Ferritin serves to store iron in a non-toxic form, to deposit it in a safe form, and to transport it to areas where it is required. The function and structure of the expressed ferritin protein varies in different cell types. This is controlled primarily by the amount and stability of messenger RNA (mRNA), but also by changes in how the mRNA is stored and how efficiently it is transcribed. One major trigger for the production of many ferritins is the mere presence of iron; an exception is the yolk ferritin of "Lymnaea sp.", which lacks an iron-responsive unit.