ESR1 and breast cancer: By using human breast cancer cell lines, with and without the estrogen receptor, grown in 3-dimension to try to mimic some in vivo conditions, we show that several known growth promoters or endocrine disruptors, 17-beta estradiol, phenol red, TCDD, and bisphenol A, did stimulate human breast cancer stem cells, as evidenced by both the number and sizes of MCF-7 mammospheres with the estrogen receptor and that metformin could suppress this stimulated MCF-7 cancer stem cell growth at non-cytotoxic concentrations.