Dr. Angie DeMichele and Dr. Lynn Henry present the latest rapid recommendation impacting two ASCO guidelines. This update focuses on testing for ESR1 mutations in patients with hormone receptor-positive, HER2-negative metastatic breast cancer, and presents treatment recommendations for patients with a detectable ESR1 mutation. Dr. DeMichele and Dr. Henry review the recent data from the EMERALD trial, discuss it's implications for practice, and ongoing developments they're monitoring for more effective therapeutic options. Read the latest update, "Testing for ESR1 Mutations to Guide Therapy for Hormone Receptor-Positive, HER2-Negative Metastatic Breast Cancer: ASCO Guideline Rapid Recommendation Update at www.asco.org/breast-cancer-guidelines. TRANSCRIPT This guideline, clinical tools, and resources are available at www.asco.org/breast-cancer-guidelines. Read the full text of the update and review authors’ disclosures of potential conflicts of interest disclosures in the Journal of Clinical Oncology, https://ascopubs.org/doi/10.1200/JCO.2300638  Brittany Harvey: Hello and welcome to the ASCO Guidelines Podcast, one of ASCO's podcasts delivering timely information to keep you up to date on the latest changes, challenges, and advances in oncology. You can find all the shows, including this one at asco.org/podcasts.  My name is Brittany Harvey, and today I'm interviewing Dr. Angie DeMichele from University of Pennsylvania, and Dr. Lynn Henry from the University of Michigan, authors on ‘Testing for ESR1 Mutations to Guide Therapy for Hormone Receptor-Positive, HER2-Negative Metastatic Breast Cancer: ASCO Guideline Rapid Recommendation Update’.  Thank you for being here, Dr. DeMichele and Dr. Henry. Dr. Angie DeMichele: It's a pleasure.  Dr. Lynn Henry: Thank you.  Brittany Harvey: Then, before we discuss this guideline, I'd like to note that ASCO takes great care in the development of its guidelines and ensuring that the ASCO conflict of interest policy is followed for each guideline. The disclosures of potential conflicts of interest for the guideline panel, including our guests on this podcast episode today, are available online with a publication of the rapid recommendation update in the Journal of Clinical Oncology, which is linked in the show notes.  So then, getting into the content of this rapid recommendation first, Dr. Henry, what prompted this rapid update, which provides updated recommendations for two ASCO guidelines? First, the ‘Biomarkers for Systemic Therapy and Metastatic Breast Cancer Guideline’, last published in 2022, and the ‘Endocrine Treatment and Targeted Therapy for Hormone Receptor-Positive, HER2-Negative Metastatic Breast Cancer Guideline’, which was last updated in 2021. Dr. Lynn Henry: Thank you, Brittany. There's been a lot of exciting news for the treatment of metastatic breast cancer in the last few years. This particular update reflects the results of the phase III EMERALD trial. This trial compared the new oral selective estrogen receptor degrader, elacestrant, to standard-of-care endocrine therapy with either fulvestrant or an aromatase inhibitor in patients with hormone receptor-positive, HER2-negative metastatic breast cancer that had previously progressed during treatment with a CDK4/6 inhibitor in combination with endocrine therapy. Compared to standard-of-care, in this trial, they showed improved progression-free survival in both the overall study population as well as specifically in the patients who had a detectable ESR1 mutation in their circulating tumor DNA. These findings were published in the Journal of Clinical Oncology in 2022, and the drug was subsequently approved by the US Food and Drug Administration in January 2023. Therefore, we felt that it was important to update the guidelines to reflect the results of this trial and the new drug approval.  Brittany Harvey: Excellent. Thank you for describing the results of that trial and the new approval.  So then, based on this data, Dr.