 Breast cancer is the commonest cancer among females in Singapore and worldwide. Approximately 60-70 of breast cancers are hormone receptor positive and thus potentially sensitive to endocrine therapy. However, both primary and acquired resistance to endocrine therapy exists, and better combinations are constantly being explored to delay endocrine resistance and improve treatment outcome. Several known mechanisms of endocrine resistance have been proposed, and include deregulation of various components of the ER pathway itself, alterations in cell cycle and cell survival signaling molecules, and the activation of escape pathways that can provide tumors with alternative proliferative and survival stimuli. 