Minimally Interventional Study on Prevalence of Emerging ESR1 Mutations in Liquid Biopsy in Three Cohorts of Patients With Breast Cancer in Comparison With Patient's Baseline ESR1 Mutation Status as Defined by Tissue Profiling.

Inclusion Criteria: Cohort 1: * BC patients, male and female, 18 years old and older, pre or post menopausal, with HR+ (ER and/or PR positive), Her-2 negative (confirmed centrally) locally advanced irresectable and/or metastatic disease * Confirmation of HR and Her-2 status may be performed in the primary tumor or in the metastatic lesion (patients with discordant results may be included) * Patients must be candidates to CDK4/6i therapy in combination with endocrine therapy in the first line setting (with or without ovarian suppression) * Patients may have received one previous line of chemotherapy in the metastatic setting, but no endocrine therapy in the metastatic setting is allowed * Patients may have received chemotherapy in the neo/adjuvant setting * Patients may have received endocrine therapy (with or without ovarian suppression) in the neo/adjuvant setting * Patients may have received a CDK4/6i in the adjuvant setting, provided they are still candidates for CDK4/6i therapy in the metastatic setting * Patients must be able to undergo a liquid biopsy procedure before starting their first line treatment * All patients must fill and sign an informed consent form. Cohort 2: * BC patients, male and felame, 18 years old and older, pre or post menopausal, with HR+ (ER and/or PR positive), Her-2 negative (confirmed centrally) locally advanced irresectable and/or metastatic disease who have progressed on a CDK4/6i in combination with endocrine therapy (with or without ovar…