Triple-negative breast cancer (TNBC), defined by the lack of expression of the estrogen receptor (ER), the progesterone receptor (PR) and the human epidermal growth factor receptor (HER2) 2, comprises 12-20% of breast cancers and is characterized by poor prognosis, early metastasis, and aggressive tumor behavior 3, 4. The gene discussed is ESR1; the disease is neoplasm.