Oligomer of 24 subunits. There are two types of subunits: L (light) chain and H (heavy) chain. The major chain can be light or heavy, depending on the species and 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 enters the spherical protein shell through pores that are formed between subunits. Mutations leading to truncation or the addition of extra residues at the C-terminus interfere with normal pore formation and with iron accumulation.