Breast cancer is the most frequently diagnosed cancer and the leading cause of death by cancer among women worldwide. The prognosis of the disease and patients’ response to different types of therapies varies in different subgroups of this heterogeneous disease. The subgroups are based on histological and molecular characteristics of the tumor, especially the expression of estrogen (ER) and progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2). Hormone-dependent breast cancer, determined predominantly by the presence of ER, is the most common type of breast cancer. Patients with hormone-dependent breast cancer have an available targeted therapy, however, tumor cells can develop resistance to the therapy, which is a major obstacle limiting the success of treatment and enabling relapse to metastatic disease.