Scientists said Thursday they have developed the first practical test to learn whether people have inherited a rare genetic defect that puts them at high risk of colon cancer.

The test, developed by Dr. Steven M. Powell and others from Johns Hopkins University, uses blood samples to detect a defective protein produced by the mutant gene, according to a report in the New England Journal of Medicine.The test remains experimental, and researchers don't know when it will be generally available or how much it might cost.

Only about 1 in 5,000 Americans carries the bad gene, but those who do are almost certain to get colon cancer.

The genetic malfunction causes a condition known as familial adenomatous polyposis. Victims have thousands of small tumors, called polyps, in their colons, and some of them eventually become cancerous.

More than 50,000 people in the United States could benefit from the test because of the possibility that the genetic defect runs in their families, the researchers said.

They said they used the test on 63 patients, and it detected the defect in 85 percent.

In an accompanying editorial, Dr. Edison T. Liu of the University of North Carolina said the test will be important to families who carry the defect.

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