SAN ANTONIO – Mutations of the estrogen receptor 1 (ESR1) gene are more common than previously thought in women with advanced estrogen receptor–positive breast cancer and are associated with poorer survival. In an interview at the San Antonio Breast Cancer Symposium, Dr. Sarat Chandarlapaty of Memorial Sloan Kettering Cancer Center, New York, discusses a mutational analysis of cell-free DNA in blood samples from the BOLERO-2 trial and how the findings may help refine therapeutic strategies.