Cloning: the facts

The aim of cloning is to produce an identical copy of an embryo. Whether it's a human or Dolly the sheep, the process is basically the same.

Researchers take DNA from one adult cell then put it into a "hollowed out" egg, often from the same person. A short jolt of electricity or other trigger is applied, the egg and DNA fuse, and the embryo cells start dividing as normal.

What happens next is controversial. There are two types of cloning - therapeutic and reproductive. Reproductive cloning aims to produce a baby, and would involve the embryo being implanted into a woman and left to grow.

Therapeutic cloning - the Korean researchers' aim - seeks to create stem cells which can grow replacement organs and cells. Effectively, we'd be able to grow our own replacement organs. To get the stem cells, the embryo is terminated and they are removed.

Haven't we had cloning before?

We've all read dozens of stories about cloned babies, and researchers have known for years that it will one day be possible.

Dr Panos Zavos, an American fertility expert, announced in January he had implanted a cloned embryo in a woman, but said last week that pregnancy had failed. Clonaid, which is linked to a UFO cult, also claimed today the birth of its sixth clone in Australia - although, like Zavos, it failed to provide any evidence.

Today's announcement is, experts believe, the first "real" evidence that a cloned embryo has been created. By publishing their research in a respected journal like Science, the Koreans also had to have their work reviewed by other scientists.

Why is this so important?

Because stem cells can be used to grow any cell or organ in the body, they could potentially cure almost any disease.

Degenerative brain diseases such as Alzheimer's and Parkinson's are likely to be the first targets. Researchers to be able to regrow damaged brain tissue, effectively reversing the effects of the disease.

Why is it controversial?

For any kind of cloning to work, embryos must be produced. Some people believe scientists are effectively playing God, and creating humans at will by doing this. For therapeutic cloning to work, embryos must be destroyed to remove the stem cells, which has caused outrage in the prolife community.

If the embryo were to be implanted in a woman, many experts believe it would lead to massive deformities in any child.