Scientists have found that people with Down's syndrome have an extra copy of a gene that may protect against lung cancer.
The discovery of the "extra line of defence" could lead to the development of drugs or treatments for the disease, which affects 40,000 people a year in the UK and 180,000 in the US.
People with Down's syndrome are less than half as likely to develop many cancers, including lung cancer, compared to the rest of the population, though they are 10 to 20 times more likely to develop childhood leukaemia.
Dr Dean Nizetic at the Centre for Applied Biology in the School of Pharmacy of the University of London led a team from the Cancer Research Campaign (CRC) in the research.
The team found that people with Down's syndrome had three copies of the key protective gene - USP25, one more than the rest of the population.
Genes are grouped into chromosomes. People with Down's syndrome have an extra copy of chromosome 21, which contains the protective gene.
The same gene is often missing from the tumour cells of people with lung cancer, so the cells have no copies at all.
Scientists think losing the gene may be an important stage in the process that leads to lung cancer.
And it is similar to other genes which are known to have safeguarding effects against cancer.
The team, partly funded by the Leukemia Research Fund and involving Imperial Cancer Research Fund scientists, was part of international research which decoded chromosome 21, a breakthrough in Down's syndrome research.
The scientists will now carry out further tests to find out if the gene definitely protects against lung cancer, and how it works.
Dr Nizetic said: "People with Down's syndrome are resistant to many forms of cancer.
Thanks to this, we may find new ways of preventing these types of disease, including lung cancer, potentially saving many lives."
Professor Gordon McVie, director general of the CRC, said the discovery of the "guardian angel" gene was a "naturally occurring situation which gave cause for optimism. It's incredibly important to isolate the presence of a lung cancer suppressant gene."
"Dr Nizetic's work is vital because if we know how people with Down's syndrome are better protected from cancer, then perhaps we can protect the rest of the population. And since losing this gene could be an important step in the development of lung cancer, its discovery might give us a new approach to treating the disease."
Carol Boys, of the Down's Syndrome Association, said: "It's heartening to know that people with Down's syndrome are protected against many common cancers.
We would welcome any further research in this area, especially if it helps us understand why people with Down's syndrome are, unfortunately, more likely to experience certain other health problems."