For the first time, scientists have edited DNA in human embryos, a highly controversial step long considered off limits.

Junjiu Huang and his colleagues at the Sun Yat-sen University in Guangzhou, China, performed a series of experiments involving 86 human embryos to see if they could make changes in a gene known as HBB, which causes the sometimes fatal blood disorder beta-thalassemia.

The report, in the journal Protein & Cell, was immediately condemned by other scientists and watchdog groups, who argue the research is unsafe, premature and raises disturbing ethical concerns.

"No researcher should have the moral warrant to flout the globally widespread policy agreement against modifying the human germline," Marcy Darnovsky of the Center for Genetics and Society, a watchdog group, wrote in an email to Shots. "This paper demonstrates the enormous safety risks that any such attempt would entail, and underlines the urgency of working to forestall other such efforts. The social dangers of creating genetically modified human beings cannot be overstated."

George Daley, a stem cell researcher at Harvard, agreed.

"Their data reinforces the...