Hormone receptor positive breast cancer is the most common type of breast cancer, comprising 70-80% of all breast cancers. Endocrine therapy is the main type of initial treatment for patients with your type of breast cancer. Endocrine therapy is treatment that tries to remove, or block certain hormones from binding to the cancer cells and thus slow or stop the growth of cancer. Although most patients with your type of breast cancer respond initially to endocrine therapies, it can lose its effectiveness. New therapies for this type of cancer are needed.