ESR1 and breast cancer: Although breast cancer has conventionally been classified based on the presence or absence of the estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2), recent studies have indicated that the genomic landscape of breast cancer extends far beyond these 3 receptors.1 Genomic profiling of tumor specimens has helped identify other targets that may serve as driver mutations for the development of breast cancer, contributing to the heterogeneity of this disease.1–4