ESR1 and triple-negative breast carcinoma: Triple-negative breast cancer (TNBC) constitutes 10%-15% of all breast cancer cases and is defined by lack of estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2).1 Compared to other subtypes, TNBC has a more aggressive natural history with higher rates of recurrence and mortality.