Ferritin, normally considered a cytoplasmic iron-storage protein, is also found in the nuclei of some cells. There is no current agreement about its function(s) in this environment. Proposals include DNA protection, provision of iron to nuclear enzymes, and regulation of transcription initiation, but evidence for these functions is scanty. We have shown previously that H-ferritin subunits can be cross-linked to chromosomal DNA in vivo ( The sensitivity of this conversion to glycerol suggests that DNA is nicked by a free radical mechanism. The rate of nicking correlates with the iron content of the ferritin and is strongly inhibited by chelators. Ferritin-dependent nicking is characterized by a kinetic lag that is not seen in control reactions containing free iron species. These results suggest that the release of iron from ferritin is an important part of the nicking mechanism. The potential role of ferritin as a protector of the genome is discussed in the context of these results.