For now, the work is confined to a laboratory. But the research, if successful, would mark another step toward turning CRISPR, a powerful form of gene editing, into a tool for medical treatment.

A Chinese scientist sparked international outrage in November when he announced that he had used the same technique to create the world's first gene-edited human babies. He said his goal was to protect them from infection with HIV, a claim that was criticized because there are safe, effective and far less controversial ways of achieving that goal.

In contrast, Dieter Egli, a developmental biologist at Columbia University, says he is conducting his experiments "for research purposes." He wants to determine whether CRISPR can safely repair mutations in human embryos to prevent genetic diseases from being passed down for generations.

So far, Egli has stopped any modified embryos from developing beyond one day so he can study them.