Do known risk factors account for all cases of breast cancer?
No. The role of the high risk breast cancer genes discovered thus far—BRCA1 and BRCA2—account for an estimated 5 to 10 percent of cases. A 2000 study of 45,000 twin pairs in Scandinavia yielded estimates that genetics could ultimately explain about one-fourth of breast cancer risk. If one identical twin is diagnosed with breast cancer, the chances are two in three that the other will not have the disease. One complicating issue is that risk factors don’t act alone; they may interact. That means it doesn’t make sense to say one or the other risk factor causes breast cancer. The culprit, for example, may be genes and the environment, rather than genes or the environment.
