Protein O-linked mannosylation is a post-translational modification that occurs in the endoplasmic reticulum and is characterized by the attachment of a mannose to the hydroxyl group of serine or threonine residues of selected proteins via an α-glycosidic bond. In D. melanogaster, O-linked mannose is not further elongated to form more complex glycan structures. Mannosyltransferases rt and tw act together as a complex to O-mannosylate proteins such as dystroglycan Dg, while Tmtc1, Tmtc2, Tmtc3 and Tmtc4 are thought to specifically O-mannosylate cadherins. (Adapted from FBrf0258752.)