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People with a lot of moles on their skin are used to be told that they are at greater risk of cancer - but, for once, they have to reason to celebrate - it may be a sign of youthfulness.

"Moley" people can look forward to a longer life than their less pigmented peers, research suggests, despite the fact that they have a marginally higher risk of developing melanoma, the most serious form of skin cancer.

A study comparing more than 1,800 twins has found that those with more moles on their skin have longer telomeres - a marker of biological ageing found in all cells.

The findings suggest that the risk of cancer is counteracted by the effects of the telomeres, which protect the chromosomes.

Scientists from the Twin Research Unit at Kings College, London, say that people with a lot of moles (more than 100) have a biological age that is six to seven years younger than those with very few (less than 25), because of the difference in the length of their telomeres.

Veronique Bataille, who led the study published in Cancer Epidemiology Biomarkers and Prevention, said: "The results are very exciting as they show for the first time that moley people who have a slightly increased risk of melanoma, on the other hand have the benefit of a reduced rate of ageing.

"This could imply susceptibility to fewer age-related diseases such as heart disease or osteoporosis. Further studies are need to confirm this."

Professor Tim Spector, head of the Twin Research Unit and co-author of the study, said: "We now plan to look in more detail at the genes which influence the numbers of moles and to see whether they also slow down the ageing process in general."

Moles are growths on the skin, usually dark, that develop from pigment-producing cells called melanocytes, and may be considered blemishes or beauty marks, depending on their size and location.

They vary in size and may be flat or raised, smooth or rough. Almost everyone has at least ten moles which commonly develop in childhood or adolescence and tend to disappear from middle age.

The average number of moles in people with white skin is 30 but some people can have as many as 400.

Doctors have suspected that they may confer some advantage because they are common in the population.

Most moles are harmless but they can occasionally turn cancerous.

Nearly half of all malignant melanomas begin in moles so a mole that looks suspicious should be removed.

As moles tend to disappear with age, scientists at the Twin Research Unit examined the relationship between the number of moles and telomere length, which is a good indicator of the rate of ageing.

Telomeres are bundles of DNA found at the end of chromosomes which protect them from disintegrating with age.

Like the plastic tips on shoelaces, they prevent the chromosome ends from fraying and sticking to each other.

Long telomeres in the white blood cells have been found to correlate with slowed ageing of many different organs including heart, muscle bone and arteries.

The results showed that those twins with longer telomeres appeared to keep their moles for longer and to have delayed ageing.

Those with shorter telomeres had fewer moles and appeared to lose them quicker with age, suggesting accelerated ageing.