From the National Breast Cancer Foundation.

Myth: If the gene mutation BRCA1 or BRCA2 is detected in your DNA, you will definitely develop breast cancer

Here’s The Truth

According to the National Cancer Institute, regarding families who are known to carry BRCA1 or BRCA2, “not every woman in such families carries a harmful BRCA1 or BRCA2 mutation, and not every cancer in such families is linked to a harmful mutation in one of these genes. Furthermore, not every woman who has a harmful BRCA1 or BRCA2 mutation will develop breast and/or ovarian cancer. But, a woman who has inherited a harmful mutation in BRCA1 or BRCA2 is about five times more likely to develop breast cancer than a woman who does not have such a mutation.”

For people who discover they have the harmful mutation, there are various proactive measures that can be done to reduce risk. These include taking a hormonal therapy called Tamoxifen or deciding to take a surgical prevention approach which is to have bilateral prophylactic mastectomies, usually done with reconstruction.  Most women will also have ovaries and fallopian tubes removed as well since there is no reliable screening test for the early stages of developing ovarian cancer.