PGR and breast cancer: Triple-negative breast cancer (TNBC) accounts for 20% of breast cancers (BCs) (Neophytou et al., 2018), where the tumorigenic cells are negative for the estrogen receptor (ER), progesterone receptor (PgR), and human epidermal growth factor receptor 2 (HER2; i.e., ER–, PgR–, HER2–) (Rakha and Chan, 2011).