CD71, also known as transferrin receptor protein 1 (TfR1), is a transmembrane glycoprotein which mediates cellular uptake of iron from transferrin, a plasma protein which transports iron in the circulation. CD71 is a homodimer of two identical transmembrane subunits linked by two disulfide bonds. CD71 is almost ubiquitously expressed, with highest expression levels on some cells and tissues, including immature erythroid cells, placental tissue, and rapidly dividing cells. Soluble CD71 present in the circulation is the truncated form of the cell surface receptor. The concentration of serum CD71 has been suggested to provide a sensitive measure of iron depletion.