Reacts with CD138 (Syndecan-1), an 85-92 kDa single chain transmembrane protein, expressed on multiple myeloma-derived cell lines and malignant plasma cell populations. It is also expressed on pre-B cells, immature B cells and plasma cells, but not on mature circulating B-lymphocytes. Syndecan-1 is a member of the family of transmembrane heparan sulfate proteoglycans. It is also expressed on some non-hematopoietic cells, including embryonic mesenchymal cells, vascular smooth muscle cells, endothelial and neural cells. CD138 binds to many extracellular matrix proteins through its heparan sulfate side-chains, like fibronectin, collagen types I, III and V, tenascin, thrombospondin, and antithrombin III. It is considered an extracellular matrix receptor that may serve as a co-receptor for fibroblast growth factor and related molecules. Monoclonal antibody DL-101 does not block the binding of clone B-B4 (another anti-syndecan-1 antibody) or Mi15 by flow cytometric analysis.