ESR1 and breast cancer: Examples of strongly over-expressed genes identified by our analyses include RARA and PGR, for which agonists/antagonist drugs are available and both of which are key regulators of ER action in breast cancer.55, 56, 57 MCF7-Y537S cells provide an important vehicle for testing these and other therapeutic strategies, for example with genome-wide or targeted siRNA screening, or screening of drug libraries.