CD138 is a transmembrane heparan sulfate proteoglycan that is mainly found in a variety of epithelial tissues and is involved in cell proliferation, cell migration, intercellular communication and cell-matrix interactions. In lymphoid tissues, CD138 is expressed in the late stages of B cell differentiation with progression to plasma cells. Anti-CD138 is a good marker for the selective identification of plasma cells, benign or malignant, in lymphoid tissues. CD138 is also expressed in a variety of epithelial cells.