The effect of genistein was recently investigated on human breast cancer cell lines, MCF-7 (high ERα/ERβ ratio), T47D (low ERα/ERβ ratio) and MDA-MB-231 (ER-negative) indicating that the treatment with genistein at physiological concentrations (1 μM) for 48 h increased the fraction of cells in the S/G2/M phases at the expense of the G1 phase in MCF-7 cells with no effects observed in T47D and MDA-MB-231 cells suggesting that a high ERα/ERβ ratio may be a marker for an augmented response to genistein [111]. This evidence concerns the gene ESR1 and breast cancer.