The MI15 antibody reacts with an extracellular epitope on CD138 (Syndecan-1), an ~85 – 95 kDa type 1 transmembrane glycoprotein expressed on the surface of pre-B cells, immature B cells, and normal and malignant plasma cells (but not mature circulating B cells), as well as on non-hematopoietic cells such as embryonic mesenchymal cells, endothelial, epithelial and neural cells. CD138 expression is used as a diagnostic marker for several types of tumors. CD138 is thought to act primarily as a receptor that modulates cell proliferation, cell migration and cell-matrix associations by linking the extracellular matrix to the cytoskeleton. Heparin sulfate and chondroitin sulfate moieties attached to CD138 associate with several proteins, including collagens, fibronectin, tenascin, thrombospondin and certain cytokines. The MI15 antibody recognizes a different epitope than that of the clone DL-101 anti-CD138 antibody but blocks binding of clone B-B4.