Triple-negative breast cancer (TNBC) is defined by the absence of estrogen-receptor (ER), progesterone-receptor (PR), and human epidermal growth factor receptor 2 (HER2) over-expression.1–4 While TNBC comprises a minority of breast cancer cases (15–20%), it results in a disproportionally higher rate of mortality. The gene discussed is ESR1; the disease is breast cancer.