The function of iron response elements (IREs). In mammalian cells, IREs are bound by iron response proteins (IRPs) when iron levels are low, blocking the production of proteins that use and store iron (including ferritin), while promoting the production of proteins that facilitate iron uptake (the transferrin receptor). (a) IRP binding to the 5′UTR of ferritin mRNA inhibits translation (top). When iron is in excess (bottom), ferritin is produced. (b) IRP binding to the 3′UTR of the transferrin receptor mRNA stimulates translation (top); no transferrin receptor is made when iron is in excess (bottom).