A method includes providing a eukaryotic cell including mutant mtDNA in which there is at least one mutation in the mtDNA, allowing the cell to come into contact with an active oxygen species or a chemical species that generates such an active oxygen species in the cell (e.g., hydrogen peroxide) and thereby changing the percentage of mutant mtDNA (mitochondrial genomic DNA) in the cell as a result of the contact. Also featured are cells obtained by the above-described method.