CD44 antigen (CD44) is a receptor for hyaluronic acid (HA). CD44 mediates cell-cell and cell-matrix interactions through its affinity for HA, and possibly also through its affinity for other ligands such as osteopontin, collagens, and matrix metalloproteinases (MMPs). The adhesion with HA plays an important role in cell migration, tumor growth and progression. CD44 is also involved in lymphocyte activation, recirculation and homing, and in hematopoiesis. CD44 is responsible for the Indian blood group system. The molecular basis of the In(A)=In1/In(B)=In2 blood group antigens is a single variation in position 46; In(B), the most frequent allele, has Arg-46.