Similarly, the increase in CK5+ cell population (linked to tumor-initiating properties and therapy resistance) and CD44hi or CD44+CD24− BCSCs has been linked to progesterone in several ER+PR+ cell lines but particularly in T47D cells, which have high PR levels, through gene amplification, even in the absence of estrogen [65, 76–78]. This evidence concerns the gene PGR and neoplasm.