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".