Just half a year after announcing the detection of an entirely novel way that cancer can begin, scientists have isolated the gene responsible for the misdeed. The discovery opens a-new phase In the genetics revolution, with all its wild promise and ethical quandaries. In its defective form, the gene predisposes people to an extremely common type of colon cancer, called hereditary nonpolyposis colorectal cancer. The familial disorder is thought to account 10 to 15 % of all colon tumors, or as many as 22,000 cases of the cancer each year. What is more, as many as 1 in 200 people , or more than a million Americans, are thought to carry the mutation, most without any knowledge of their inherent risk. The finding thus holds significance for a far greater number of people than any other genes isolated to date, including those for other rarer forms of cancer. Colon cancer is the second most frequently fatal malignancy in America after, lung cancer, killing about 64,000 people each year. "For those who carry the gene, the risk of getting colon cancer or one of a handful of other malignancies is 70 to 90 % , and many will develop tumors before the age of 50. Scientists predicted that a genetic test for people in families, known to be at risk for colon tumors could be ready within 6 months....