It can be categorized as either positive or negative for the immunohistochemical expression of three hormone receptors: the estrogen receptor (ER), the progesterone receptor (PR), and the human epidermal growth factor receptor 2 (HER2); cancer may be positive or negative for any of these, and those that are negative for all three are termed triple-negative breast cancer (TNBC). This evidence concerns the gene PGR and cancer.