PGR and breast neoplasm: 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.