In early June, American scientists announced that they had, for the first time, used a precise genome-editing technique known as ‘base editing’ to modify the genomes of human embryos. The scientific community has reacted with a mixture of enthusiasm and alarm.
Positive reactions focus on the possibility of ‘correcting’ harmful mutations that cause a whole range of hereditary diseases whilst the embryo is still in its early stages. Negative comments express concerns that this technology could be misused in an attempt to create embryos with specific characteristics, such as above-average intelligence or a better memory – or even to modify moral and volitional traits.
Behind this groundbreaking research is the Swiss-born biologist Dieter Egli, who, after studying at Harvard, now works at Columbia University in New York. He led a team of scientists (including the Czech biochemist Štěpán Jeřábek), who published a study on 1 June on the preprint portal bioRxiv detailing their success in editing the genomes of embryos. This means that the work has not yet undergone peer review. However, Egli has such an excellent reputation and this study “appears to be sound”, so lively discussion about it (and, above all, its implications) has already begun.
Base editing is a method of precision genome editing that allows a single ‘letter’ in the DNA – specifically, a single nitrogenous base – to be changed without the need to cut the DNA. It works by attaching a special enzyme (known as a deaminase) to the genome, linked to a modified version of Cas9, which does not cut the DNA but merely attaches to it as a guide. The enzyme then chemically converts one base into another – for example, cytosine (C) into thymine (T), or adenine (A) into guanine (G). The entire process takes place directly at the site, gently and without breaking the DNA strand.
The key difference compared to CRISPR-Cas9 is that conventional CRISPR physically cuts the DNA (creating a so-called double-strand break), and repair then depends on cellular repair mechanisms, which are imprecise and can cause unwanted mutations or the insertion/deletion of parts of the sequence. Base editing bypasses this ‘cut’ – it edits directly in situ, without breaking the strand, and is therefore significantly more precise and safer. The disadvantage, however, is its limited repertoire: it can only change four types of bases, whereas CRISPR is capable of performing any complex genome modifications – including the insertion and deletion of larger sections.
Source: National Library of Medicine
What makes this work so groundbreaking? Gene editing in embryos is nothing new. It looked promising many years ago, but as scientists were able to study it more closely, they realised that such interventions posed a number of problems. The methods used to date were not precise enough, which sometimes led to the loss of modified chromosomes. Because of this, the technology could not be put to much practical use, as the risk of unexpected side effects was too great.
A number of healthcare companies were established, seeking to use this technology to treat hereditary diseases. In 2023, the US Food and Drug Administration (FDA) approved a CRISPR-based treatment for sickle cell anaemia, but due to shortcomings, the genetic modification of embryos has not yet become as widespread as had been anticipated.
However, according to a number of experts who have already studied it, the new method represents a fundamental change. “This is a conceptual shift that really has the potential to move this field forward,” gynaecologist Emre Seli from Yale University told the scientific journal *Nature*. “It will go down in history in a positive light – as less reckless, more cautious and more ethical than previous attempts,” added geneticist Greg Neely from the University of Sydney to the same journal. Neither of them was involved in the research. Other scientists have also given the research a similarly enthusiastic review in the New York Times. The newspaper was the first to draw attention to the study.
However, a number of experts have expressed concerns about the potential consequences of this work. For example, Stanford lawyer Hank Greely, who specialises in the medical ethics of modern technologies, warns that this study could inspire billionaires to use it as a basis for modifying the genetic code of ‘their’ embryos. “For a few million dollars, it would be possible to set up a laboratory for artificial insemination and genetic testing and get started. (…) And the result could be truly sick children,” warns Greely.
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After all, it wouldn’t be the first time someone has attempted to genetically modify human embryos – and it didn’t end well. In 2018, a Chinese doctor attempted to illegally modify embryos to protect them against HIV. Ultimately, this experiment resulted in the birth of two girls who may, in fact, have genetic problems. The doctor eventually ended up behind bars for three years.
The newly described base editing technique is a gene-editing tool that enables scientists to make precise changes to individual letters in DNA. It is also more advanced than the older CRISPR method – it is not as invasive in terms of DNA manipulation. This significantly reduces the risk of unwanted genetic changes, although it cannot yet rule them out entirely.
Egli’s team used base editing on human embryos to alter a single letter of the genetic code in three genes. One of these helps regulate levels of ‘bad’ cholesterol in the blood, whilst the other two genes are involved in the production of foetal haemoglobin. The researchers are investigating whether these two genes can be edited to treat blood disorders such as sickle cell anaemia and thalassaemia.
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Overall, the edits worked satisfactorily and, by all accounts, better than with CRISPR. However, ‘unexpected new letters’ also appeared in some cells. According to Egli, many of these issues have already been mitigated, but the technology is not yet ready for clinical use.
According to him, the main problem is that cell division could also have been halted during the editing process. “These base editors can have harmful effects on the embryo. Why would we use them when we don’t fully understand them?” he says. In its current form, “it cannot be used. It’s as clear as day,” he adds.
According to him, the aforementioned concerns about misuse are therefore entirely premature at this stage, and there is little point in addressing them at present. Egli will continue his research; he has secured funding from the private company Nucleus Genomic – as the US federal government is legally prohibited from funding research on human embryos.
Link to the study
An article written by Tomáš Karlík (CT), initially published on 8 June 2026 at 10:45 (CEST)