Significant doubts have emerged about claims from a Chinese scientist that he has helped make the world’s first genetically edited babies.
Prof He Jiankui says the twin girls, born a few weeks ago, had their DNA altered as embryos to prevent them from contracting HIV.
His claims, filmed by Associated Press, are unverified and have sparked outrage from other scientists, who have called the idea monstrous.
Such work is banned in most countries.
Gene editing could potentially help avoid heritable diseases by deleting or changing troublesome coding in embryos.
But experts worry meddling with the genome of an embryo could cause harm not only to the individual but also future generations that inherit these same changes.
And many countries, including the UK, have laws that prevent the use of genome editing in embryos for assisted reproduction in humans.
Scientists can do gene editing research on discarded IVF embryos, as long as they are destroyed immediately afterwards and not used to make a baby.
But Prof He, who was educated at Stanford in the US and works from a lab in the southern Chinese city of Shenzhen, says he used gene-editing tools to make two twin baby girls, known as “Lulu” and “Nana”.
In a video, he claims to have eliminated a gene called CCR5 to make the girls resistant to HIV should they ever come into contact with the virus.
He says his work is about creating children who would not suffer from diseases, rather than making designer babies with bespoke eye colour or a high IQ.
“I understand my work will be controversial – but I believe families need this technology and I’m willing to take the criticism for them,” he says in the video.
However, several organisations, including a hospital, linked to the claim have denied any involvement.
And other scientists say if the reports are true, Prof He has gone too far, experimenting on healthy embryos without justification.
Dr Dusko Ilic, an expert in stem cell science at King’s College London, said: “If this can be called ethical, then their perception of ethics is very different to the rest of the world’s.”
He argues that HIV is highly treatable and that if the infection is kept under control with drugs, then there is almost no risk of the parents passing it on to the baby anyway.
Prof Julian Savulescu, an expert in ethics at the University of Oxford, said: “If true, this experiment is monstrous. The embryos were healthy – no known diseases.
“Gene editing itself is experimental and is still associated with off-target mutations, capable of causing genetic problems early and later in life, including the development of cancer.
“There are many effective ways to prevent HIV in healthy individuals – for example, protected sex. And there are effective treatments if one does contract it.
“This experiment exposes healthy normal children to risks of gene editing for no real necessary benefit.”
Scientists say baby gene editing may one day be justifiable, but that more checks and measures are needed before allowing it.