The cloned embryos were used as a source of stem cells, which can make new heart muscle, bone, brain tissue or any other type of cell in the body. However, researchers say other sources of stem cells may be easier, cheaper and less controversial. Opponents say it is unethical to experiment on human embryos and have called for a ban. Stem cells are one of the great hopes for medicine. Being able to create new tissue might be able to heal the damage caused by a heart attack or repair a severed spinal cord. There are already trials taking place using stem cells taken from donated embryos to restore people's sight. However, these donated cells do not match the patient so they would be rejected by the body. Cloning bypasses this problem. The technique used - somatic cell nuclear transfer - has been well-known since Dolly the sheep became the first mammal to be cloned, in 1996.Skin cells were taken from an adult and the genetic information was placed inside a donor egg which had been stripped of its own DNA. Electricity was used to encourage the egg to develop into an embryo.