The W3D5 antibody reacts with SUSD2 (sushi domain containing 2) which is expressed by bone marrow mesenchymal stem cells, vessel-surrounding smooth muscle cells, and a few neoplastic cell lines. SUSD2 is a recently identified type I transmembrane protein of 820 amino acids consisting of a large extracellular region containing a Somatomedin B (SMB), an Adhesion-associated domain in MUC4 and Other Proteins (AMOP), a von Willebrand factor (vWF), and a F1c Sushi domain. Mesenchymal stem cells (MSCs) were originally named bone marrow stromal cells. They are a population of adult stem cells with a large capacity for self-renewal and multipotency for differentiation into a variety of cell types including osteoblasts, chondrocytes, myocytes, adipocytes, beta-pancreatic islets cells, and certain neuronal cells. MSCs reside in many cells and tissues, such as bone marrow, placenta, adipose tissue, adult peripheral blood, fetal blood, skin, liver, and lung. Besides their plasticity for tissue repair, MSCs also exhibit powerful immunosuppressive activity and play an important role in supporting hematopoiesis.