BRCA1 and breast cancer: The BRCA1 protein is a necessary factor for the transactivation of ESR1. Previous studies demonstrated that the transfection of ERα-positive BC cells with siRNA against BRCA1 silenced the expression of ESR1, whereas the ectopic expression of a wild-type BRCA1 construct into the BRCA1-mutated/ERα-negative HCC1937 cell line rescued the ERα protein [52].