The BRCA gene test is a blood test that uses DNA analysis to identify harmful changes (mutations) in either one of the two breast cancer susceptibility genes — BRCA1 and BRCA2. Women who have inherited mutations in these genes face a much higher risk of developing breast cancer and ovarian cancer compared with the general population.
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The BRCA gene test is offered only to people who are likely to have an inherited mutation, based on personal or family history, or who have specific types of breast cancer. The BRCA gene test isn't routinely performed on women at average risk of breast and ovarian cancers.
Having a BRCA gene mutation is uncommon. Inherited BRCA gene mutations are responsible for about 5 percent of breast cancers and about 10 to 15 percent of ovarian cancers.
From a BRCA gene test, you learn whether you carry an inherited BRCA gene mutation and receive an estimate of your personal risk of breast cancer and ovarian cancer. Genetic counseling is an important part of the BRCA gene test process.
Source: http://www.mayoclinic.org/tests-procedures/brca-gene-test/basics/definition/prc-20020361