Ferritin is a protein whose principal role within cells is the storage of iron in a nontoxic, but bioavailable, form. The assembled ferritin molecule, often referred to as a nanocage, can store up to 4,500 atoms of iron (for review see ref. 1). In contrast to cytosolic ferritin, serum ferritin is relatively iron-poor and may contain only a few atoms of iron. Ferritin does not have an obvious leader sequence, and the mechanism of its release from cells is unclear. Serum ferritin is increased during inflammation, which suggests that it may play a role in modulating inflammatory effects (2, 3). In a recent issue of PNAS, Coffman et al. (4) provide compelling evidence that serum ferritin regulates vascular remodeling and angiogenesis, demonstrating a role for serum ferritin in cell proliferation.