PGR and breast cancer: Previous studies demonstrated that HCAR3 is overexpressed in human breast cancer tissues, primary tumor cells, and representative breast cancer cell lines—BT-474, HCC1954, and HCC38—encompassing the major molecular subtypes: progesterone receptor (PR)+/estrogen receptor (ER)+/HER2+, PR−/ER−/HER2+, and triple-negative, respectively [10,27].