The DL-101 monoclonal antibody specifically reacts with human CD138, a 85-92 kDA type I heparan sulfate proteoglycan. CD138 is known as Syndecan-1 and is expressed on pre-B cells, immature B cells, plasma cells, and malignant plasma cells, to the exclusion of mature circulating B cells. Syndecan-1 is also expressed on vascular smooth muscle, endothelial, neural, and embryonic mesenchymal cells. It is involved in cellular proliferation, cellular migration, and acts as an extracellular matrix receptor. Among the hematopoietic elements, CD138 is a useful marker for plasma cells.