Transferrin receptors are membrane glycoproteins that serve as the gateway for circulating transferrin iron to the interior of all body cells. In addition to erythroid precursors that contain 80% of the total body receptor mass, rapidly dividing cells and the placenta contain a high density of receptors. The synthesis of ferritin and transferrin receptors is precisely regulated by a c0mmon mechanism involving the iron response protein and a nucleotide sequence termed the iron response element.