However, significant phenotypic differences were observed between ER+ cancers with or without loss of wt BRCA1. Assuming phenotype is linked to gene function, these phenotypic differences suggest that ER+ cancers with a wt BRCA1 allele are likely to also have retained BRCA1 function and have not inactivated wt BRCA1 by an alternative mechanism. This evidence concerns the gene BRCA1 and cancer.