The living tissue inside an animal has been regressed back into an embryonic state for the first time, Spanish researchers say.
They believe it could lead to new ways of repairing the body, for example after a heart attack. However, the study published in the journal Nature, showed the technique led to tumours forming in mice.
Stem cell experts said it was a "cool" study, but would need to be much more controlled before leading to therapies.
When an egg is first fertilised, it has the potential to develop into every tissue in the human body, from brain cells to skin.
That flexibility is lost as an embryo develops. However, transforming adult tissues back into an embryonic-like state may lead to treatments that can regenerate a weakened heart, or the light-sensing cells in the eye or even the brain after a stroke.