Base editing of human embryos works-but mosaicism and safety keep it from the clinic

For the first time, researchers used base editing to precisely alter single letters in human embryo DNA—and the edits stuck, though not uniformly. Dieter Egli, a developmental cell biologist at Columbia University, and his team posted their results on bioRxiv on June 1, 2026, reporting single-nucleotide changes at three genes in early-stage human embryos.

Three genes, one base editor, uneven results

Egli’s group targeted PCSK9 (which regulates LDL cholesterol), HBG1, and HBG2 (involved in fetal hemoglobin). For each, they swapped an A for a G at a specific site. The PCSK9 edit switched the gene off, mimicking natural loss-of-function variants known to reduce coronary heart disease risk. The HBG1/HBG2 edits reproduced a protective mutation that ameliorates sickle cell disease and thalassemia.

But the edits didn’t take in every cell. Mosaicism—some cells carrying the new letter, others retaining the old—was present. Egli says his team has since improved their protocols to reduce mosaicism, but the preprint data show the problem exists.

Safety ceiling: mRNA toxicity stops cell division

At excessive doses of the mRNA that delivers the base editor, embryos stopped dividing. “You can’t use it. It’s as clear as day and night,” Egli told Nature. The technique’s own developer says it’s not ready for the clinic. That’s a stronger warning than most press releases from biotech companies.

Even if the mosaicism and toxicity issues get solved, critics argue the entire enterprise may be misguided. Fyodor Urnov at UC Berkeley calls embryo editing for disease prevention “a solution in search of a problem,” noting that IVF and preimplantation genetic screening already let couples avoid passing on many genetic conditions. Hank Greely, a bioethicist at Stanford, worries that wealthy individuals could set up an IVF lab and a genetic testing lab “for probably a handful of millions of dollars and start doing this,” with the potential result being “really sick kids.”

The shadow of 2018

The field still operates under the memory of He Jiankui, who used first-generation CRISPR-Cas9 on embryos in 2018, resulting in the first gene-edited babies, and later spent three years in prison. Base editing is cleaner than CRISPR-Cas9 (it doesn’t cut both DNA strands), but it’s not error-free, and the ethical landscape hasn’t changed. Greg Neely at the University of Sydney called this work “less reckless, more careful and ethical than previous attempts,” yet he and others fear it could normalize embryo editing for non-medical enhancement—the “baby improvement” movement.

What this work actually enables, for now, is better understanding of base editing’s limits in human embryos. Until the mRNA toxicity window is characterized and mosaicism is eliminated, any talk of clinical application is premature—and Egli himself says so.

Source: First precise genome editing of human embryos triggers praise and alarm
Domain: nature.com