If true, it would be a profound leap of science and ethics.
A US scientist said he took part in the work in China, but this kind of gene editing is banned in the United States because the DNA changes can pass to future generations and it risks harming other genes.
Many mainstream scientists think it's too unsafe to try, and some denounced the Chinese report as human experimentation.
The researcher, He Jiankui of Shenzhen, said he altered embryos for seven couples during fertility treatments, with one pregnancy resulting thus far. He said his goal was not to cure or prevent an inherited disease, but to try to bestow a trait that few people naturally have — an ability to resist possible future infection with HIV, the AIDS virus.
He said the parents involved declined to be identified or interviewed, and he would not say where they live or where the work was done.
There is no independent confirmation of He's claim, and it has not been published in a journal, where it would be vetted by other experts. He revealed it Monday in Hong Kong to one of the organisers of an international conference on gene editing that is set to begin on Tuesday, and earlier in exclusive interviews with The Associated Press.
"I feel a strong responsibility that it's not just to make a first, but also make it an example," He told the AP. "Society will decide what to do next" in terms of allowing or forbidding such science.
Some scientists were astounded to hear of the claim and strongly condemned it.
It's "unconscionable ... an experiment on human beings that is not morally or ethically defensible," said Dr Kiran Musunuru, a University of Pennsylvania gene editing expert and editor of a genetics journal.
"This is far too premature," said Dr Eric Topol, who heads the Scripps Research Translational Institute in California. "We're dealing with the operating instructions of a human being. It's a big deal."
However, one famed geneticist, Harvard University's George Church, defended attempting gene editing for HIV, which he called "a major and growing public health threat."
"I think this is justifiable," Church said of that goal.
In recent years scientists have discovered a relatively easy way to edit genes, the strands of DNA that govern the body, using a tool that makes it possible to supply a needed gene or disable one that's causing problems.
It's only recently been tried in adults to treat deadly diseases, and the changes are confined to that person. Editing sperm, eggs or embryos is different — the changes can be inherited. In the US, it's not allowed except for lab research. China outlaws human cloning but not specifically gene editing.
He Jiankui studied at Rice and Stanford universities in the US before opening a lab at Southern University of Science and Technology of China in Shenzhen, where he also has two genetics companies.
The US scientist who worked with him on this project after He returned to China was physics and bioengineering professor Michael Deem, who was his adviser at Rice. Deem also holds what he called "a small stake" in and serves on the scientific advisory boards of He's two companies.
The Chinese scientist said he chose to try embryo gene editing for HIV because those infections are a big problem in China. He sought to disable a gene called CCR5 that forms a protein doorway allowing HIV, the virus that causes AIDS, to enter a cell.
All the men in the project had HIV and all the women did not, but the gene editing was not aimed at preventing the small risk of transmission, He said. The fathers had their infections deeply suppressed by standard HIV medicines and there are simple ways to keep them from infecting offspring.
Instead, the appeal was to offer couples affected by HIV a chance to have a child that might be protected from a similar fate.
He said the gene editing occurred during IVF, or lab dish fertilisation. First, sperm was "washed" to separate it from semen, the fluid where HIV can lurk. A single sperm was placed into a single egg to create an embryo. Then the gene editing tool was used.
When the embryos were 3 to 5 days old, a few cells were removed and checked for editing. Couples could choose whether to use edited or unedited embryos for pregnancy attempts. Eleven embryos were used in six attempts before the twin pregnancy was achieved, He said.
Tests suggest that one twin had both copies of the intended gene altered and the other twin had just one altered, with no evidence of harm to other genes, He said. People with one copy can still get HIV.
Several scientists reviewed materials that He provided to the AP and said tests so far are insufficient to draw conclusions.
It's unclear whether participants fully understood the purpose and potential risks and benefits. For example, consent forms called the project an "AIDS vaccine development" programme.
The study participants are not ethicists, He said, but "are as much authorities on what is correct and what is wrong because it's their life on the line."
"I believe this is going to help the families and their children," He said. If it causes unwanted side effects or harm, "I would feel the same pain as they do and it's going to be my own responsibility."
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