Gene scientists on Wednesday said that in lab-dish cells, they had found a way to switch off the rogue chromosome that causes Down's syndrome.

The breakthrough opens up the tantalising goal of therapy for Down's, they said, cautioning that years of work lie ahead before this aim is reached -- if, in fact, it is attainable.

Down's syndrome is the world's leading genetically caused mental disease, accounting for around one in 600 live births in the United States.

It also carries with it a heightened risk of heart defects, leukaemia, immune-system malfunction and premature Alzheimer's disease.

The disease, formally called trisomy 21, is caused by an additional chromosome 21, which has a cascade of unexplained impacts on brain development and body function.

Acting on a hunch, scientists at the University of Massachusetts Medical School reported that they had inserted a gene into this unwanted third chromosome and, in effect, used it like an off switch.

It is the first time that correction has been achieved for an entire chromosome, a coil of DNA that is studded with hundreds of genes, the protein-making codes to build and sustain life.

"Our hope is that for individuals living with Down's syndrome this proof-of-principle opens up multiple exciting new avenues for studying the disorder now, and brings into the realm of consideration research on the concept of 'chromosome therapy' in the future," said Jeanne Lawrence, a professor of cell and developmental biology.

People without Down's are born with 23 pairs of chromosomes, including two sex chromosomes, which pair up as two X chromosomes for females and an X and Y chromosome for males.

The team noted that, in early female embryos, a special gene called XIST comes into play, silencing one of the two X chromosomes so that they do not over-function.

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Lab success: Chromosome for Down's is switched off

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