Predictive gene tests for cancer are designed to identify persons who have inherited a gene mutation that may result in cancer. First candidates are families who have participated in linkage analysis studies, where the tests have been specifically tailored to pick out the gene, or gene markers, in their DNA.
Once a gene has been isolated and a test developed, testing becomes feasible in broader populations. The first candidates might be members of other very high-risk families that have had several affected members over at least two generations. Next might be persons with a family history that is less marked - perhaps one or two relatives with the disease.
Soon genetic tests for some types of inherited colon and breast cancer may be offered to the public. The targets of the tests would remain the same: individuals whose body cells carry the disease-causing mutation. These would be people who have inherited the mutant gene, including those whose family history is not apparent (for example, a woman who has acquired the breast cancer susceptibility gene through her father's side of the family). It would also include people in whom the gene mutated very early in embryonic development.
It is important to remember that predictive gene tests will be able to identify only a small proportion of the people who will get, for example, breast or colon cancer. Most cancers are not inherited, and most people who get cancer, whether or not they have relatives with it, do not have an inherited mutation.