This isn't about editing genes, it is about replacing mitochondria. Not the same thing at all. Mitochondria are like bacteria in the way they replicate. There a

This isn't about editing genes, it is about replacing mitochondria. Not the same thing at all. Mitochondria are like bacteria in the way they replicate. There are hundreds of them in a cell. They can pass components around between one another, can split and fuse. They are integrated with quality control mechanisms that cull the herd. It is a complicated picture. As the article notes the procedure isn't effectively clearing out all of the old mitochondria, and dynamics can favor them in the future. It is known that variations in mitochondrial DNA produce radical differences in competition between mitochondrial strains in the cell. That is how deletions affecting OXPHOS machinery cause one mutant to take over the whole population very quickly - some differences produce mitochondria that either replicate better or resist quality control more effectively. That one is one of the causes of aging, but the same principle exists for other differences between mitochondrial genomes. If you put two or more in a cell and let them fight it out, hard to say in advance what the outcome will be given present knowledge. So, basically, the people working on mitochondrial replacement need to make their tools for cleaning out the old mitochondria more efficient. If 100% success is achieved, that genome isn't coming back. Alternatively, actual gene therapy might be a better approach - though challenging if you want to edit mitochondrial genomes, as you have the same problem of coverage and competition. There is allotopic expression, moving mitochondrial genes to the cell nucleus, which is feasible via today's gene therapy. Given the amount of work needed to copy mitochondrial genes into the nucleus, however, the challenge being how to alter them so as to get the proteins produced back to the mitochondria, something that has been achieved for three genes so far, it might be more cost-effective to work on better clearance and replacement technologies for the near term of assisted reproduction needs.