MCF-7 cells were placed in phenol red-free DMEM supplemented with 5% dextran-coated charcoal-treated FBS for 48 h before plating. MCF-7 cells (1000) were plated in six well plates. Forty eight hours later, cells were treated with DMSO (vehicle), E2, daidzein, daidzein analogues (at 1 μM and 10 μM) and 4-OH-TAM (at 10 nM and 100 nM) (A) and E2, E2 + daidzein, E2 + daidzein analogues (at 1 μM and 10 μM) and E2 + 4-OH-TAM (at 10 nM and 100 nM) (B). Colonies of > 50 cells were counted as positive. Results are normalized to percentage of clonogenic survival from DMSO control cells. The values are the means and the SEM of triplicates from a single experiment and representative for at least three independent experiments. Significant difference from vehicle control (A) and E2 (B), *p < 0.05, **p < 0.01 and *** p < 0.001; Tukey test.