 The majority of women diagnosed with breast cancer are post-menopausal, of which up to 75 are estrogen ER andor progesterone receptor PR positive. Even in pre-menopausal breast cancer over half of all patients will have expression of these hormone receptors. Thus therapeutic strategies targeting the estrogen receptor or its ligand are the most common treatment offered in breast cancer. Despite substantial benefits now demonstrated with selective estrogen receptor modulators e.g. tamoxifen and aromatase inhibitors e.g. anastrazole, letrozole and exemestane, a significant proportion of patients will still unfortunately have or develop resistance to these hormonal therapies. 