What do we know about heredity and breast cancer?

Breast cancer is a common disease. Each year, approximately 200,000 women in the United States are diagnosed with breast cancer, and one in nine American women will develop breast cancer in her lifetime. But hereditary breast cancer - caused by a mutant gene passed from parents to their children - is rare. Estimates of the incidence of hereditary breast cancer range from between 5 to 10 percent to as many as 27 percent of all breast cancers.

In 1994, the first gene associated with breast cancer - BRCA1 (for BReast CAncer1) was identified on chromosome 17. A year later, a second gene associated with breast cancer - BRCA2 - was discovered on chromosome 13. When individuals carry a mutated form of either BRCA1 or BRCA2, they have an increased risk of developing breast or ovarian cancer at some point in their lives. Children of parents with a BRCA1 or BRCA2 mutation have a 50 percent chance of inheriting the gene mutation.