NEW YORK — The embryos were not perfect. Some cells carried the edit. Others did not. That is the reality of the latest advance in human gene editing, and it is the problem researchers are still trying to solve.
Dr. Dieter Egli of Columbia University used a technique called base editing to swap individual letters in the DNA of human embryos. It is more precise than older methods, which cut both strands of the DNA helix. Base editing makes a single chemical change. But precision does not guarantee perfection.
In some embryos, the editing molecules simply missed their target. The result was a genetic patchwork — some cells changed, others untouched. Scientists call this mosaicism. If such an embryo were allowed to develop into a baby, the mixture of edited and unedited cells could cause medical problems. That is a hard ceiling on any talk of clinical use.
Mainstream scientific bodies have been clear: germline editing is not safe enough for pregnancies. Not now. That position hardened after 2018, when a Chinese scientist announced he had created the first gene-edited babies. The announcement drew international condemnation. Countries moved to restrict or ban clinical use. The field has been under a shadow ever since.
Dr. Egli’s work does not change that. It is a step, not a breakthrough to the clinic. The New York Times reported the findings. The work shows progress in the lab, but it also shows how far the technology still has to go before anyone can responsibly use it to prevent genetic disease in a real child.
Base editing is a refinement. Older gene-editing tools cut DNA at a specific spot and relied on the cell’s own repair machinery to make a change. That process was messy. It could introduce unintended mutations. Base editing does not cut the DNA. It chemically converts one letter into another — for example, changing a C to a T. That is cleaner. But it is not foolproof.
The problem of mosaicism is not unique to base editing. It has dogged gene editing in embryos from the start. The editing tool is introduced at a very early stage, often when the embryo is just a few cells. If the tool does not reach every cell, or if it works in some cells but not others, the result is a mix. That mix is a risk. No one knows exactly what health problems it could cause, but the unknowns are enough to stop any responsible doctor from trying.
Dr. Egli and his team are pushing ahead. The potential is real. Thousands of genetic diseases are caused by a single wrong letter in the DNA. If you could fix that letter in an embryo, the child would be born free of the disease. That is the promise. It is a powerful one.
But the gap between promise and practice remains wide. The Chinese scientist who announced gene-edited babies in 2018 did not follow accepted scientific protocols. He was widely condemned. The scientific community has since tried to draw a clear line: basic research into germline editing is acceptable; clinical use is not. Dr. Egli’s work sits on the research side of that line.
The embryos in the study were not allowed to develop. They were destroyed after a few days. That is standard for this kind of research. No one is proposing to implant them. The goal is to learn, to refine the technique, to reduce the error rate, to solve the mosaicism problem. Only then, perhaps, could the conversation about clinical use begin in earnest.
That conversation will not be purely scientific. It will be ethical, legal, and political. Different countries have different rules. Some ban germline editing outright. Others allow research under strict oversight. The United States does not ban it, but federal funding cannot be used for work that destroys embryos. Private funding fills the gap.
Dr. Egli’s work is a reminder that the science is moving faster than the rules. The embryos are not perfect. The technique is not ready. But the research continues, driven by the hope of curing disease and the weight of the ethical questions that come with it.
