new screening test for colon cancer, based on detecting a gene that is disrupted very early in the course of the disease, could prevent many thousands of deaths each year, researchers are reporting today.

The test, developed by researchers at the Johns Hopkins Oncology Center in Baltimore and other institutions, works by pulling out minute quantities of DNA from the colon cells that are shed in a stool sample and examining a gene called APC. Human cells must generally disrupt a series of their growth control genes before becoming cancerous. APC is one of the first genes to go haywire in colon cancer.

A reliable way to detect that the gene has changed or mutated could save many of the half million people, including 57,000 Americans, who die of colon cancer each year.

"Many of these deaths are really entirely preventable if you can detect tumors early," said Dr. Bert Vogelstein, the leader of the team that developed the APC gene test. "APC mutations initiate tumors 20 to 30 years before a full-blown malignancy, and that provides a huge window of opportunity." Their report appears today in The New England Journal of Medicine.

So far, the test has been given to a small number of patients. It needs to be applied to large populations, with a proven reduction in mortality, before clinicians can be confident in it, said Dr. Robert Kurtz, a gastroenterologist at Memorial Sloan-Kettering Cancer Center in Manhattan.

Dr. Vogelstein and his fellow researchers say they detected APC gene mutations in almost 60 percent of patients with colon cancer and got no false positives in patients without the disease. The lack of false positives is important because patients who receive a positive result in the current occult blood test must then undergo colonoscopy, an expensive and invasive procedure.

"It is a terrific tour de force to be able to detect those mutations in such small quantities," said Dr. Kenneth Offit, a geneticist at Sloan-Kettering.

Most mutations in the APC gene change the DNA programming to produce a much shorter than normal protein, which cannot perform its usual growth-control function. The new test extracts the DNA for the APC gene, both from normal and cancerous cells, and makes the DNA manufacture protein. The telltale appearance of truncated proteins indicates that the gene has been mutated and that the patient has at least the first signs of colon cancer.

A special feature of the test is that it detects minute amounts of human DNA in stool samples, distinguishing it from the copious bacterial DNA. This feature of the test was developed by Exact Sciences, a company in Maynard, Mass., that has licensed the Vogelstein team's APC test.

Don M. Hardison, the company's chief executive, said he hoped to market the test as early as 2004. The company is working on another genetic test that searches stool samples for 21 mutations in three genes that are commonly found in colon cancer. This test, which will be marketed next year, catches some 70 percent of cases with only 5 percent false positives, Mr. Hardison said.

The two tests will be complementary, he said, and together they could catch almost all cases of colon cancer if people over 50 took them on an annual basis. "We have a chance to make a tremendous impact on saving lives by referring the right people to get colonoscopy," he said.

Colonoscopy, in which a threading tube is inserted into the colon to look for polyps, is an accurate way of detecting colon cancer but is very unpopular because of its cost and discomfort.

The occult blood test, another principal screening method, is known to save lives but gives many false positives, all of which must be followed by colonoscopy.

The promise of genetic tests is that they can be adjusted so as to maximize both sensitivity, the percentage of cases detected, and specificity, the avoidance of false positives.