Deletion of YDL120, the yeast homologue of the human gene responsible for Friedreich's ataxia, elicits decreased cellular respiration associated with decreased cytochrome c oxidase activity and, in certain nuclear backgrounds, mitochondrial DNA is lost. In the null mutants, the cellular growth is highly sensitive to oxidants, such as H2O2, iron and copper. However, only ferrous sulfate elicits loss of mitochondrial DNA. Mitochondria of the null mutants contain 10 times more iron than wild-type. The neurodegeneration observed in Friedreich's ataxia can be well explained on the basis of a mitochondrial iron overload responsible for an increased production of highly toxic free radicals.