BRCA1 and BRCA2 were the first breast cancer genes to be identified. Mutated forms of these genes are believed to be responsible for about half the cases of inherited breast cancer, especially those that occur in younger women. Both are tumor suppressor genes.
Mutations of the breast cancer gene BRCA1, like those of BRCA2, are responsible mainly for instances of hereditary breast cancer. They seldom appear involved in the sporadic, non-inherited breast cancer -- the 95% of breast cancer that does not run in families.
Both BRCA1 and BRCA2 are large, complex genes. The symbol BRCA stands for BReast CAncer.