Scientists are a step closer to discovering why radiation causes cancer.
Radiation damages cells
They have identified a molecule which appears to play a crucial role in protecting cells against radiation.
It could help scientists understand how accidental or deliberate exposure to radiation from a nuclear attack or "dirty bomb" could affect cells.
The discovery could also have implications for the treatment of cancer, the UK researchers say, because targeting cancer cells with drugs to inactivate the molecule could make them much more sensitive to the effects of radiotherapy.
Exposing human cells to radiation sets off an emergency response system, which repairs mutated genes or kills off cells that have suffered irreparable damage.
This system is a key defence against the development of cancer.
The research was carried out by scientists at the Wellcome Trust/Cancer Research UK Institute of Cancer and Developmental Biology at Cambridge University.
Scientists grew cells in the laboratory and studied the effects of exposing them to beams of radiation.
It may suggest how the effects of radiation might be curtailed following a possible nuclear attack
Professor Steve Jackson, Cancer Research UK
It was discovered that a protein molecule called MDC1 played a key role in helping cells to detect and repair radiation damage.
If the molecule was prevented from doing its job, cells were no longer able to defend themselves against radiation, making them susceptible to developing a range of genetic errors.
The MDC1 molecule works together with others to detect damage to DNA and prevent cells from dividing until their genes have been repaired.
After cells have been exposed to radiation, MDC1 helps guide the other molecule to places within the DNA where damage has occurred, kick-starting the repair process.
Radiotherapy treatment harnesses the damaging effects of radiation to cause lethal damage to the DNA of cancer cells.
If the MDC1 can be prevented from working, it could make the treatment even more effective, the researchers say.
Professor Steve Jackson, who led the research, said: "The molecule we've discovered seems to play a crucial role in recognising genetic damage and setting into motion the chain of events that will lead to its repair.
"By helping cells to patch up radiation damage, we speculate that it is acting as a key barrier against the development of cancer."
He added: "Understanding what happens within our cells when they're pounded with radiation is important for a whole range of reasons, not least for predicting the effects of cancer radiotherapy.
"In addition, it may suggest how the effects of radiation might be curtailed following a possible nuclear attack.
"Some individuals may be naturally more resistant to the radioactivity than others, and studying these people could give us an important insight into ways of reducing the risk of cancer from radioactive exposure."
He said: "There's a real potential here to improve radiotherapy by inactivating the molecule and leaving cancer cells sensitised to radiation.
"At the moment, we're investigating how much of the new molecule is found within tumours, so that we can see whether differences in MDC1 levels may influence clinical outcome."
Sir Paul Nurse, Cancer Research UK's chief executive, said: "Our bodies are constantly being bombarded by background levels of radioactivity, but early on in the history of life, cells evolved systems to protect themselves from its effects.
"Studying these fundamental systems is crucial for understanding how radiation can cause cancer, why some people may be more prone to the disease than others and above all how we can kill cancer cells more effectively.
"This new research is an important advance and we hope it will have real clinical applications."
The research is published in the journal Nature.