Scientists say a discovery about how cancer cells "live forever" could lead to new treatments.
Normal cells have a set lifespan
Healthy cells have timers so they only live as long as they are needed.
Cancer cells do not have these timers and so can keep on replicating, say researchers from Cancer Research UK, writing in the journal Cell.
And they warn that if this 'timer' could be applied to healthy cells, slowing their ageing process, they could then become cancerous themselves.
Cell ageing is controlled by strips of DNA at the end of chromosomes.
These DNA strips, called telomeres, get shorter every time a cell divides until there is nothing left - and the cell knows it is time to die.
But cancer cells have ways of blocking the ageing process.
It was known that many use an enzyme called telomerase to rebuild their telomeres.
However, scientists knew this could not be the only mechanism, as cancer cells can achieve immortality without it.
'Shaking off shackles'
In this study, researchers at the Cancer Research UK London Research Institute, used a technique called immunofluorescence to look at what various molecules in cancer cells were doing.
One particular molecule, called RAD51D, appeared repeatedly at the site of the telomeres, suggesting it was interacting in some way with the timer mechanism.
When researchers blocked the activity of RAD51D they found substantial damage to the telomeres and other parts of the genome - the telltale signs of accelerated ageing.
They suggest this shows that the molecule usually acts as a protective "cap" for the telomeres in cancer cells, preventing the normal telomere shortening.
This then slows the normal cell-ageing process and allows cancer cells to keep on growing and dividing eternally - ignoring the normal constraints on lifespan.
The team suggest the finding could provide a new route for attacking up to 10% of tumours.
Dr Madalena Tarsounas, who led the study, said: "Cancer has an amazing ability to shake off the shackles of ageing and death, which is one of the reasons why it can be so hard to treat.
"Understanding how cancer cells remain eternally young has been a key focus of research for more than a decade, so it's particularly exciting to have made such a striking discovery."
She added: "We have found evidence of a completely new mechanism for stopping the clock on a cancer cell's timer and preventing its lifespan from ticking down.
"It raises the possibility of starting the clock again and making cancer cells susceptible to death once more."
She added: "As well as opening the way to new types of treatment for cancer, our study has shed light on the complex but intriguing processes which control how and when we get old."
Pamela Goldberg, Chief Executive of Breast Cancer Campaign, says: "By avoiding one of the normally inescapable cell mechanisms, cell death, cancer cells are able to have unique control in the human body.
"This research has discovered an important element which appears to be essential in sidestepping this cell death process and if we can in some way control this we may be able to stop the growth and spread of cancer cells."
Professor Robert Souhami, of Cancer Research UK, added: "Ageing, at least at a cellular level, isn't just a question of running out of steam but a carefully planned process to keep the number of old and new cells in balance.
"Cancer cells are adept at slipping the constraints of the ageing process, but this highly significant study points to ways of making them mortal, and vulnerable, once more."