ESR1 and breast cancer: TNBCs are characterized by a lack of receptor expressions (estrogen receptor [ER]; progesterone receptor [PR]) and lack of expression of human epidermal growth factor receptor 2 (ERBB2 gene, also known as HER2), which is typically seen as a driver of tumourigenesis through amplification and surface cell membrane overexpression, but in rare cases for breast cancer may also be due to an activating mutation(s) [2].