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More than a quarter of a middle-aged person's skin may have already made the first steps towards cancer, a study suggests.

Analysis of samples from 55- to 73-year-olds found more than 100 DNA mutations linked to cancer in every 1 sq cm (0.1 sq in) of skin.

The team, at the Sanger Institute, near Cambridge, said the results were "surprising".

Experts said prevention was the best defence against damage from the sun.

Skin cancer is one of the most common cancers.

Ultraviolet-radiation from sunlight bombards our skin and transforms it from healthy to cancerous tissue.

Seeds of cancer

Many of the mutations that culminate in skin cancer are already known, but the team wanted to know when they first started to appear.

The researchers analysed excess skin that had been removed from the eyelids of four patients.

They then drilled down deeply into the skin's DNA to discover the very first steps being taken on the journey to cancer.

Dr Peter Campbell, the head of cancer genetics at Sanger, told the BBC News website: "The most surprising thing is just the scale, that a quarter to a third of cells had these cancerous mutations is way higher than we'd expect, but these cells are functioning normally."

However, it would take multiple mutations - nobody is sure exactly how many - to culminate in a tumour.

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The results, published in the journal Science, did show there were some subtle changes in the way the mildly mutated cells were behaving.

They were growing more quickly than other skin cells.

Dr Campbell said: "It certainly changes my sun worshipping, but I don't think we should be terrified.

"It drives home the message that these mutations accumulate throughout life, and the best prevention is a lifetime of attention to the damage from sun exposure."

The findings may be a warning to people trying to develop new cancer drugs, which often target the changes that make a cancer "unique".

"We have not really had any insight into whether those cancerous changes occur in normal cells as well, a treatment that kills 20-30% of normal cells would potentially be a lot of collateral damage," Dr Campbell said.

Dr Bav Shergill, from the British Association of Dermatologists, said: "Whilst the body's immune system can prove quite effective at removing mutated cells, it is important to remember that some of cells aren't removed and mutate into cancers.

"Prevention is the first line of defence; wearing protective clothing, seeking shade and choosing a sunscreen with an SPF [sun protection factor] of at least 30 are all good sun safety practices."

Dr Alan Worsley, from Cancer Research UK, said: "Research like this could help uncover which specific mistakes are more likely to push a damaged skin cell into becoming a cancer.

"Although we all need some sun, avoid sunburn and skin damage when the sun is strong by spending time in the shade, covering up with clothing and using plenty of sunscreen with at least SPF 15 and four or more stars."