Metformin prevents transformation of MCF10A-ER-Src cells. A, Number of cells grown in the presence or absence of 1 μM 4-hydroxy tamoxifen (TAM) with the indicated concentrations of metformin for 24 hours. B, Phase-contrast images of cells grown in the presence or absence of 0.1 mM metformin and/or TAM for 36 hours. C, Wound-healing/invasion response assay of cells grown in the presence or absence of 0.1 mM metformin and/or TAM. D, Relative number of foci, colonies in soft agar, and mammospheres in untreated or TAM-treated cells in the presence of the indicated concentration of metformin.