In a technique called spindle nuclear transfer, the nucleus of a donor egg is removed and the DNA of another woman's egg is injected.

The New York-based doctor who helped a couple have a child using DNA from three people has been told by the U.S. Food and Drug Administration that he can’t go ahead with clinical trials meant to test the technique.

Last year, John Zhang, the founder of New Hope Fertility Center, pioneered a new type of in-vitro fertilization that involves transferring DNA from the mother’s egg into a hollowed-out egg donated by a younger woman. But the work violates federal legislation that forbids implanting genetically modified embryos, so after fertilizing the egg with the father’s sperm, Zhang went to Mexico, where he inserted the embryo into the mother’s womb. A healthy baby boy was born in April 2016.

On Friday, the FDA sent Zhang a strongly worded letter saying he must stop marketing the technique in the U.S. The move is a sign that the federal government is taking a hard line against the development of technologies to make genetically altered babies, even in cases where doing so would prevent severe disease.

According to the FDA, Zhang then requested a meeting with the agency to ask permission to carry out a clinical trial using the technique in the U.S. The agency subsequently denied the meeting. Zhang has since been marketing his fertility procedure to women with certain genetic diseases and older women having trouble conceiving through a new company called Darwin Life. The creation of the company was first reported by MIT Technology Review.

Modifying embryos in a lab is not illegal under U.S. law as long as federal funds are not used to carry out the work. But implanting one in a woman’s womb so that a baby can develop is prohibited. Since December 2015, Congress has barred the FDA from accepting research applications that involve a human embryo that has been "intentionally created or modified to include a heritable genetic modification." The FDA letter also says Zhang did not have permission to export the embryo. New Hope Fertility Center did not respond to a request for comment.

Zhang’s technology, called "spindle nuclear transfer," was initially performed on the eggs of a Jordanian woman to prevent her from passing on a serious neurological disease to her children. The woman and her husband previously had two children who died from the condition, called Leigh syndrome, which is caused by faulty mitochondria.

As a result of the procedure, the woman gave birth to a child that technically has three genetic parents, since healthy mitochondria in the hollowed-out donor egg has its own DNA. The baby wasn’t the first to be born with three genetic parents—more than a dozen babies were born with an older three-person IVF technique in the 1990s before the FDA intervened and decided to regulate the procedure like a new drug. The treatment was never approved.

Some evidence suggests that malfunctioning mitochondria are one reason why older women can’t easily produce viable eggs, which is why Zhang thinks using a young egg will help. The idea is controversial, though, since it’s unknown whether children born using this method will have long-term side effects. Zhang told MIT Technology Review in June that he was in the process of assessing a handful of women over 40 who may be able to benefit, but that he would have to go outside the U.S. to do the procedure.

A similar treatment was approved in the U.K. late last year, but only when a couple is at very high risk of having a child with a life-threatening genetic disease. Clinics wishing to perform the procedure must first obtain a license from the U.K. government.

Leigh Turner, a bioethicist at the University of Minnesota who first called attention to the FDA letter to Zhang, says prohibitions on this kind of research encourage rogue scientists to "look for settings with lower regulatory standards."

"The U.S. ban on genetically modifying embryos merits reassessment," he says. "There will always be countries with lax laws, or an inability to enforce regulations, and some entrepreneurs will flock to such jurisdictions and try to take advantage of them."

Naomi Cahn, a professor at the George Washington University Law School who specializes in reproductive technology, says she isn’t surprised by the FDA’s reaction to Zhang’s company, Darwin Life. But she worries that by restricting the FDA from even considering applications for clinical trials that involve altering embryos, the U.S. will lose its edge in science and medical research.

"Other countries are ahead of us on this," she says. "We need to be more sensitive to what’s going on internationally with this technology."