PGR and breast cancer: Although molecular staging further divides breast cancer into ER (estrogen receptor)-positive and/or PR (progesterone receptor)-positive luminal types, Her2 (human epidermal growth factor receptor 2)-positive, and triple-negative breast cancer (TNBC) types, as well as concomitant endocrine and targeted therapies, it is still difficult to improve therapeutic outcomes for some types of breast cancer, such as endocrine-resistant breast cancer, inflammatory breast cancers, and TNBC2–4.