CD166, also known as the CD6 ligand or the Activated Leukocyte Cell Adhesion Molecule (ALCAM), is a 100-105 kD transmembrane glycoprotein. It belongs to the Ig superfamily of proteins and expressed on activated T cells, activated monocytes, epithelial cells, fibroblasts, and neurons. CD166 plays an important role in mediating adhesion interactions between thymic epithelial cells and CD6+ cells during intrathymic T cell development. Recently CD166 has also been used as a potential cancer stem cell marker. The antibody reacts with human activated leukocyte cell adhesion molecule (ALCAM).