Among available human breast cancer cell lines, only MCF-7 and BT474 [25] demonstrate SRC-3 gene amplification and express high levels of the protein while the balance of breast cancer cell lines tested (10 ER+ and one ER-negative (ER−)) have no gene amplification and moderate to low SRC-3 protein expression [26,27]. This evidence concerns the gene NCOA3 and breast cancer.