Scientists have identified a molecule that allows special cells from embryos, called stem cells, to multiply without limit.
Stem cells from embryos can turn into any other cell
The UK researchers have dubbed the molecule Nanog, after the mythological Celtic land of the ever young.
Stem cells found in embryos are special because they can turn into almost any type of cell in the body, whether it is a heart cell, skin cell or brain cell.
Research into these cells is expected to lead to revolutionary new treatments for a range of conditions from Parkinson's Disease to heart failure and diabetes.
But the work is controversial because it involves using cells taken from embryos.
This discovery by a team at the Institute for Stem Cell Research at Edinburgh University could help to overcome that problem.
Their finding could ultimately enable scientists to transform stem cells from adults into cells that have all the characteristics of those taken from embryos.
Dr Ian Chambers, who isolated the molecule, said: "Nanog seems to be a master molecule that makes embryonic stem cells grow in the laboratory.
Nanog seems to be a master gene
"In effect, this makes stem cells immortal. Being Scottish, I therefore chose the name after the Tir na nOg legend."
Professor Austin Smith, who heads the Edinburgh research team, described the finding as exciting.
He said the discovery unlocked the secrets of embryonic stem cells.
"This new molecule that we call Nanog, or forever young, is expressed only in these embryonic stem cells.
"This molecule alone is powerful enough to define these cells as embryonic stem cells. It is the master molecule."
He said the discovery could help scientists to develop treatments for a variety of conditions.
"If Nanog has the same effect in humans as we have found in mice, this will be a key step in developing embryonic stem cells for medical treatments."
The findings were published in the journal Cell, alongside another paper by researchers from the Nara Institute of Science and Technology in Japan.
The two groups realised they discovered the same molecule last year and have since collaborated to bring this work to completion.