The goal of the retrospective phase of this study is to validate the specificity and the sensitivity of two Real-Time quantitative PCR based techniques in order to support the gold-standard approaches used to evaluate the HER2 gene status in breast cancer. Furthemore, HER2-low patients are enrolled in a prospective phase, both in early stage and in case of recurrence, in order to monitorate HER2 expression levels changes under treatments by using liquid biopsy. The first aim is to be able to distinguish responders vs non-responders earlier than the standard monitoring through tumoral markers and medical examination.