ESR1 and breast cancer: Burstein HJ, DeMichele A, Somerfield MR, Henry NL; Biomarker Testing and Endocrine and Targeted Therapy in Metastatic Breast Cancer Expert Panels: Testing for ESR1 mutations to guide therapy for hormone receptor-positive, human epidermal growth factor receptor 2-negative metastatic breast cancer: ASCO Guideline Rapid Recommendation Update.