Syndecan-4 is a transmembrane (type I) heparan sulfate proteoglycan containing 198aa and belonging to the syndecan proteoglycan family. Syndecan-4 functions as a receptor or coreceptor in intracellular signaling. Initially, Syndecan-4 was isolated as ryudocan, from rat endothelial cells, and from human epithelial and fibroblastic cells, as amphiglycan. The syndecan heparan sulfate chains of Syndecan-4 binds to HIV-1 gp120 and thus functions as trans HIV receptors. This cell surface proteoglycan usually exists as a homodimer that interacts with CDCP1, with GIPC PDZ domain and with NUDT16L1/SDOS. Reports suggest that Syndecan-4 is often upregulated in skin dermis after wounding, thus playing an important role in wound healing and angiogenesis. It is also needed in satellite cell function and in the development and regeneration of skeletal muscle.