Although the use of long-term antiestrogen therapy to antagonize estrogen action in breast cancers has proven to be a very flourishing strategy for the treatment of ERα-positive breast cancer for post-menopausal women, it is also clear that the development of an improved antiestrogen therapy is essential to overcome the frequent problem of tumor resistance to hormonal therapy (Ring and Dowsett, 2004; Weinberg et al., 2005). Here, ESR1 is linked to breast cancer.