Speculation swirled after reports came out last week that a group of U.S. scientists had CRISPR’d human embryos for the first time. The results of that initial study have now been released and the findings are quite astonishing.

Shoukhrat Mitalipov and his colleagues from Oregon Health and Science University have successfully used the CRISPR Cas9 gene editing technology to wipe out a genetically inherited heart mutation in embryos.

Until now, reports of studies using CRISPR on humans had been from outside the country, and the results were not good. China has been particularly bold about using the technique on humans. Chinese scientists conducted a similar experiment using CRISPR on embryos in 2015, but those embryos reportedly resulted in something called “mosaicism,” which are off-target genetic mutations that happen when the changes to DNA are adopted by only some of the cells. The issue sparked an international ethical debate around the use of the technology on humans.

Undaunted, scientists from Sichuan University then moved on to fully grown adults, using CRISPR to try and cure a virulent form of lung cancer.

Embryonic results in the Oregon experiment, however, seemed to have turned out much better.

“The results indicate that the approach is effective and the CRISPR–Cas9 targeting very accurate,

providing some assurance regarding safety concerns; moreover, there was no evidence of off-target mutations. These findings suggest that this approach might potentially have applications for the correction of heritable mutations in human embryos in conjunction with PGD,” a statement from Nature, which published the scientific findings today, reads.

Mitalipov and his colleagues were able to avoid the previous mistakes made by the Chinese scientists by injecting the Cas9 enzyme (which acts as a sort of scissors for DNA fragments) into the sperm and eggs at the same time. It’s unclear how many embryos were successfully treated (we’ve asked), but it seems this method worked.

According to the release, “There were no off-target mutations.”

The practice of germline editing using CRISPR is still hotly contested, however. Last year, the U.S. Congress ruled against using CRISPR in clinical trials to edit embryos. The team was able to make this discovery, despite the Congressional block, by only growing them for a few days. They were never meant for implantation inside a womb or to grow to become human babies.

But many scientists go beyond concerns of mosaicism, considering germline editing to be unethical, and argue it could lead to designer babies.

On the flip side of that is the potential to wipe out deadly diseases before a baby takes its first breath. The heart disease eliminated in Mitalipov’s lab, hypertrophic cardiomyopathy (HCM), affects one in every 500 people.

Will we see more U.S. experiments using CRISPR after these initial results? Who knows? But the study is promising — and inches ever closer to a brave new world.