People with Down's tend to develop fewer tumours
Scientists may have solved the mystery of why people with Down's syndrome seem to have a lower risk of some cancers.
The extra copy of chromosome 21 which causes Down's appears to contain a gene that protects from solid cancerous tumours, tests on mice suggest.
The gene seems to interfere with signals a tumour relies on to grow. The finding raises hope of new ways to prevent and treat cancer.
The study by the Children's Hospital of Boston appears in the journal Nature.
Humans usually have two copies of the 23 chromosomes that together contain all our genetic information, one from each parent.
Down's syndrome is a genetic disorder which results from the presence of an extra, third copy of chromosome 21.
It has been known for some time that individuals with Down's syndrome get certain types of cancer less often than those without the condition.
However, the reason why has been unclear.
The latest study showed that having an extra copy of one of the genes located on chromosome 21 - a gene called Dscr1 - is sufficient to slow cancer growth in mice.
The gene seems to work in combination with another gene also found on chromosome 21 to interfere with the signals a tumour relies upon to stimulate growth of its own blood vessels.
Without those vessels feeding the tumour with its own supply of blood it cannot thrive.
Writing in the journal, the researchers, led by Dr Sandra Ryeom, said: "It is, perhaps, inspiring that the Down's syndrome population provides us with new insight into mechanisms that regulate cancer growth and, by so doing, identifies potential targets for tumour prevention and therapy."
Dr Kairbaan Hodivala-Dilke, a Cancer Research UK scientist at Queen Mary, University of London, said: "This finding raises several important questions about the roles of other chromosome 21 genes that might help regulate tumour growth.
"The next stage is to think about how we might be able to exploit this research to improve cancer treatments in the future."
Stuart Mills, of the Down's Syndrome Association, said: "We have known for some time that people with Down's syndrome have lower incidences of cancer, apart from leukaemia, than the rest of the population.
"This is one of the first studies to examine the reasons why, and we welcome its findings. We will be following further research with great interest."