Although it has been reported that (–)-xanthatin can exert anti-proliferative effects on breast cancer cells with unknown mechanisms, the findings of this study show that (–)-xanthatin targets mammospheres, which are enriched in breast cancer stem cells (BCSCs) derived from the luminal A type MCF-7 cell line (positive for estrogen and progesterone receptors). This evidence concerns the gene PGR and breast cancer.