Japanese scientists may be on the track of one of the great dreams of regenerative medicine: making an adult cell revert into an embryonic stem cell. If their results are confirmed and if the technique also works with human cells, it could defuse the bitter ethical and political debate about embryo research. Shinya Yamanaka and Kazutoshi Takahashi, of Kyoto University, found that four factors, or genes, turned the adult cells into cells which behave like embryonic stem cells. These passed the basic ID test: when injected under the skin of healthy mice, they formed teratomas, or tumours from the three germ layers of the body. Up to now it has been thought impossible to create an embryonic stem cell without resorting to cloning.

Yamanaka and Takahashi's work is still preliminary, of course. An American cloning expert, Robert Lanza, says that the experiment was exciting, but inconclusive. It required serious genetic modification of the cells, which could lead to cancers at some stage, he cautioned.