RESEARCH ARTICLE Calcitriol restores antiestro v c

Full list of author information is available at the end of the articleBackground Breast cancer is a heterogeneous disease, encompass-ing a number of distinct biological entities that are associated with a variety of pathological and clinical features [1]. The gene expression profile of breast cancer allows to classify this disease in five groups, two of them estrogen receptor (ER)-positive (luminal A and B) and three ER-negative (normal breast-like, human epidermal growth factor receptor- 2 (HER2) and basal-like) [2]. Approximately 30 % of all breast tumors do not express ER, a protein with both prog-nostic and predictive values. Indeed, the presence of ERα correlates with increased disease-free survival and better prognosis. Importantly, ERα-positive breast cancers respond appropriately to endocrine therapies [3-5]. Tamoxifen is the most common and effective therapy in pre- and postmenopausal patients affected with ER-positive tumors, since a long-term use of thi