In the first known U.S. study, scientists have successfully corrected a genetic mutation — one which causes the heart muscle to be unusually thick — in human embryos. The MIT Technology Review first learned of the results last week and called them "a milestone." Down the line, the research could advance to clinical trials in which researchers work with parents to correct mutations in embryos, but that raises the ethical question: If something goes wrong, what do you do?

Why it matters: This was "a cleaner study" and a more successful use of the gene-editing technique CRISPR than we've seen before, bioethicist Jessica Berg told Axios. The researchers, based in Oregon, corrected the genetic mutation in one cell and passed that correction to more embryonic cells, as they began to divide. Past studies carried out in China found errors in some cells after division.

• One existing option for parents is pre-implantation genetic diagnosis (PGD), a method which indicates the presence of genetic mutations in embryos. Researchers who conducted this study say their gene-correcting method could be used in conjunction with PGD in the future to treat embryos. But Jennifer Doudna, one of the creators of CRISPR, says "It may be possible in the not-too-distant future to avoid embryo editing altogether by editing gametes [egg and sperm cells] prior to embryogenesis."
• Difficult questions will arise at each stage of the research, Berg says. If an embryo is genetically modified, but there is a mistake, to discard it raises questions of abortion. Another ethical question is the idea of "changing progeny," and the implications of that for society, Michael Werner, executive director at the Alliance for Regenerative Medicine, told Axios. And the Oregon scientists only demonstrated that the genetic flaw was successfully corrected to a certain number of divisions. Berg asks, "How much evidence do you need before you can say this really works?"
• We don't like the word "editing," says Shoukhrat Mitalipov, the study's lead researcher. They prefer "correcting": the study isolated one genetic flaw and fixed it. "It is nowhere near designing babies," Berg says. "[The researchers] weren't saying … 'we wanted a taller baby or a smarter baby' … that was nothing like this."