The estrogen-dependent nature of breast cancer was first reported in 1896 with the publication of George Beatson's observations on the regression of breast cancer following oophorectomy. Endocrine therapy, targeting ER either directly by selective estrogen receptor modulators (SERMs) and pure antagonists or indirectly by aromatase inhibitors (AIs) that block estrogen production, remains the mainstay of treatment of hormone-sensitive breast cancer in the adjuvant and metastatic settings.