The United Kingdom has announced regulations for a new fertilization method that could create babies using sperm and eggs from three people.
Doctors said the technique could eliminate debilitating and potentially fatal diseases passed from mother to child.
In the United States, the Food and Drug Administration is also considering the technique.
"We want to replace these mutated genes, which by nature have become pathogenic to humans," Dr. Shoukhrat Mitalipov, the leading researcher in the field, explained.
"We're reversing them back to normal, so I don't understand why you would be opposing that," he added.
But opponents in both the United Kingdom and the United States said it is unethical. They question whether the government should sanction the creation of genetically modified people, or so-called "designer babies."
"It does initially look like something you would want to support," Marcy Darnovsky, director of the Center for Genetics and Society, said. "But the safety concerns and the social and ethical concerns are really overriding."