How Do Abnormal Genes Cause Cancer?

In their normal form, BRCA1 and BRCA2 genes prevent breast cancer by producing a protein that stops cells from growing out of control. Every person (women and men alike) has two copies of each of these genes in most cells in her or his body.

As long as at least one gene in each pair is working properly, breast cells function normally. But if both copies of a breast gene are abnormal, they no longer can stop abnormal growth. In this case, breast cells multiply much more than normal. And some can invade healthy tissue. The result is what we call invasive breast cancer. (In non-invasive breast cancer, cells grow too much but haven't started to invade normal surrounding healthy tissue.)

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 an abnormal gene passed on by a parent. They have one abnormal gene from that 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 — toxic exposure, 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 gene will still work to control cell growth and prevent cancer. But if that normal gene breaks down, for whatever reason, then cancer results