U.S. regulators are telling a fertility doctor in New York to stop marketing an experimental procedure where a baby is created with DNA from three people rather than just two. Authorities worry that such a procedure, which involves a mother, father, and egg donor, could result in certain genetic diseases.

The doctor in question, John Zhang, helped a Jordanian couple conceive a baby last year. The FDA says that Zhang promised not to use the technology in the U.S. again without permission, but his companies continue to promote the procedure, which is not approved in the U.S. and has been barred by Congress.

This procedue was aimed at allowing the mother to have a child but not risk giving it Leigh syndrome, a severe neurological disorder that she risks passing down due to her DNA. The technique would involve removing some of the mother’s DNA from an egg, leaving the tainted DNA behind, and putting it into a healthy donor egg, which is fertilized. As a result, the baby has DNA from all three people.

“For some years, reproductive specialists have been able to deselect genetically affected embryos with mitochondrial disease, using sophisticated diagnostic procedures in the IVF laboratory,” Professor Bart Fauser, Editor-in-Chief of RBMO, said in a past statement. “Now, for the first time, an egg with abnormal mitochondria can be changed to contain mostly normal mitochondria from a healthy egg donor. This is a major change of technology and an obvious advantage for women who are at risk of passing such diseases on to the next generation.”