The ferritin complex is an oligomer of 24 subunits with light and heavy chains. The major chain can be light or heavy, depending on the tissue type. The functional molecule forms a roughly spherical shell with a diameter of 12 nm and contains a central cavity into which the insoluble mineral iron core is deposited. Iron metabolism provides a useful example of gene expression translational control. Increased iron levels stimulate the synthesis of the iron-binding protein, ferritin, without any corresponding increase in the amount of ferritin mRNA. The 5?-UTR of both ferritin heavy chain mRNA and light chain mRNA contain a single iron-response element (IRE), a specific cis-acting regulatory sequence which forms a hairpin structure.