A family of transmembrane glycoproteins that contain a short cytoplasmic domain, a single-span transmembrane domain, and an extracellular domain with heparin sulfate and chondroitin SULFATE chains. syndecans interact with a variety of heparin-binding intercellular signaling peptides and proteins and may play a role in modulating cellular signaling during embryonic development, tumorigenesis, and angiogenesis.