Transport along the secretory pathway is largely signal-mediated. Proteins in the secretory pathway can be covalently modified with various carbohydrate structures, most commonly O-glycans, N-glycans and/or proteoglycans. Carbohydrate modifications can change the physical properties of proteins or can function as specific recognition epitopes. Glycosylation can act as an apical sorting signal in polarized epithelial cells and provide a signal for surface transport in non-polarized fibroblasts. Homologues of leguminous plant lectins have been identified in yeast, fruitflies, worms and humans. Intracellular lectins are candidate receptors in the secretory pathway to mediate concentration of cargo in carrier vesicles.