The BRCA 1 and 2 gene mutations, and a handful of other (rarer) gene mutations, account for about 10% of all breast cancer cases. The abnormal genes affect about 1 in 400 people and can be inherited from either or both parents.
BRCA gene mutations increase the lifetime risk of breast cancer in women to between 50 and 87% (depending on the study). The lifetime risk of breast cancer in the general population is about 12%.
Due to the publicity BRCA has received in the media lately, most people think it only affects women. That's not true. Men can be affected too. Men carrying an abnormal BRCA gene have a 5-10% risk of getting breast cancer at some point in their lives. Unaffected men only have a 0.1% lifetime risk.