Although essential, iron is also potentially toxic to cells. Therefore it is important that cells maintain optimal iron levels, which is controlled by three proteins: transferrin receptor, ferritin and iron binding proteins. Transferrin receptors bind iron and transport it into cells; ferritin is an intracellular protein that provides a storage site for iron. The regulation of both transferrin and ferritin are under the control of Iron Binding Protein (IBP) which binds to iron response elements (IREs) at the 5â end of ferritin mRNA and the 3â end of transferrin receptor mRNA. In thinking about the roles of these proteins in maintaining intracellular iron levels, propose a mechanism for how IBP might be linked to the stability/translation of both the transferrin and ferritin mRNA under both low and high intracellular levels of iron.