“We’re not saying this is going to be used tomorrow in the clinics,” said Dieter Egli, the Columbia University geneticist whose team has just edited human embryos more precisely than anyone had before.
Egli and his colleagues posted their study online on June 4, 2026, and it is under review at a scientific journal. They used base editing, a newer tool that swaps individual genetic letters. Earlier CRISPR methods, which cut DNA out, tended to cause damage, and the team found none of the extensive harm associated with them.
What the team did
They targeted two genes. PCSK9 can carry mutations that push up LDL cholesterol and the risk of heart disease. HBG directs hemoglobin production in fetuses. The editors went into fertilized eggs and into two-cell embryos donated by parents, and both genes were successfully changed. In some experiments, both changed in the same embryo.
It was not clean. Now and then the editing molecules missed their target DNA, so some cells kept the original gene while others carried the edit. Those embryos were mosaics, and cells with different versions of one gene could have caused medical problems if the embryos had grown into babies.
Egli’s earlier work explains why this matters. In 2020, his group used CRISPR on embryos carrying a faulty copy of a gene called EYS, which causes hereditary blindness. The repair failed in about half of them. Some embryos lost long stretches of DNA and some lost the whole chromosome. “It had absolutely catastrophic consequences,” he said. David Liu of Harvard and colleagues had by then created base editing in 2016. Last year, a baby was cured of a potentially lethal genetic disorder after receiving a customized set of base editing molecules.
Reactions
Paula Amato, a fertility expert at Oregon Health & Science University who was not part of the study, called the method “promising” but wants to see the final published results. Ana Iltis, a bioethicist at Wake Forest University, warned that judging the safety of base-edited embryos takes more than checking for damaged chromosomes. “It is possible that some of the potentially harmful effects would not be evident until after birth,” she said.
Fyodor Urnov, a geneticist at the University of California, Berkeley, said the results matched earlier base-editing work in living cells. For embryos, though, he called the idea novel and risky. Regular I.V.F. already screens embryos for genetic abnormalities. “Do we do what we’ve done safely and effectively 15 million times since 1978, or do we instead try something that we can never truly de-risk, and where the risks are clear?” he asked. He also said the work gives “baby improvers” a how-to manual.
Nathan Treff, chief clinical officer of Nucleus Genomics and a co-author, said fixing disease-causing mutations could let I.V.F. patients use embryos they would otherwise discard. “There’s still work to do before getting to that point, but this research gets us closer,” he said. Nucleus Genomics, founded in 2021, screens I.V.F. embryos for thousands of genetic disorders and will fund the next stage, since the federal government does not pay for embryo research of this kind. Critics have accused the company of promoting a biotech version of eugenics, which it rejects.
The next studies will look at avoiding mosaics and test base editing on embryos of about 100 cells, the stage at which fertility clinics typically freeze and test them. Egli says each extra gene raises the risk of failure. “I think you can probably combine three or four, maybe even five, but I think there’s a limit,” he said. “Where that limit is remains to be determined.”
