ERα and ERβ bind to various estrogen molecules as their selective ligands. Different estrogen molecules may differ in their specific affinities for α or β isoform of ER. 17-β-E2binds to both ERα and ERβ almost equivalently [27]. The estrogen metabolite estrone, raloxifene (selective estrogen receptor modulator or SERM) and ethinyl estradiol (EE, a synthetic estrogen used as a major component of contraceptive pills) bind preferentially to ERα.