Her condition, a defective version of a gene called BRCA1, is not common. Mutations in that gene and another called BRCA2 are estimated to cause only 5 percent to 10 percent of breast cancers and 10 percent to 15 percent of ovarian cancers among white women in the United States. The mutations are found in other racial and ethnic groups as well, but it is not known how common they are in those groups. Women who carry these mutations have, on average, about a 65 percent risk of developing breast cancer, as opposed to a risk of about 12 percent for most women. For some mutation carriers, the risk is even higher; Ms. Jolie, who described her condition and medical choices in an Op-Ed article in The New York Times on Tuesday, said that the estimate for her was 87 percent. Most women with breast cancer do not have BRCA mutations and do not need to be tested for them, doctors say. But for women who have breast cancer and do have mutations, knowing their status can help them make further treatment decisions, such as whether to have an unaffected breast or their ovaries removed. Women who should consider asking a doctor about testing are those who have breast cancer before age 50; those who have a family history of both breast and ovarian cancer; or those who have many close relatives with breast cancer, especially if it developed when they were young. Any woman with ovarian cancer should consider being tested, as should Jewish women with breast or ovarian cancer. Men with breast cancer and their families should also inquire about the possibility of a genetic predisposition to the disease. Because the cancer risks for carriers are so high, women with the mutations are often advised to have their breasts and ovaries removed as a preventive measure. It is generally considered safe to wait long enough to have children before having the ovaries removed, but the operation should be done by age 40, said Dr. Susan M. Domchek, an expert on the genetics of breast and ovarian cancer at the University of Pennsylvania and the executive director of its Basser Research Center, which specializes in BRCA mutations. Ms. Jolie, 37, said that she herself had a 50 percent risk of ovarian cancer. “I started with the breasts, as my risk of breast cancer is higher than my risk of ovarian cancer, and the surgery is more complex,” she wrote. Removing the breasts is not the only option, Dr. Domchek said. Some women with BRCA mutations choose close monitoring with mammograms and M.R.I. scans once a year, staggered so that they have one scan or the other every six months. But those tests, unlike the surgery, do not prevent breast cancer, but do offer a chance to find it early. For some women, certain drugs can lower the risk of breast cancer, but not as much as preventive mastectomy.