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First, current gene tests cannot provide a satisfactory answer for everyone who seems to be at risk for inherited breast or colon cancer. In some families, multiple cases may reflect shared environmental exposures rather than inherited susceptibility. Even when an inherited gene is to blame, it is not necessarily the test gene; the BRCA1 gene mutation, for example, is found in only about half of the families with hereditary breast cancer. Second, despite major advances in DNA technology, identifying mutations remains a great challenge. Many of the genes of greatest interest to researchers are enormous, containing many thousands of bases. Mutations can occur anywhere, and searching through long stretches of DNA is difficult. In addition, a single gene can have numerous mutations, not all of them equally influential. The cystic fibrosis gene, for instance, can display any one of more than 300 different mutations, which cause varying degrees of disease; some seem to cause no symptoms at all. Thus, a positive test does not guarantee that disease is imminent, while a negative test - since it evaluates only the more common mutations - cannot completely rule it out. Furthermore, predictive tests deal in probabilities, not certainties. One person with a given gene, even one that is dominant like the hereditary breast cancer gene, may develop disease, while another person remains healthy, and no one yet knows why. A gene may respond to the commands of other genes or be switched on by an environmental factor such as sunlight. Perhaps the most important limitation of gene testing is that test information often is not matched by state-of-the-art diagnostics and therapies. Many diseases and many types of cancer still lack optimal screening procedures; it is often not possible to detect an early cancer even in an individual with a known predisposition. In inherited breast cancer, frequent screening with mammography offers the best chance of early detection, but falls short of prevention. Moreover, mammography is least effective in the glandular breasts of young women, the very ones at greatest risk from an inherited susceptibility. For the moment, the best assurance of prevention may lie in drastic and costly surgery to remove the breasts - but even a total mastectomy can leave some breast cells behind. As for the ovarian cancer that threatens high-risk families, available screening measures often cannot discover disease in time. Here, too, women in high-risk families often opt for prophylactic surgery to remove the ovaries. To date, however, neither type of prophylactic surgery has been proven to prevent completely the occurrence of cancer. Scientists are actively studying interventions aimed at the prevention of cancer. For example, ongoing clinical trials are evaluating the use of tamoxifen, an anticancer drug, as a breast cancer preventive. However, such approaches are still in the realm of research.