The primary objective is to compare the phenotype and genotype of the primary tumor with those of its metastases in order to optimize the treatment of metastatic disease, in patients presenting with first metastatic progression of breast cancer.

Inclusion Criteria:

To be eligible to participate in the study, patients must fulfill all of the following criteria:

Female patients. Age ≥ 18 years. ECOG performance status ≤ 2. Metastatic breast carcinoma, either at diagnosis or at first metastatic relapse.

Available FFPE +/- frozen primary tumor samples. Evaluable metastatic disease. Metastatic disease outside any previous radiotherapy field (e.g. sub-clavicular or internal mammary lymph nodes).

Metastatic disease accessible to either percutaneous or surgical sampling. Signed written informed consent (approved by an Independent Ethics Committee and obtained prior to any study-specific screening procedure).

Social and psychological welfare in concordance with compliance to the study.

Exclusion Criteria:

To be eligible to participate in the study, patients must fulfill none of the following criteria:

Bilateral or multifocal breast cancer. Isolated local or contralateral relapse.

Solitary bone and/or brain metastatic disease unless :

Metastatic sites eligible for a therapeutic surgery. Metastatic sites sampled for diagnosis purpose. Past or current history of malignant neoplasms, except for curatively treated basal and squamous cell carcinoma of the skin. Carcinoma in situ of the cervix. Any coagulopathy contraindicating tumor biopsy. Presence of a contraindication to general anesthesia, if required.

Sponsor will share de-identified data sets. Documents generated under the project will be disseminated in accordance with Institut Curie policies.

Data requests can be submitted starting 9 months after last article publication and will be made accessible for up to 12 months.

Access to trial individual participant data can be requested by qualified researchers engaging in independent scientific research, and will be provided following review and approval of a research proposal and Statistical Analysis Plan (SAP) and execution of a data sharing agreement (DSA).