South Korean and U.S. scientists succeeded in changing a mutant gene that causes genetic diseases to normal in a just-fertilized human embryo. Whereas before they cut out the entire problematic gene, this time they changed only a single base, a part of it. It is like fixing a single typo instead of rewriting the entire sentence.
A team led by Dieter Egli, a professor at Columbia University in the United States, said on the 1st (local time) that it "succeeded in correcting the base of a mutant gene that causes heart disease and anemia in an early-stage human embryo," in a post on the preprint server bioRxiv.
Domestic researchers also took part, including Woo Jae-sung, a group leader at the Institute for Basic Science (IBS) Center for Biomolecular and Cellular Structure, and Bae Sang-soo, a professor at Seoul National University College of Medicine. The results were posted online but have not yet undergone peer review or been published as a formal paper.
Conventional gene editing risked cutting the wrong place, but with precision editing like this, such errors can be prevented. The scientific community said the door has opened to treating fatal genetic diseases before birth, but also raised concerns that the technique could be abused to produce designer babies whose intelligence or appearance is altered at will.
◇Changing a single letter instead of the sentence of a gene
Egli's team edited the PCK9, HBG1 and HBG2 genes in a human embryo that had just been fertilized and divided into two cells. Mutations in the PCK9 gene can raise blood cholesterol levels and cause heart disease. The HBG1 and HBG2 genes are involved in producing hemoglobin, which carries oxygen in the blood. The team said that correcting HBG variant genes could treat blood disorders such as sickle cell disease and thalassemia.
There have been previous cases of editing genes in human embryos. He Jiankui of Southern University of Science and Technology in Shenzhen, China, said in 2018 that three healthy babies were born after he edited a gene that causes AIDS (acquired immune deficiency syndrome) at the embryo stage. The method used then was the CRISPR-Cas9 gene scissors. Scientists criticized the technology as too risky to be applied to humans. He served three years in prison in China for illegal medical practice.
The CRISPR-Cas9 gene scissors used by He Jiankui consist of a guide RNA, which locates and zips onto the target DNA sequence, and Cas9, an enzyme protein that cuts at the bound site. Because CRISPR-Cas9 cuts both strands of DNA, errors occurred in which unwanted regions were added or deleted during the editing process.
Egli blocked such problems using next-generation gene scissors called base editing. DNA is a kind of blueprint that governs all life processes in the human body. Two strands interlock like a zipper to form a double helix, and the teeth of that zipper are the four bases A (adenine), G (guanine), C (cytosine) and T (thymine). Genetic diseases arise when the order of these bases differs from normal. Egli changed the A base to G in three genes in the embryo. The team cultured the base-edited human embryos to the blastocyst stage on days 5–6 post-fertilization, before implantation. In some blastocysts, cells were taken for analysis, and embryonic stem cell lines were also established.
◇Only partial success; premature for clinical use
Gene editing has already been approved as a therapy for genetic diseases in adults. In 2023, the U.S. Food and Drug Administration (FDA) approved a CRISPR-Cas9-based therapy for sickle cell disease. Last year in the United States, doctors attempted a world-first personalized gene edit in a 7-month-old baby to treat a rare genetic disorder.
But editing genes in embryos in the womb is a different matter. In adults, even if editing is not perfect in every cell, it may not pose a major problem. For example, in liver disease, treatment is possible if only one-fifth of hepatocytes are properly edited. In embryos, however, editing must be perfect because they later differentiate into all the cells of the body.
The new results showed limits on that point as well. In the PCK9 gene, base editing that changed A to G occurred in 19 of 25 embryonic cells. Three-quarters succeeded. Around PCSK9, there were no unwanted base changes, insertions or deletions.
By contrast, base editing occurred in 52% of cells for HBG1 and 68% for HBG2. Unwanted changes also appeared. In HBG1, changes were found in which one or two Gs were added. The embryo became a mosaic with genes differing by cell. If a baby grows from this state, cells with edited bases and cells without them will coexist.
The problem is that there is currently no way to test whether an edited embryo is a mosaic. Hospitals now biopsy a single cell to test whether an in vitro–fertilized embryo carries a genetic disease. But for a mosaic embryo, testing just one cell is not enough. Scientists say the genes should be edited at the sperm and egg stage, not in the embryo, to block such problems.
Egli said, "This demonstrates that base editing can achieve more precise gene editing than CRISPR-Cas9," but added, "Clinical application is still premature because off-target changes and mosaic embryo issues remain."
◇Concerns it could be used to "enhance" babies
The scientific community was split over the study. Emre Seli, a professor at Yale School of Medicine, told Nature on the 5th that "this work is a conceptual shift with the potential to advance embryo gene editing." Greg Neely of the University of Sydney also said, "This study will be viewed positively by history as less reckless, more cautious and ethical than previous approaches."
By contrast, Fyodor Urnov of the University of California, Berkeley (UC Berkeley), said, "Given that in vitro fertilization and genetic testing are already used to prevent genetic diseases, this work will only impact gene editing for 'baby enhancement.'" In other words, base editing in embryos could be abused not to block disease but to select desired traits such as intelligence or appearance.
Egli collaborated with Nucleus Genomics, a New York embryo testing company. In November last year, the company sparked controversy with a New York City subway ad reading, "Have the best baby." The company's CEO said that traits like intelligence and height are influenced by the complex interplay of numerous genes, making base editing impossible, but concerns about potential abuse persist. Some warn that scientists may have opened a "Pandora's box" that leads to unforeseen consequences.
References
bioRxiv (2026), DOI: https://doi.org/10.64898/2026.05.30.728989
New England Journal of Medicine (2025), DOI: https://doi.org/10.1056/NEJMoa2504747