BRCA1 and breast carcinoma: Levels of BRCA1 expression have also been reported to be low in women with sporadic breast cancers that have basal-like features (Turner et al., 2007, Mueller and Roskelley, 2003), and may be related to the frequent loss of heterozygosity at the breast cancer tumor susceptibility locus on chromosome 17q21 (Staff et al., 2003) and/or BRCA1 promoter hypermethylation (Birgisdottir et al., 2006) or increased expression of negative regulatory factors (Turner et al., 2007, Garcia et al., 2011, Wu et al., 2012a).