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Leading scientists have called for a ban on research to "re-engineer the human species" for the next five years.

Two babies were born last year in China who had been genetically altered to protect them against HIV.

The scientists, writing in the journal Nature, said "major speed bumps" needed to be put in front of such research.

The US National Institutes of Health said a ban should start immediately, but others have criticised the idea.

How do you re-engineer the human species?

It all comes down to the ability of scientists to manipulate DNA.

Our DNA contains the instructions for building and running the human body including traits such as height, hair colour and risk of diseases.

A relatively new tool called CRISPR has allowed scientists to quickly and cheaply alter DNA and has led to an explosion in gene editing technologies.

They have the potential to be used to "re-engineer" us by correcting faults in DNA that cause disease or even to enhance the human body.

What happened in China?

Image copyright Getty Images Image caption Prof He Jiankui created the first gene-edited babies.

In November last year, Prof He Jiankui announced the birth of twin girls known as Lulu and Nana.

But what made them different to any other child was that their DNA had been tweaked in a lab.

Gene-editing tools were used to give them protection against HIV by, effectively, changing the locks on the door that HIV uses to infect our cells.

Prof He made the announcement in a video saying: "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 performed this using his own money, in his own time and against Chinese regulations.

Why is gene-editing babies controversial?

The full consequences of gene-editing babies are uncertain, but the effects are permanent.

Any modifications are passed on down through the generations, introducing a lasting change to the human race.

But the science is so new that it is unknown whether it is safe, and there may be unintended consequences of altering seemingly simple parts of human DNA.

Prof He altered the CCR5 gene in the twins to protect them against HIV, but this gene also has also has a role in fighting flu and potentially intelligence.

The science has also raced far ahead of the public debate about what is acceptable - is it OK to genetically modify an embryo to stop a child growing up with a fatal disease? What about "enhancements" that have no medical benefit?

What are the researchers calling for?

They say gene-editing has "implications for the entire species" and decisions about its use cannot be made by individual scientists.

And that it was up to the world to decide whether gene-editing should be completely banned, used only medically or even adopted for wide-scale human enhancement.

For now, they want a global moratorium (a temporary ban) on using gene-editing for sperm, eggs and embryos that would be used to create a baby.

They want this ban to last for five years.

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It would not affect using gene-editing for research or for using gene-editing in adults when any changes could not be passed on to subsequent generations.

Then, they argue it is up to individual nations to make decisions "with due respect to the opinions of humankind".

They say:

two years' notice should be given of any intent to create gene-edited babies

it must be justified scientifically, technically, medically and morally

and there must be broad consensus in the country to proceed

The 18 researchers who have called for the moratorium include some of the leading figures in the field including Feng Zhang and Emmanuelle Charpentier, who made the key discoveries that have made gene-editing human cells possible.

Does everyone agree?

No.

Dr Francis Collins, the director of the US National Institutes of Health, said a moratorium should be "put into effect immediately" in light of the "irresponsible and unethical research in China".

He said: "Research on the potential to alter the very biological essence of humanity raises profound safety, ethical, and philosophical issues."

Dr Helen O'Neill, programme director of Reproductive Science and Women's Health at University College London, said there was already a global ban.

She said: "Currently, there are (as there was in China) legal and ethical measures in place globally which regulate the use of gametes and embryos.

"Let's not forget that He Jiankui broke many rules… it was not that he did this because the law allowed it.

"Naming a 'moratorium' sheds a negative light on the potential for germline genome editing."

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