For the first time, scientists in the US have successfully edited the DNA of viable human embryos using the powerful gene-editing tool CRISPR. Gaining the ability to edit human DNA is the first step toward one day allowing scientists to prevent babies from being born with incurable diseases or disabilities. But further success with this kind of research is likely to raise the heated discussion on the ethical implications of genetically altering human embryos. The story was first reported last week by MIT Technology Review.

The research — which was published today in the journal Nature — was led by Shoukhrat Mitalipov of Oregon Health and Science University. It involved editing dozens of viable embryos and effectively correcting a gene mutation that causes a heart condition called hypertrophic cardiomyopathy, which can lead to sudden death. The embryos were developed for only a few days and were not implanted. Without implantation, embryos cannot develop into babies.

“It feels a bit like a ‘one small step for (hu)mans, one giant leap for (hu)mankind’ moment,” Jennifer Doudna, a CRISPR pioneer at the University of California, Berkley, who was not involved in the research, wrote in an email to The New York Times. “I expect these results will be encouraging to those who hope to use human embryo editing for either research or eventual clinical purposes.”

This research is controversial

Human embryos have been edited with CRISPR before, only in China. In the US, this kind of research is much more controversial: there’s even a ban on using National Institutes of Health funding for research using gene-editing technologies in human embryos. In February, however, a committee created by the National Academy of Sciences and the National Academy of Medicine endorsed the use of genetic engineering on human embryos when there isn’t a “reasonable alternative” available, and only to eliminate serious diseases.

There are many concerns around genetically engineering humans. CRISPR is a very precise gene-editing tool, but it can sometimes lead to editing errors. So some fear that small mistakes could lead to permanent problems in the human gene pool. There are also ethical concerns: bioethicists fear that gene-editing will lead to a world where parents will be able to customize their own “designer baby,” complete with specific traits.

These “super-baby” concerns could be worked out easily, says Arthur Caplan, a bioethicist at New York University. “If you don’t want eugenics, you just draw a line and stop there,” Caplan tells The Verge. Scientists and bioethicists should agree on rules on what should and should not be done, and then make sure that editors of scientific journals enforce them. Research into how to create designer babies should not be published. Or you could have the National Academy of Sciences work with industry and Congress to lay out a review committee and permit funding.

“If America were to take the lead both in terms of working with journals, working with private foundations, with patient groups, and working with state and federal government, I think you’d get collaboration from the rest of the world,” Caplan says.

“If you don’t want eugenics, you just draw a line and stop there.”

Engineered humans are still far away into the future. But Mitalipov’s research is getting us closer: he and his team were able to edit some of the embryos precisely, without creating undesired mutations. They also avoided another problem: in experiments in China, the desired DNA changes were picked up only by some cells, not all the cells of an embryo — an effect called mosaicism. That makes gene-editing unsafe. But Mitalipov was able to avoid mosaicism, ensuring that all cells were mutation-free.

In the study, scientists fertilized eggs with the sperm of a man who has the mutation that causes the heart condition. They then used the gene-editing tool CRISPR-Cas9 to cut the mutated sequence on the male MYBPC3 gene, which was responsible for hypertrophic cardiomyopathy. Surprisingly, the male gene copied the healthy sequence from the female gene; it didn’t use a synthetic repair template injected by the scientists. It’s not exactly clear why this happened, but it shows a way that embryos repair themselves that wasn’t known before, according to The New York Times.

The technique worked best when the egg was injected with sperm and CRISPR-Cas9 at the same time (instead of being fertilized and then injected with CRISPR-Cas9). Of the 58 edited embryos, 42 had two mutation-free copies of the gene in every cell; 16 had unwanted mutations.

So the technique isn’t perfect yet, but the research shows just how far gene editing has come — and makes the prospect of engineered, disease-free humans more science fact than science fiction.

Update August 2nd 1:40 PM ET: The story has been updated to add details about the research, which was first reported last week by MIT Technological Review.