Genetics

About 5% to 10% of breast cancers are thought to be hereditary, caused by abnormal genes passed from parent to child.
Genes are particles in cells, contained in chromosomes, and made of DNA (deoxyribonucleic acid). DNA contains the instructions for building proteins. And proteins control the structure and function of all the cells that make up your body.
Think of your genes as an instruction manual for cell growth and function. Abnormalities in the DNA are like typographical errors. They may provide the wrong set of instructions, leading to faulty cell growth or function. In any one person, if there is an error in a gene, that same mistake will appear in all the cells that contain the same gene. This is like having an instruction manual in which all the copies have the same typographical error.

BRCA1 and BRCA2 genes

Most inherited cases of breast cancer are associated with two abnormal genes: BRCA1 (BReast CAncer gene one) and BRCA2 (BReast CAncer gene two).
Everyone has BRCA1 and BRCA2 genes. The function of the BRCA genes is to repair cell damage and keep breast cells growing normally. But when these genes contain abnormalities or mutations that are passed from generation to generation, the genes don't function normally and breast cancer risk increases. Abnormal BRCA1 and BRCA2 genes may account for up to 10% of all breast cancers, or 1 out of every 10 cases.
Having an abnormal BRCA1 or BRCA2 gene doesn't mean you will be diagnosed with breast cancer. Researchers are learning that other mutations in pieces of chromosomes -- called SNPs (single nucleotide polymorphisms) -- may be linked to higher breast cancer risk in women with an abnormal BRCA1 gene as well as women who didn't inherit an abnormal breast cancer gene.
Women who are diagnosed with breast cancer and have an abnormal BRCA1 or BRCA2 gene often have a family history of breast cancer, ovarian cancer, and other cancers. Still, most people who develop breast cancer did not inherit an abnormal breast cancer gene and have no family history of the disease.
You are substantially more likely to have an abnormal breast cancer gene if:

• You have blood relatives (grandmothers, mother, sisters, aunts) on either your mother's or father's side of the family who had breast cancer diagnosed before age 50.
• There is both breast and ovarian cancer in your family, particularly in a single individual.
• There are other gland-related cancers in your family such as pancreatic, colon, and thyroid cancers.
• Women in your family have had cancer in both breasts.
• You are of Ashkenazi Jewish (Eastern European) heritage.
• You are African American and have been diagnosed with breast cancer at age 35 or younger.
• A man in your family has had breast cancer.

If one family member has an abnormal breast cancer gene, it does not mean that all family members will have it.
The average woman in the United States has about a 1 in 8, or a 12-13%, risk of developing breast cancer in her lifetime. Women who have an abnormal BRCA1 or BRCA2 gene (or both) can have up to an 80% risk of being diagnosed with breast cancer during their lifetimes. Breast cancers associated with an abnormal BRCA1 or BRCA2 gene tend to develop in younger women and occur more often in both breasts than cancers in women without these abnormal genes.
Women with an abnormal BRCA1 or BRCA2 gene also have an increased risk of developing ovarian, colon, pancreatic, and thyroid cancers, as well as melanoma.
Men who have an abnormal BRCA2 gene have a higher risk for breast cancer than men who don't -- about 8% by the time they're 80 years old. This is about 80 times greater than average.
Men with an abnormal BRCA1 gene have a slightly higher risk of prostate cancer. Men with an abnormal BRCA2 gene are 7 times more likely than men without the abnormal gene to develop prostate cancer. Other cancer risks, such as cancer of the skin or digestive tract, also may be slightly higher in men with abnormal BRCA1 or BRCA2 genes.