The ESR1 Mutation in the Estrogen Receptor

The ESR1 mutations are usually seen under the selective pressure of aromatase inhibitors in the metastatic setting. If we've got them at baseline in a newly diagnosed metastatic patient before they start new therapy, it's about 4% as reported in Mona Lisa 2 trial. But if you look at in a pre-treated metastatic setting patient, this is about 40% in the plasma. Kind of tells you that these sub-colonial mutations are really something that we deal with in our patients. And we want to think about novel endocrine agents that can actually act on these.