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Earlier this year, Chinese scientists caused an international furor when they reported that they had taken 86 human embryos and attempted to modify the gene that causes β-thalassaemia, a blood disorder that is potentially fatal.

It was the first time edits had been confirmed to have been done on reproductive cells and the news caused deep divisions within the scientific community. Some expressed optimism and hope that such research could eventually lead to the eradication of genetic diseases from the face of the Earth. Others were horrified — warning that genetically modifying humans is unsafe and could have devastating consequences on future generations of our race that no one can foresee.

[The rumors were true: Scientists edited the genomes of human embryos for the first time]

Now another group of scientists — this time from Britain — is seeking permission to conduct similar experiments, raising the stakes for the technology.

Kathy Niakan, a stem-cell researcher at the Francis Crick Institute in London, has told the country's regulators that her work will focus on trying to understand what genes are at play during the first few days after fertilization, according to a...