The purpose of the study is to determine the frequency ESR1 mutations by Digital PCR in patient with metastatic breast cancer.

Breast cancer is the most common cancer in woman. Aromatase inhibitors have demonstrated a real efficacy however a resistance to treatment exists.

ESR1 mutations appear like involved in the mechanism of resistance to aromatase inhibitors treatment.

The purpose of the study is to determine the frequency ESR1 mutations by Digital PCR in patient with metastatic breast cancer.

The significance of the Digital PCR technique will be determined first in plasma issued from healthy volunteers.

At the initiation of aromatase inhibitors treatment patient with metastatic breast cancer will be included in the study. During their follow-up visit every 3 months), their status towards their disease will be collected and a plasma will be collected too.

When the patient progress clinically or radiologically the plasma concomitant to this progression will be analysed by Digital PCR to detect ESR1 mutations.

The patient will be followed during 2 years.

Inclusion Criteria:

• Woman aged superior to 18 years old
• Inform consent signed
• Metastatic breast cancer or loco-regionnaly advanced breast cancer
• Inoperable
• With an indication to treat with aromatase inhibitor
• Treatment with aromatase inhibitors innitiated at the inclusion or at least 6 months before inclusion with a stable disease
• Without precedent treatment or with treatment by chemotherapy/tamoxifen/faslodex or aromatase inhibitors in a adjuvant treatment and with a time frame of 2 years between last treatment with aromatase inhibitors and metastatic evolution
• Treatment by aromatase inhibitors alone or in combination with a targeted therapy (trastuzumab +/- pertuzumab, bevacizumab, everolimus)

Exclusion Criteria:

• No inform consent signed
• Patient under guardianship, curatorship
• Psychosocial disorder
• No affiliated or beneficiary of a social benefit system