Prospective Observational Study of Antitumor Activity Correlation Between Hormonal Therapy and Expression miRNA100

Inclusion Criteria: * Ability to provide written informed consent * Age greater than 18 years * histological diagnosis of invasive carcinoma of the breast * X-ray evidence (mammography and / or ultrasound) strongly suggestive for the presence of invasive breast cancer (BIRADS 4c or BIRADS 5) of greater than 15mm diameter. * Carcinoma stage I (if diameter\> 15 mm) or II, unresectable, or * Carcinoma in stage II or III, operable as a result of presurgical therapy, or * Carcinoma in IV debut stage, asymptomatic, with primary operable breast cancer, or as a result of presurgical therapy * Positivity for the estrogen receptor and / or to the progestin defined as the expression of one or both hormone receptors in ≥10% of tumor cells * Negativity for HER2 * Cell proliferation, defined as the percentage of Ki67 positive tumor cells,\> 5% (corresponding to a value of 1.79 after log-normal transformation) * Postmenopausal status * Ability to take an oral therapy, in the absence of malabsorption syndrome known, previous stomach surgery or the small intestine * Ability to perform the staging examinations and screening provided for in the * Protocol * Eastern Cooperative Oncology Group (ECOG) = 0 * The patients will have a normal bone marrow function, liver and kidney Exclusion Criteria: * Previous treatment for breast cancer with chemotherapy, lapatinib, trastuzumab, letrozole, anastrozole, exemestane or tamoxifen. * Other cancers diagnosed in the last five years, with the exception of basal cell or squamous cell carcinoma of the skin, melanoma in situ or cervical cancer in situ. * Premenopausal * Known hypersensitivity to letrozole or to any of its excipients (for example: women with rare hereditary problems of galactose intolerance, the Lapp lactase deficiency or mal absorption of glucose / galactose). * Evidence of a severe and poorly controlled systemic disease affecting the lung, heart, liver, kidney that may compromise adherence to treatment or an extended follow-up. * Inflammatory bowel disease not controlled (eg Crohn's disease, ulcerative colitis). * Active infection and / or inadequately controlled. * Alteration of mental status, dementia, or any psychiatric condition that might impair the ability to consciously sign the informed consent. * Breast cancer triple negative, or negative for both HER2 overexpression for the expression of hormone receptors.