A scientist in China claims to have created the world’s first genetically edited babies, in a potentially ground-breaking and controversial medical first.

If true, it would be a profound leap of science and ethics. This kind of gene editing is banned in most countries as the technology is still experimental and DNA changes can pass to future generations, potentially with unforeseen side-effects.

Many mainstream scientists think it is too unsafe to try, and some denounced the Chinese report as human experimentation.

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The researcher, He Jiankui of Southern University of Science and Technology in Shenzhen, said he altered embryos for seven couples during fertility treatments, with one pregnancy resulting so 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.

Q&A What is Crispr? Show Hide Crispr, or to give it its full name, Crispr-Cas9, allows scientists to precisely target and edit pieces of the genome. Crispr is a guide molecule made of RNA, that allows a specific site of interest on the DNA double helix to be targeted. The RNA molecule is attached to a bacterial enzyme called Cas9 that works like a pair of 'molecular scissors' to cut the DNA at the exact point required. This allows scientists to cut, paste and delete individual letters of genetic code.

He said the parents involved declined to be identified or interviewed, and he would not say where they lived 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 on Monday in Hong Kong to one of the organisers of an international conference on gene editing that is due to begin on Tuesday, and earlier in 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 said. “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 was “unconscionable … an experiment on human beings that is not morally or ethically defensible,” said Dr Kiran Musunuru, a University of Pennsylvania gene-editing expert.

“If true, this experiment is monstrous,” said Julian Savulescu, a professor of practical ethics at the University of Oxford. “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. In many other places in the world, this would be illegal punishable by imprisonment.”

In recent years, scientists have discovered a relatively easy way to edit genes, the strands of DNA that govern the body. The tool, called Crispr-Cas9, makes it possible to operate on DNA to supply a needed gene or disable one that is causing problems.

It has only recently been tried in adults to treat deadly diseases, and the changes are confined to that person. If sperm, eggs or embryos were to be edited, the changes could then be inherited.

He Jiankui studied at Rice and Stanford universities in the US before returning to his homeland to open a lab at Southern University of Science and Technology of China in Shenzhen, where he also has two genetics companies.

He said he practised editing mice, monkey and human embryos in the lab for several years and has applied for patents on his methods. He said he chose embryo gene editing for HIV because these infections are a major problem in China. He sought to disable a gene called CCR5 that forms a protein doorway that allows HIV, the virus that causes Aids, to enter a cell.

All of the men in the project had HIV and all of 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 that do not involve altering genes. 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 in vitro fertilisation. First, sperm was “washed” to separate it from semen, in which HIV can lurk. A single sperm was placed into a single egg to create an embryo. Then the gene-editing tool was added. When the embryos were three to five days old, a few cells were removed and checked for editing. Couples could choose whether to use edited or unedited embryos for pregnancy attempts. In all, 16 of 22 embryos were edited, and 11 embryos were used in six implant 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 immediate evidence of harm to other genes, He said. People with one copy of the gene can still get HIV.

Musunuru said that even if editing worked perfectly, people without normal CCR5 genes faced higher risks of contracting certain other viruses, such as West Nile, and of dying from flu. Since there are many ways to prevent HIV infection and it is treatable if it occurs, those other medical risks are a concern.

There also are questions about the way He said he proceeded. He gave official notice of his work long after he said he started it, on 8 November. It is also unclear whether participants fully understood the purpose and potential risks and benefits; for example, consent forms called the project an Aids vaccine development programme.

He said he personally made the goals clear and told participants that embryo gene editing had never been tried before and carried risks. He said he also would provide insurance coverage for any children conceived through the project and plans medical follow-up until the children are 18, and longer if they agree once they are adults.

“I believe this is going to help the families and their children,” He said. If it caused unwanted side-effects or harm, “I would feel the same pain as they do and it’s going to be my own responsibility”.

Dr Sarah Chan, a bioethicist at the University of Edinburgh, said that if true, the experiment was “of grave ethical concern”.

“Whether or not the veracity of these reports is eventually borne out, making such claims in a way that seems deliberately designed to provoke maximum controversy and shock value is irresponsible and unethical,” she said.

“The claim made by those responsible for the research is that the babies have been genome edited in an attempt to make them immune to HIV. The lifetime risk of contracting HIV is extremely low in the first place; there are other means of prevention and it is no longer an incurable, inevitably terminal disease. Putting these children at such drastic risk for such a marginal gain is unjustifiable.”