A team of researchers in Oregon have become the first in the US to attempt genetically altering human embryos, according to reporting by MIT Technology Review. The attempt is said to represent an advance in the safety and efficacy of methods used to correct genetic defects that spur disease.

Until now, the only three published reports of human embryo gene editing were from researchers in China. But their experiments—using a gene-editing method called CRISPR —caused “off-target” genetic changes, basically sloppy changes in the DNA that were not intended. Also, not all the cells in the embryos were successfully edited, causing an effect called “mosaicism.” Together, the problems suggested that the technique was not advanced enough to safely alter human embryos without unintended or incomplete genetic consequences.

Scientists familiar with the new US work told MIT Technology Review that the Oregon team has improved these issues. They’re said to have shown in experiments with “many tens” of human embryos that they can correct genetic mutations that cause disease while avoiding mosaicism and off-target effects. Their improved method allows for earlier delivery of CRISPR into cells at the same time sperm fertilize an egg.

The team of researchers, led by Shoukhrat Mitalipov of Oregon Health and Science University, created the embryos using sperm donated by men with inherited mutations that cause a disease. But it is not clear what disease or genes were edited. None of the embryos were allowed to develop for more than few days and were never intended for implantation.

When reached by MIT Technology Review, Mitalipov declined to comment on the results, which he said are pending publication.

An unnamed scientist familiar with the work told MIT Technology Review that it was “proof of principle that it can work. They significantly reduced mosaicism. I don’t think it’s the start of clinical trials yet, but it does take it further than anyone has before.”

The news follows a report earlier this year by the National Academy of Sciences and the National Academy of Medicine, which essentially gave a green light to researchers to use gene editing to treat or prevent inherited disease. The report—authored by 22 of the world’s leading experts on genetics, bioethics, medicine, and law—was cautious though, suggesting the technique only be used under tight oversight, with stringent precautions, and when all other options are exhausted. The panel opposed any efforts to create “designer babies” or use gene editing for anything other than eliminating disease.

Ethicists have long feared that such gene-editing technology could be used for nefarious or questionable purposes. In the US, no clinical trials involving edited human embryos are currently allowed to proceed. However, there are many countries without such legal restrictions.