This figure illustrates MCF-7 human breast carcinomas cells expressing the estrogen receptor and forming mammospheres after exposure to a solvent, estrogen (E2), the epigenetic tumor promoter TCDD, and the epigenetic toxicant bisphenol A, with or without exposure to metformin (BPA). It is clear that E2, TCDD, and BPA cause enhanced growth of the mammospheres compared to the control mammospheres, while co-treatment with two concentrations of non-cytotoxic metformin significantly inhibited the growth stimulation of E2, TCDD, and BPA. Permission granted from PLOS ONE. In: Jung, J.-W., et al. [136].