An estimated 25 to 30 percent of breast cancers are familial – which means that a family member passes along multiple genetic mutations that increase the risk of getting the disease, says cancer genetics expert Kenneth Offit of Memorial Sloan Kettering. However, only about one-third of hereditary breast cancers are associated with a BRCA mutation, suggesting that many other genes are involved. Researchers at Memorial Sloan Kettering are currently studying 30 other genetic mutations that may also increase the risk of breast cancer.
In some ethnic groups, such as Ashkenazi Jews, the proportion of breast and ovarian cancers due to a BRCA mutation is significantly higher, says Dr. Offit. In addition, most women with ovarian cancer who have a strong family history of both breast and ovarian cancer have a BRCA mutation.