This study evaluates the Impact of DihydroPyrimidine Dehydrogenase (DPD) activity on the efficacy of Capecitabine in patients with metastatic breast cancer. The DPD phenotype before the initiation of treatment will be assess and then the patient will be follow up during the treatment with Capecitabine up to 24 month.

Inclusion Criteria:

• Age over 18,
• Performance status 0 to 2,
• Patients with metastatic HER2 negative breast cancer,
• Patients eligible for capecitabine monotherapy at a dose of 2000 mg / m² / day, 14 days every 21 days,
• Determination of Uracil level performed according to national recommendations,
• Patients with at least one lesion evaluable according to the RECIST criteria 1.1, or presenting at least 1 hypermetabolic lesion on PET-TDM according to PERCIST 1.0 criteria. In the case of single cutaneous metastasis (s), it is required to make photographs of lesions with a measure of the lesions using a ruler,
• Patients receiving social coverage.

Exclusion Criteria:

• Performance status> 2,
• Contraindication to capecitabine monotherapy at a dose of 2000 mg / m² / day, 14 days every 21 days,
• Presence of untreated or uncontrolled symptomatic cerebral or leptomeningeal metastases (unstable corticosteroid requirements) and / or non-clinically stable in the 3 months prior to inclusion,
• History of cancer, with the exception of cancers in complete remission for more than 5 years, totally resected cutaneous basal cell carcinoma, in situ carcinoma or in situ cervical epithelioma treated,
• Vulnerable people