US scientists say they have moved a step closer to being able to treat disorders caused by an extra chromosome.
They have "switched off" the chromosome that causes the symptoms of Down's syndrome in human cells in the lab.
The research, published in Nature, could one day lead to new medical treatments for the condition.
Future work may be of real benefit to people with Down's syndrome, said the UK Down's Syndrome Association.
Humans are born with 23 pairs of chromosomes, including two sex chromosomes, making a total of 46 in each cell.
People with Down's syndrome have three - rather than two - copies of chromosome 21.
This causes symptoms such as learning disabilities and early-onset Alzheimer's disease, as well as a greater risk of blood disorders and heart defects.
Gene therapy, which uses genes to treat illnesses, has been attempted for problems caused by a single defective gene. But until now, the idea of being able to silence the effects of a whole chromosome had appeared beyond the realms of possibility, even in the lab.
Now scientists at the University of Massachusetts Medical School have shown that, in theory, this might be possible but would take decades of research.
A team led by Dr Jeanne Lawrence inserted a gene called XIST into the stem cells of a person with Down's syndrome grown in the lab.
The gene plays a role in normal cell development by switching off one of the two X chromosomes present in female embryos, ensuring daughters avoid a double dose of X chromosome genes.
The experiments showed that the gene was able to silence the extra copy of chromosome 21, helping correct unusual patterns of growth in the cells.
Dr Lawrence told BBC News: "The research means that we have a new way - right away - to study the cellular basis for Down's syndrome, that could help identify drugs for Down's syndrome.
"At the same time we have made it conceivable - not necessarily possible or effective, that still needs to be proven - but conceivable that you could use just a single gene to correct the over-expression of the whole chromosome. So it makes genetic therapy for Down's syndrome more conceivable where it really wasn't before."
Commenting on the study, Carol Boys, chief executive of the Down's Syndrome Association, said it was exciting new research from a very well-respected team.
"The findings could have serious implications for future work that may be of real benefit to people with Down's syndrome," she said.
"We are a very long way from understanding how these findings might translate into clinical applications but it could be that they will be of great assistance in the search for conventional treatments for some of the health conditions that affect people with Down's syndrome."
Dr Lucy Raymond, from the department of medical genetics at the University of Cambridge, said the group had demonstrated an important proof of concept.
"This is an exciting breakthrough, but this process is still at a very early [cellular] stage and we are nowhere near seeing this procedure being used in the treatment of Down's syndrome in people."