Similarly, molecularly, breast tumors can be defined by whether or not they express estrogen receptor (ER), progesterone receptor (PR), or human epidermal growth factor receptor 2 (HER2), and among these subtypes, triple-negative breast cancer (TNBC) is defined by the absence of estrogen (ER) and progesterone (PR) receptors and human epidermal growth factor (HER2) expression. This evidence concerns the gene ESR1 and breast neoplasm.