Reactive oxygen species (ROS) and DNA double-strand breaks can result from mitochondrial defects and external sources, such as ionizing radiation. If not repaired properly, pathogenic mutations are generated. Human diseases resulting from inherited mitochondrial defects manifest in organs that physiologically require a high level of ATP synthesis. These diseases are clinically challenging, but new experimental clinical therapies include gene editing and mitochondrial transplants. Pathogenic ROS-associated cellular damage includes DNA double-strand breaks, and mouse models are now available to...