Germline alterations of the BRCA1 tumor suppressor gene have been implicated at least in half of familial breast cancers. Nevertheless, in sporadic breast cancer no mutation of this gene has been characterized to date. In sporadic breast tumors, other BRCA1 gene loss of function mechanisms, such as down-regulation of gene expression, have been suggested. In an effort to better understand the relationship between BRCA1 expression and malignant transformation, we have adapted the new real-time quantitative PCR method based on a 5' nuclease assay and the use of doubly labeled fluorescent TaqMan probes to quantify BRCA1 mRNA. We have compared expression of BRCA1 mRNA with or without exon 11 in the normal breast epithelial cell line MCF10a and in three cancer cell lines (MCF-7, MDA-MB231 and HBL100) by comparing two methods of quantification: the comparative CT and the standard curve. We found that the full length BRCA1 mRNA, which encodes the functional nuclear protein, was down-regulated in tumor cells when compared with MCF10a cells.