PGR and breast cancer: Estrogen receptor and progesterone receptor (PR) are both critical steroid HRs routinely employed to predict endocrine therapy response and prognosis in breast cancer (14–16); thus, the HR status of breast cancer could be further subdivided into ER-positive/PR-positive, ER-positive/PR-negative, ER-negative/PR-positive, and ER-negative/PR-negative subpopulations theoretically.