LONDON – A pioneering IVF technique combining DNA from three people to protect a baby from a rare genetic disease has been used in Britain, leading to a healthy cohort of eight babies with no sign of serious disease, scientists said Wednesday.

Four girls and four boys, including one set of twins, were born healthy after scientists used the treatment to prevent mothers with mutations in their mitochondrial DNA from transmitting the condition to their children, scientists at Newcastle University in northern England said in a statement Wednesday.

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The authors say it represents the first study into an entire cohort of babies and paves the way for further research on their health outcomes, as well as improved medical techniques for that specific treatment – which is granted approval in the United Kingdom on a case-by-case basis.

Mitochondria are commonly known as the “powerhouse of the cell” and produce energy required for major parts of the body to function. However, small mutations in mitochondrial DNA can affect tissues with high-energy demands such as the heart, muscle and brain, causing devastating disease and, in some cases, death.

Mitochondrial DNA is inherited from the mother, and although males can be affected, they do not transmit the disease, researchers said. About 1 in 5,000 babies are born worldwide each year with mutations that can cause the disease, researchers said.

Now, scientists have detailed how an IVF technique called pronuclear transfer has been used to combine the DNA of three people to reduce the risk of mitochondrial disease being passed down the generations, in accompanying studies published in the New England Journal of Medicine on Wednesday.

The technique uses 99.9 percent of the DNA from a man and woman, with another 0.1 percent from a second woman’s donor egg. It works by transplanting the nuclear genome of an egg from the mother with the condition – which contains genes essential for individual characteristics like hair color and height – to an egg donated by an unaffected woman that has had its nuclear genome removed, researchers said.

The resulting embryo inherits nuclear DNA from its two parents, but the mitochondrial DNA comes from the donated egg, researchers said. The treatment was offered to certain women at very high risk of passing on serious mitochondrial disease, in accordance with U.K. regulations that assess each application for the procedure on a case-by-case basis.

The eight infants – who range in age from newborn to over 2 years old – were assessed to be healthy, meeting developmental milestones and reported levels of mitochondrial disease-causing mutations that were undetectable or at levels unlikely to cause disease, the Newcastle University statement said. Three of the babies had levels of disease-causing mitochondrial DNA mutations of up to 20 percent, which is still below the 80 percent threshold for clinical disease, it added.

Doug Turnbull, a neurologist at Newcastle University who co-wrote the study, said it was the first to document a “cohort” of children who had received the treatment. He said it is the result of an extremely cautious approach by scientists and regulators that has been more than two decades in the making.

“People have used very similar techniques, but nobody’s quite used this particular technique,” he told The Washington Post in a phone interview Thursday. “It’s just absolutely critical when you’re doing a new technique to be cautious and to make sure … it’s as safe and efficient as possible.”

The procedure has also raised concern from some, including religious groups, about its ethics and the fear that it could open the door to further genetic modification.

Peter Thompson, chief executive of the Human Fertilization and Embryology Authority, which regulates the process in Britain, said that only people with a “very high risk” of passing on a serious mitochondrial disease are eligible for the treatment and that every application is assessed individually. As of July 1, 35 patients have been granted approval by U.K. authorities to proceed with the treatment since it was first licensed in 2017.

“These robust but flexible regulatory processes allow the technique to be used safely for the purposes that Parliament agreed in 2015,” Thompson said in a statement in response to Wednesday’s news.

Scientists have cautiously welcomed the findings, while stressing the importance of long-term monitoring and raising the prospect of whether the procedure offers advantages over embryo screening for genetic disease. Others have raised the issue of cost in the long-term project that is supported by Britain’s National Health Service and medical charity the Wellcome Trust, among other groups.

Mary Herbert, who is professor of reproductive biology at Newcastle University and lead author of the research paper, said “the findings give grounds for optimism” but further research is needed to “bridge the gap” between reducing risk of mitochondrial disease and preventing it.

Turnbull said the team is also looking to improve medical techniques and follow up with the children involved for as long as possible to track their health outcomes. He said researchers are offering health assessments for five years, but it “would be lovely to be able to follow them up much longer.”

Joanna Poulton, a professor in mitochondrial genetics at the University of Oxford, who was not involved in the research, said “time will tell” whether the treatment results in “dramatic clinical advance.”

The births come amid a wider boom in genomic sequencing and IVF start-ups that have sparked a wider debate about the ethics and science behind embryo screening and genetic preselection.

In the United States, those undertaking IVF typically test for rare genetic disorders stemming from a single gene mutation, such as cystic fibrosis, or chromosomal abnormalities such as Down syndrome. The use of donor mitochondria, however, is not permitted under U.S. regulations. In Britain, the creation of babies using DNA from three people was first made legal in 2015, hailed for its ability to prevent serious disease being passed on.

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