But removal of these cells kills the human beings from which they are taken. Therapeutic cloning means killing. Charities such as Life would like to see more work done to follow up research showing that adult stem cells, instead of embryonic ones, could be used to develop tissues and organs. In 1997 Dolly the sheep became the world's first mammal cloned from an adult cell. It was part of a project to try to make human recipients less likely to reject pig organs after a transplant operation.
Researchers try to alter pig genes and then clone the modified pigs as a source of organs.
Some pig parts like valves are currently used in areas like heart transplants. But funding for the controversial project was recently halted amid fears that pig viruses could be passed on to humans. The technique used to produce Dolly, and other clones since, involved removing the genetic material from an egg and replacing it with the nucleus of a cell from an adult sheep. The reconstructed eggs were then grown in a lab for a few days before being implanted in a womb to develop into a clone. However, Dolly was the only success, an achievement rate far below 1 per cent.