Fig. 7. Model for syndecan function in axon guidance signaling. Syndecan (SDN-1) on the neural growth cone exposes HS chains with `neuron-specific' sugar modifications, here simplified by 6Osulfations. The HS side chains of syndecan bind specific guidance cues (e.g. SLT-1) and modulate signaling by cognate high-affinity receptors, such as the SAX-3/Robo receptor (co-receptor function). Genetic analysis suggests that parallel signaling to syndecan relies on distinct sugar motifs on additional HSPGs in a context-dependent manner. In PVQ interneurons, this parallel pathway involves HS with epimerization (green), whereas in motoneuron development it involves both epimerization and 6O-sulfation. The HS modification patterns may thus define the specificity of different ligand/receptor interactions; A and B represent different ligands.