Frataxin is a ubiquitous mitochondrial iron-binding protein involved in the biosynthesis of Fe/S clusters and heme. Its deficiency causes Friedreich's ataxia, a severe neurodegenerative disease. Mitochondrial ferritin is another major iron-binding protein, abundant in the testis and in sideroblasts from patients with sideroblastic anemia. We previously showed that its expression rescued the defects caused by frataxin deficiency in the yeast. To verify if this occurs also in mammals, we silenced frataxin in HeLa cells.