BRCA1 and breast cancer: Evidence that normal mammary epithelial cells express BRCA1 mRNAa while cancer cells express both mRNAa and mRNAb led to the proposal that low levels of BRCA1 in the sporadic form of breast cancer could be due, at least in part, to promoter switch that favors expression of the translationally repressed BRCA1 mRNAb [22].