Cell surface proteoglycan that bears heparan sulfate. Required for axonal and myotube guidance, is a necessary component of slit/robo signaling and is required in the slit target cells. In 13-16 hours embryos, expressed in lymph glands, peripheral and central nervous system and basal surfaces of gut epithelia. Sdc and robo are coexpressed in domains adjacent to slit; in tracheal pits and midline glia cells. Flies exhibit aberrant midline guidance of axons and establishment of muscle pattern. Belongs to the syndecan proteoglycan family.