For the first time in the United States, scientists have edited the genes of human embryos, a controversial step toward someday helping babies avoid inherited diseases.

According to MIT Technology Review, the experiment was just an exercise in science — the embryos were not allowed to develop for more than a few days and were never intended to be implanted into a womb. The publication first reported the news on Wednesday.

Officials at Oregon Health & Science University confirmed that the work took place there and said results would be published in a journal soon. It is thought to be the first such work in the U.S.; previous experiments like this have been reported from China.

The Oregon scientists reportedly used a technique called CRISPR, which allows specific sections of DNA to be altered or replaced. It’s much more precise than some types of gene therapy that cannot ensure that desired changes will take place exactly where and as intended. With gene editing, the changes are permanent and would be passed down to any offspring.

The approach holds great potential to avoid many genetic diseases, but has raised fears of “designer babies” if done for less lofty reasons, such as producing desirable traits.

Earlier this year, the National Academy of Sciences and National Academy of Medicine said in a report that altering the genes of embryos might be OK if done under strict criteria and aimed at preventing serious disease.

And last year, Britain said some of its scientists could edit embryo genes to better understand human development.