CD138 / Syndecan-1 is a transmembrane heparin sulphate proteoglycan which is made up of one core protein and five glycosaminoglycan. The function of CD138 is largely unknown but is expected to play a role in cell adhesion. It is expressed on the surface of pre-B cells and plasma cells but is absent from mature B cells. It is a selective marker for B cell lymphoblastic leukemia and lymphoplasmocytoid leukemia. It is lost from the apoptotic myeloma cells; hence is a useful marker for viable myeloma cells. In addition, CD138 is expressed by simple and stratified epithelia, fibroblasts, keratinocytes and endothelial cells.