Scientists from Korea and the United States succeeded in converting a mutation-causing gene that triggers genetic diseases to normal in just-fertilized human embryos. Before, they would cut out the entire problematic gene, but this time they changed only a single base, one part of it. It is like fixing a single typo instead of rewriting the whole sentence.
A research team led by Dieter Egli, a professor at Columbia University in the United States, said it succeeded in correcting the base of a mutation-causing gene that leads to heart disease and anemia in early-stage human embryos, according to a paper posted on June 1 (local time) on the preprint server bioRxiv.
Domestic researchers also joined the study, including U Jae-sung, group leader at the Institute for Basic Science (IBS) Center for Biomolecular and Cellular Structure, and Bae Sang-su, a professor at Seoul National University College of Medicine. The findings were released online, but they have not yet undergone peer review or been published as a formal paper.
Conventional gene editing risked cutting the wrong site, but with precise editing like this, such errors can be prevented. The scientific community said the work opens the door to treating deadly genetic diseases before birth, while also raising concerns it could be misused to produce so-called designer babies by altering intelligence or appearance at will.
◇ Change a single letter instead of the sentence of a gene
Egli's team edited the PCK9, HBG1, and HBG2 genes in human embryos that had split into two cells immediately after fertilization. A mutation 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 researchers said correcting HBG variants 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, at the embryo stage, a gene that causes AIDS (acquired immune deficiency syndrome). The method used then was the CRISPR-Cas9 gene scissors. Scientists criticized the technology as too dangerous for use in humans. He served three years in prison in China for illegal medical practices.
The CRISPR-Cas9 gene scissors used by He Jiankui consist of a guide RNA that finds and binds, like a zipper, to the problematic gene's DNA, and Cas9, an enzyme protein that cuts the binding site. Because CRISPR-Cas9 cuts both strands of DNA, errors such as unintended insertions or deletions sometimes occurred during the editing process.
Egli blocked these problems by using a next-generation editor 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 the 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 of the embryo. The team cultured the edited human embryos to the blastocyst stage, on days 5–6 after fertilization, before implantation. In some blastocysts they took cells for analysis and also established embryonic stem cell lines.
◇ Only partial success, clinical use is premature
Gene editing has already been approved as a treatment technology 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 the world's first personalized gene editing in a 7-month-old infant to treat a rare genetic disorder.
But editing genes in embryos in the womb is a different level. In adults, it is not a big problem if gene editing is not perfect in every cell. For example, in liver disease, treatment is possible if only one-fifth of liver cells are properly edited. In embryos, however, the editing must be flawless because they later differentiate into all the cells of the human body.
The results here showed limits on that point. In the PCK9 gene, base editing that changed A to G occurred in 19 of 25 embryonic cells, a three-quarters success rate. 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, an extra G—one or two—was added. The embryos became mosaics with different genes in different cells. If a baby developed from this state, cells with corrected bases and cells without the correction would coexist.
The problem is that there is currently no way to test whether an edited embryo is a mosaic. Hospitals now take one cell from an in vitro–fertilized embryo to test for genetic disease. But for mosaic embryos, examining just one cell is not enough. Scientists say editing should be done at the sperm and egg stage, not in embryos, to prevent such problems.
Egli said, "This proves that base editing can achieve more precise gene editing than CRISPR-Cas9," while noting, "Clinical application is still premature because off-target changes and mosaic embryo issues remain."
◇ Concern it could be misused for "baby enhancement"
The scientific community split over the study. Emre Seli, a professor at Yale School of Medicine, told Nature on June 5 that the work is a conceptual shift with the potential to advance embryo gene editing. Greg Neely of the University of Sydney also said the study will be viewed positively in history as less reckless, more cautious, and more ethical than previous methods.
On the other hand, Fyodor Urnov of the University of California, Berkeley (UC Berkeley), said the research will only affect gene editing for "baby enhancement," noting that in vitro fertilization and genetic testing are already used to prevent genetic diseases. In other words, base editing in embryos could be misused not to block disease but to select traits like intelligence or appearance.
Egli conducted the research with New York–based embryo testing company Nucleus Genomics. In November last year, the company sparked controversy with a New York City subway ad saying, "Have the best baby." The company's chief said traits such as intelligence or height are influenced by the complex interplay of many genes, making base editing impossible, but concerns about potential misuse persist. Some warn that scientists may have opened a "Pandora's box" that could lead 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