The clone 3A6, a mouse monoclonal antibody, recognizes a ~100 kD transmembrane glycoprotein domain known as CD166 (the CD6 ligand) or the Activated Leukocyte Cell Adhesion Molecule (ALCAM). CD166 belongs to the immunoglobulin superfamily (IgSF) and it is widely expressed on activated lymphocytes and monocytes as well as other non-hematopoietic cells such as epithelial cells, fibroblasts and nerve cells. CD166 plays various roles in mediating physiological processes including angiogenesis, immune response and cell migration during the neuronal develop-ment. Overexpression of CD166 contributes positive indication of MM progression and that purified soluble CD166 effectively inhibits tumor progression. CD166 might be a potential therapeutic target against hematopoietic malignancy including MM.