In a U.S. first, a team of biologists has edited a human embryo's DNA. The technique has been used before by scientists in China, but never in the United States, where the ethical debate over editing embryos rages on with no consensus in sight. And according to the U.S. team, their trial has achieved an unprecedented level of success.

There's not much information on exactly what was edited, because there is not yet a published paper to accompany the research. MIT Technology Review first reported on Wednesday that Shoukhrat Mitalipov, a biologist at Oregon Health and Science University, had used CRISPR to remove genes from an early stage embryo. More importantly, they reported that the rate of mosaicism in these embryos was lower than in previous attempts.

Mosaicism happens when you use CRISPR on an embryo that already has multiple cells. CRISPR can hunt down specific genes and snip them out, but with multiple cells—each with its own copy of the genetic code—there's no guarantee that the targeted gene will be edited out everywhere. Even if it works in nine out of 10 cells, that one unedited cell will go on to produce millions of other cells. In many cases, those errant genes could do enough damage to render the treatment ineffective. Mitalipov's team reportedly avoided that problem by performing CRISPR on a fertilized egg just after sperm made contact, such that there was only one cell to edit.