BRCA1 and breast cancer: Of note, however, although mutations within the BRCA1 gene are rare (2–3%) in such sporadic cases [2], the level of BRCA1 protein in cancerous breast cells of 30–40% of the sporadic breast cancers is markedly reduced by various non-mutational down-regulating mechanisms [4], indicating that BRCA1 malfunction is a major risk factor which is associated with sporadic breast cancers as well.