ESR1 and breast carcinoma: Breast cancer is the number one cause of cancer in women in the United States and the second cause of cancer death in this population.1 Of these, triple-negative breast cancer (TNBC) is the most aggressive molecular subtype.2,3 It lacks the three typical therapeutic targets: estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor 2 receptor (HER-2).