The two women reportedly carry genes that cause a condition called MERRF syndrome -- a rare disorder affecting the muscles and nervous system that can result in recurrent seizures, movement issues and dementia. The genes that cause the syndrome are transferred to offspring through a mother's mitochondrial DNA, and that's where mitochondrial replacement therapy comes in. To remove the risk of MERRF, doctors use the father's sperm to fertilize an egg from both the mother and a donor who doesn't carry the genes. Then, the nucleus, which carries most of our genetic material, is removed from the donor egg and replaced with the nucleus from the mother's egg. This way, the mitochondria with the tainted genes are left behind in the mother's egg, which won't be used. All of the genes that determine what we look like are provided by the mother and father, while healthy mitochondria, which contain only 0.2 percent of our genes, are provided by the donor. The result is an embryo with three people's genetic material.

In 2016, a baby was born from a similar technique. It differed in that it swapped the mother and donor nuclei before the eggs were fertilized rather than after, because the parents objected to wasting an embryo due to religious regions. But it was successful. It was done in Mexico since the US, where the doctor who performed the treatment was based, doesn't allow the procedure. A similar technique called cytoplasmic transfer also results in three-parent children and it has been performed dozens of times around the world. Instead of swapping nuclei, mitochondria from a donor is injected into a mother's egg. It was pioneered in the 1990s by US embryologist Jacques Cohen, but the US Food and Drug Administration halted use of the procedure due to safety concerns. While it's unclear if it was a result of the treatment, a small number of fetuses and children born through cytoplasmic transfer developed genetic and developmental disorders.

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."