PGR and breast cancer: Breast cancer can be classified into the following phenotypes: estrogen receptor (ER)-positive (luminal, normal breast-like); human epidermal growth factor receptor (HER) 2-overexpressing; and basal-like, based on cDNA microarray analysis.9–11 The basal-like subtype displays characteristics of myoepithelial cells/basal cells, and often corresponds to so-called ER-negative/progesterone receptor (PR)-negative/HER2-negative (triple-negative) breast cancer (TNBC).