Cell surface adhesion protein CD44 is a ubiquitously expressed type I transmembrane protein that has important functions related to cell-cell adhesion and extracellular matrix interactions. The transmembrane protein is post-translationally modified at multiple sites by glycosylation and phosphorylation. CD44 ligands include hyaluronic acid, collagens, laminins, osteopontin, serglycin, and fibronectin. CD44 has been implicated in inflammatory cell functions, as well as in tumor growth and metastasis. CD44 is overexpressed in many types of cancer; the interaction between CD44 and HER2 has been linked to an increase in ovarian carcinoma cell growth. CD44 interacts with ezrin, radixin, and moesin to link the actin cytoskeleton to the plasma membrane and the extracellular matrix. These interactions are critical for CD44 function in cell-cell adhesion and cell motility.