Human cloning has been used to produce early embryos, marking a "significant step" for medicine, say US scientists.
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.
The study, published in the journal Cell, used methods like those that produced Dolly the sheep in the UK.
However, researchers say other sources of stem cells may be easier, cheaper and less controversial.
Campaigners opposed to the use of embryonic stem cells argue that all embryos, whether created in the lab or not, have the potential to go on to become a fully-fledged human, and as such it is morally wrong to experiment on them.