PGR and breast cancer: Generally, breast cancer is mainly classified into the Luminal-A (ER+/PRhigh/HER2−), Luminal-B (ER+/PRlow/HER2−), HER2-riched (ER−/PR-/HER2+) and Basal-like (ER−/PR−/HER2−, also known as TNBC) subtypes according to the expression pattern of estrogen receptor (ER), progesterone receptor (PR) and human epidermal growth factor receptor 2 (HER2) (21, 22).