. Based on the monosaccharide that is attached first to the protein, O-glycans can be classified into O-linked N-acetylgalactosamine (GalNAc), O-linked-mannose, O-linked-fucose, O-linked-glucose, O-linked-xylose, O-linked-galactose and O-linked N-acetylglucosamine (GlcNAc) glycans. These modifications are mediated by glycosyltransferases and occur primarily in the endoplasmic reticulum or the Golgi apparatus targeting selected secreted and membrane proteins; however, protein O-GlcNAcylation occurs mainly in the cytosol and the nucleus. The number and type of O-glycans varies according to the target protein with some of the sugars also undergoing further modification (e.g. sulfation). These post-translational modifications are essential for protein folding, stability, trafficking, activity, protein-protein interaction and for several processes including cell adhesion and signalling. In D. melanogaster, except for O-linked glycosaminoglycans, O-glycan chains are shorter, often composed of one or two saccharides, and use glucuronic acid (GlcA) in place of sialic acid for capping oligosaccharide chains. (Adapted from FBrf0243014.)