CD44 is a broadly distributed transmembrane glycoprotein that plays a critical role in a variety of cellular behaviors, including adhesion, migration, invasion, and survival. CD44 mediates cell–cell and cell– matrix interactions in a large part through its affinity for hyaluronan (HA), a glycosaminoglycan constituent of extracellular matrices, but also potentially through its affinity for other ligands such as osteopontin, collagens, and matrix metalloproteinases (MMPs).