PGR and breast cancer: Breast cancer can be classified into different molecular subtypes based on the expression of the estrogen receptor (ER), progesterone receptor (PR) and the human epidermal growth factor receptor 2 (HER2, also known as HER2/neu or ErbB2) or the lack of the expression of these receptors (triple‐negative breast cancer, TNBC) (Langlands et al., 2013).