Breast cancer treatment emerged as the model for tailored therapy with the introduction of antihormonal modalities and the measurement of estrogen receptors as a clinically applicable biomarker . In the adjuvant setting tamoxifen treatment is administered to reduce the risk of recurrence and death in estrogen receptor (ER)-positive breast cancer . It has been confirmed by multiple clinical trials that ER-negative tumors almost entirely fail to respond to antihormonal agents [3,4]. While in the routine setting expression of ER in cancer cells is used for the indication of patients likely to benefit from endocrine therapies, approximately half of ER-positive patients still fail to respond to hormonal therapy . This significant ratio of nonresponders has prompted numerous studies seeking new and additional biomarkers for ER-positive tumors.