ESR1 and breast cancer: Metaplastic breast carcinoma (MpBC) is a rare morphological subtype of breast cancer, representing 0.2–5% of invasive cases.1 Historically, over 70% of MpBC cases are classified as triple-negative breast cancer (TNBC) and therefore lack clinically targetable biomarkers such as estrogen receptor (ER), progesterone receptor (PR), or human epidermal growth factor receptor 2 (HER2).6 As a result, cytotoxic chemotherapy is often the primary systemic therapeutic option available.