ESR1 and breast cancer: The previous phase III clinical trials, PALOMA-2 and PALOMA-3, demonstrated that palbociclib prolonged PFS and OS in combination with endocrine therapy, letrozole in first-line ER+/HER2− advanced breast cancer patients [9], or fulvestrant in HR+/HER2− metastatic breast cancer patients with disease progression after previous endocrine therapy as second- or later-line treatment [10].