In 1994 and 1995, researchers identified two so-called breast cancer genes, named BRCA1 and BRCA2. Women who inherit either of these genes have a 90 percent risk of developing breast cancer sometime during their lives. The genes are particularly common among Jewish women of Eastern European descent. But inheriting the BRCA1 or BRCA2 genes does not mean you will definitely develop breast cancer. It could mean just the opposite.
Genes have functions - usually ones that enhance survival.
According to Mary-Claire King, Ph.D., a geneticist at the University of Washington, Seattle, normal BRCA1 and BRCA2 genes control the cell proliferation characteristic of breast cancer. BRCA2 genes keep cancers in check, but damaged genes promote their spread. These same genes also direct the cells that make milk, instructing them when to turn on and turn off. That could be advantageous when it comes to nurturing infants. The answer is a mutation, or permanent change, caused by damage, re-arrangement, or deletion of part of the gene.