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Genes are nature's blueprints for every living thing. Genes come in pairs: one set of genes is passed down (or inherited) from your mother and the other set from your father. Genes determine how your body will function and grow, as well as the color of your hair and eyes.
Some genes do not function properly because there is a mistake in them. If a gene has a mistake, it is said to be mutated or altered. In fact, all people have altered forms of some genes. Some alterations can increase your risk for certain illnesses, such as cancer. In recent years, gene alterations have been found in some families with a history of breast cancer. Some women in these families also have had ovarian cancer.
These alterations are most often found in genes named BRCA1 and BRCA2 (BReast CAncer Gene 1 and BReast CAncer Gene 2). Both men and women have BRCA1 and BRCA2 genes, so alterations in these genes can be passed down from either the mother or the father. It is likely that more genes like these will be discovered in the future.
A woman with a BRCA1 or BRCA2 alteration is more likely to develop breast cancer than is a woman without an alteration. However, not every woman who has an altered BRCA1 or BRCA2 gene will get this type of cancer, because genes are not the only factor that affects a person's risk for cancer. Therefore, an altered gene is not sufficient by itself to cause cancer.
Most cases of breast cancer do not involve an altered BRCA1 or BRCA2 gene. At most, 1 in 10 breast cancer cases involves an inherited altered gene, and not all inherited breast cancer involves BRCA1 or BRCA2.
Although breast cancer is rare even in men with an altered gene, men with an altered BRCA2 gene have higher rates of breast cancer than men without an altered gene. Men with an altered BRCA1 or BRCA2 gene may also have a slightly increased risk of prostate cancer. Even if a man never develops cancer, he can pass the altered gene to his sons and daughters.