CD71,also known as transferrin receptor protein 1 (TfR1),is a transmembrane glycoprotein composed of two disulfide-linked monomers,each of 90 kDa molecular weight. Each monomer binds one holo-transferrin molecule creating an iron-Tf-TfR complex which enters the cell by endocytosis. CD71 is present on actively proliferating cells and is essential for iron transport into proliferating cells.