Interestingly, in many reported ER-selective bioassays, such as the proliferation of breast cancer cell lines, gene reporter assays in mammalian or non-mammalian cells, and ER binding assays, it was found that most phytoestrogens preferentially interact with ERβ and display high specificity toward ERβ transactivation [55,56,57]. This evidence concerns the gene ESR2 and breast cancer.