How Does Breast Cancer Start?

DNA mutation.

Mutations in DNA -- the genetic instructions found in the cells of the body -- is what causes breast cancer to start or develop. The majority of these DNA gene mutations are acquired, which means that for some unknown reason one breast cancer cell goes awry, becomes cancerous and multiplies. Other gene mutations are inherited, meaning they are passed down from one generation to another.

Acquired Gene Mutations

More than 85 percent of all breast cancers are caused by acquired gene mutations of an unknown cause. An example of a form of breast cancer that is caused by an acquired gene mutation is HER2-positive (human epidermal growth factor-2) breast cancer.

In this form of breast cancer, a woman has too many copies of the HER-2 gene for an unknown reason. Approximately 20 to 25 percent of all breast cancers are HER2-positive. Unfortunately, the HER2-positive form of the breast cancer has a greater likelihood of recurring after first remission and is associated with a decrease in survival as compared to breast cancer that is HER2-negative.

Inherited Gene Mutations

Scientific research has revealed that certain inherited genes can increase the risk of developing breast cancer in some women. Two well-known and publicized inherited gene mutations are the BRCA1 and BRCA2 genes. In women with these mutations, the risk of developing breast cancer can be as high as 80 percent.

There are other known inherited gene mutations such as ATM, p53, CHEK2, PTEN and CDH1, which also can increase the risk of developing breast cancer. However, these gene mutations are more rare than the BRCA1 and BRCA2 genetic mutations.

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