Start with 117. That is how many edited embryo cells the Columbia team checked for chromosome damage, and every one still had intact copies of the two chromosomes holding the target genes. Earlier work with standard CRISPR-Cas9 in embryos kept losing entire chromosomes or large pieces of them, Earth.com reported. The bottom line: base editing passed that test. It did not pass the next one.
The work comes from Dieter Egli, who studies developmental cell biology at Columbia University, and Stepan Jerabek. They posted it on the bioRxiv preprint server on 1 June 2026, which means it has not yet been peer reviewed. The embryos were never intended for implantation or pregnancy.
What was edited, and how
The team used adenine base editors, which change a single DNA letter and cut only one strand, rather than both as Cas9 does. They made A-to-G edits in PCSK9, a gene tied to cholesterol, and in the fetal hemoglobin genes HBG1 and HBG2, which matter for sickle cell disease and beta-thalassemia. Neither target was chosen to cure anyone; both copy changes already seen in some healthy people.
The edits landed often. In many embryos, three-quarters of cells carried the change. Some embryos grew to the blastocyst stage, a hollow ball of cells that appears about five days in, and from three of them the team derived stem cell lines that kept the edit.
Delivery mattered. When the editor was supplied as messenger RNA, all 19 embryos stalled within days. Supplied as the finished protein, about a third reached the blastocyst stage. The researchers suspect the embryos may treat foreign RNA as a threat, a response not seen before in human embryos.
Where it falls short
Mosaicism is the sticking point. In embryos edited at the one-cell stage it showed up in most cases: some cells carry the edit, others do not. That leaves open what a child would inherit. The authors also found that off-target editing differed considerably depending on the guide RNA. They concluded that base editing looks far less damaging to the genome than Cas9 in human embryos, but that unresolved mosaicism, off-target activity and more safety studies keep it out of the clinic. Egli has said that pushing it there now would be premature.
Reaction split along familiar lines. A Nature News commentary quoted some experts calling it a major technical step, while others warned that precise embryo editing could speed commercial interest in germline modification before the technology is understood. The shadow of 2018 and the He Jiankui scandal hangs over any such work. Supporters distinguish this study from that experiment. Others fear the opposite: that wealthy families could be tempted to edit embryos for enhancement, before safety is established.
Under today’s guidelines, neither of these edits could be used in an actual pregnancy.
Sources
- News: Base Editing Rewrites Embryo Genomes Precisely – CRISPR Medicine
- Scientists edit human embryos without damaging chromosomes
- Scientists edited DNA in human embryos; the breakthrough is real, but so are the risk
