A prospective, single-centre, proof-of-concept pilot study in patients with metastatic breast cancers (MBC) (whatever the immunohistochemical subtype) treated at the IUCT-O. Eligible patients will be selected and informed of this study during a medical consultation for cancer that has metastasised, has relapsed or is progressing metastatically, by medical oncologists at the Oncopole Claudius Regaud (OCR). Then, with the patient's agreement and before the start of anti-tumour treatment, a blood sample will be taken to detect DP-circulating cells. A breast cancer tumour sample (non-bone metastasis or, failing that, primary tumour) must be available (FFPE archived tumour block).

Each patient will participate in the study for one day.

60 patients will be included in this interventional study.

Inclusion Criteria:

1. Patients with metastatic breast cancer eligible for 1st, 2nd or 3rd line treatment at stage IV, regardless of immunohistochemical subtype (triple-negative, RH+/HER2-negative or HER2-positive).
2. Patient with metastatic disease or metastatic relapse or progression who has not yet started 1st line treatment for metastatic disease or line 2 or line 3.
3. Tumour sample available (archived tumour block): non-bone metastasis preferred if available or, failing this, primary breast tumour.
4. Age ≥ 18 years and WHO ≤ 2.
5. Patient affiliated to a French Social Security scheme.
6. Patient having signed his/her informed consent prior to inclusion in the study and prior to any specific procedure for the study.

Exclusion Criteria:

1. Associated pathology(ies) likely to prevent the study procedure from running smoothly.
2. Any psychological, family, geographical or sociological condition that prevents compliance with the medical monitoring and/or procedures set out in the study protocol.
3. Patient who has forfeited his/her freedom by administrative or legal award or who is under legal protection (curatorship and guardianship, protection of justice).
4. Patient who has presented with another solid tumour (excluding carcinoma in situ of the breast or cervix) within 5 years.
5. Pregnant patient.