Syndecan-1 (SYND1 or SDC1) is also known as CD antigen CD138, is a transmembrane (type I) heparan sulfate proteoglycan and is a member of the syndecan proteoglycan family. The syndecans mediate cell binding, cell signaling, and cytoskeletal organization and syndecan receptors are required for internalization of the HIV-1 tat protein. The syndecan-1 / SDC1 protein functions as an integral membrane protein and participates in cell proliferation, cell migration and cell-matrix interactions via its receptor for extracellular matrix proteins. It is a useful marker for plasma cells, but only if the cells tested are already known to be derived from blood.