Characterization of Innate Immune System in Patients With Luminal Advanced Breast Cancer

After enrolment, patients will be assigned to one of the following cohorts according to their level of hormone-resistance: * Cohort A (hormone-sensitive disease). * Cohort B (hormone-resistant disease). In both cohorts it will be collected residual pre-treatment tumor samples from metastatic lesions, preferably obtained after last treatment just prior to study entry, and/or primary breast tumor; as well as serial blood samples at baseline, after 6 weeks, at the same time of radiological re-evaluation (3 months after ET initiation) and at progressive disease (PD) which will be used for serial analytic determinations of the expression profiles of Natural Killer (NK) cells, other lymphoid innate cells, NKG2D ligands, cytokines and other possible biomarkers; in addition to obtain local data of hemogram and estradiol, Follicle Stimulating Hormone (FSH) and Luteinizing Hormone (LH) levels. Patients who have participated in the study during their first line of ET are eligible to participate after progression, when they initiate the second line of ET (in this case, inclusion/exclusion criteria should be checked newly and Informed Consent Form (ICF) signed again). From the control population it will be collected blood and local data (e.g. hemogram, estradiol, FSH and LH levels), in a single time-point after ICF signature.