Mitochondrial diseases aren't curable and many are very hard to treat, so treatments like mitochondrial replacement therapy stand to help a lot of people and save lives. "It is absolutely fantastic that we have got to this point in such a well-regulated and controlled way," Sian Harding, director of the British Heart Foundation Imperial Cardiac Regenerative Medicine Centre in London, told the Guardian. "It is going to be so important now to follow up and understand whether this is successful and how we can take it forward. If you don't follow up the children, we just won't know whether this is the right thing to do."