Iron-dependent coordinated control of ferritin and transferrin receptor expression at the translational level. IRP changes its binding affinity for the IRE depending on intracellular iron availability. IREs are a family of stem-loop sequences capable of modulating either mRNA translation or stabilization/degradation, according to their location on 5′- or 3′-UTR in the mRNAs. A single IRE is also located in the 5′-UTR of erythroid δ-aminolevulinate synthase (not shown). As for ferritin, IRP blocks mRNA translation when intracellular iron is low, to avoid iron utilization for heme synthesis. See the introduction and reference 4 for a review.