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Another chromosome down, more to go

In the fast and furious race to decode the human genome, the smallest entrant—human chromosome 21—is second to cross the finish line. The result could eventually be a big victory for people with Down's syndrome, scientists say.

Down's syndrome is the most common genetic disorder; 1 in every 700 babies is born with it. It's caused by an extra copy of chromosome 21, so the disorder is also known as trisomy 21. Embryos with three copies of other chromosomes, except X and Y, don't survive to birth.

"There's something special about 21," says Ian Dunham of the Sanger Centre in Cambridge, England. That something special, scientists have long suspected, is a paucity of genes. "We've known for the past few years that this chromosome would have less than its share," says Roger H. Reeves, a geneticist at Johns Hopkins Medical Institutions in Baltimore, Md.

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