ESR1 and breast carcinoma: Triple-negative breast cancer (TNBC) encompasses a subset of breast cancers that lack expression of the estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2).1,2 TNBCs account for 10 to 20% of newly diagnosed breast cancer cases and are associated with higher incidence of visceral metastases, higher risk of early recurrence and worse prognosis.3,4