The CD44 antigens are transmembrane glycoproteins and members of the hyaladherin family of hyaluronan-binding proteins. Multiple CD44 isoforms have been described, the predominant form being CD44S, a glycoprotein of 85 kDa. CD44 is present on most cells or tissues, but not on platelets, hepatocytes, cardiac muscle, kidney tubular epithelium, testis and skin portions. The human blood group antigens Ina/b reside on CD44.