CD166 is a type 1 transmembrane glycoprotein, which is anchored to the actin cytoskeleton through its intracellular domain, whereas its extracellular part binds its ligand CD6. CD166 functions as an adhesion molecule expressed during development of many organs, as well as in hematopoiesis (pluripotent stem cells), endothelial and epithelial linings, endometrium, blastocysts, and in the central and peripheral nervous system (on neurons), as well as on activated T cells and activated monocytes. Multiple isoforms of CD166 have been found.