BRCA 1 and 2 are uncommon mutations. Only 2-3% of women with breast cancer have them. However, it is useful to know if you have the gene in your family because their risk of getting breast cancer in their lifetime is 60%, as compared to 18% for someone without the mutation. Doctors use the following aid to assist in their decisions to test for the mutation.
The predictions rely strongly on breast cancer family history <50 years of age, and strong family history of breast and ovarian cancers (and also prostate in male relatives).