Mitochondria replacement is promising, because it opens the door to stripping away mitochondrial disorders, which affect around one in 6,500 people, and include heart conditions and muscular dystrophy. The mitochondria are the sources of chemical power in the cells, but if not functioning correctly can leave developing babies short-changed for energy, and as a result health issues.

Where the replacement therapy steps in is in switching out the faulty mitochondria for fully-functioning alternatives. The donor egg’s mitochondria has its own genetic material, and so while the bulk of the baby’s DNA is that of its parents, from the transplanted nucleus, the all-important “cellular power packs” lack those flaws found in the parents’ versions.