CD138, also known as Syndecan-1 (SDC1), is a member of the transmembrane heparan sulfate proteoglycan family. CD138 acts as an extracellular matrix receptor and participates in many cellular functions, including cell-cell adhesion and cell-matrix adhesion. CD138 is expressed on plasma cells and various epithelial cell types. Altered CD138 expression has been described in many different tumor types. CD138 can be shed form the cell surface and exist as a soluble form, which has paracrine and autocrine functions and functions as competitor of transmembrane CD138 for growth factors and other extracellular ligands.