In their normal state, BRCA1 and BRCA2 genes prevent breast cancer by producing a protein that stops cells from growing out of control. Every person (women and men) has two copies of each of these genes in most cells in their body. As long as at least one of the genes in each pair is working properly, breast cells function normally.
However, if both copies of the gene have mutations, abnormal cell growth can no longer be prevented. When abnormal growth occurs, breast cells begin to multiply at very rapid rates. Some of the extra cells can invade healthy breast tissue, causing invasive breast cancer. In non-invasive breast cancer, abnormal cell growth occurs, but the surrounding breast tissue is not invaded.
All breast cancers are caused by abnormal genes. The abnormalities, or mutations, in the genes can be either inherited or acquired:
Inherited genetic abnormality. Some people are born with one abnormal gene from one parent and one normal gene from the other parent.
Acquired (or non-hereditary) genetic abnormality. A gene can became abnormal as a result of "wear and tear", through an error in how the gene reproduces, or from a variety of other factors, such as exposure to toxins, environmental effects, diet, hormonal influences, or unknown causes. Acquired genetic abnormalities account for 85% to 90% of breast cancers.
Whether you inherited an abnormal breast cancer gene or acquired it, if you have one normal gene, that normal gene will still work to control cell growth and prevent cancer. But, if circumstances cause that normal gene to malfunction or break down, cancer may result.