The results, in general, demonstrated that metformin reduced the expression of Oct4 in E2- and TCDD- treated human breast cancer stem cells in MCF-7 mammospheres, but not in the bisphenol A-treated mammospheres, suggesting a different mechanism of action of the bisphenol A on the breast cancer stem cells self-renewal ability. This evidence concerns the gene POU5F1 and breast carcinoma.