Researchers in the United States say the chromosomal abnormality that affects people with Down's syndrome also protects them against cancer.
People with Down's syndrome have an extra copy of the chromosome 21, one of the long coiled strands of DNA and associated proteins that carry the genes; instead of two copies they have three.
People with Down's suffer from mental retardation, have distinct facial and other physical characteristics and a higher risk of some diseases but they are protected against cancer, atherosclerosis and diabetic retinopathy which can cause blindness in people with diabetes and, like atherosclerosis, is associated with blood vessel function.
Some research has suggested that having this trisomy, may lessen the risk of cancer as Down's sufferers have less cancer than most other people.
However, the research has never been conclusive so a team at Johns Hopkins University in Baltimore, Maryland, set out to test the theory in mice genetically engineered to have the equivalent of Down's and human colon cancer and to produce extra amounts of Ets2.
They say the more Ets2 the mice had, the less likely they were to develop colon cancer.
The researchers believe their new study provides clear evidence that a chromosome 21 trisomy can be protective and could lead to new forms of cancer treatment.
The scientists led by Dr. Roger Reeves found a gene on chromosome 21 called Ets2 which appeared to prevent mice developing cancer which they say could be explained by having three copies of the gene - one on each chromosome - instead of two.
They say having more copies of a gene increases the "dose" of the protein that the gene produces and three copies of Ets2 might protect against cancer developing while two does not.
This they say is something of a surprise as in general Ets2 is usually thought to encourage the growth of tumours.
The research is published in the journal Nature.