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.

Patients WITHOUT HR+ (ER- and/or PR-positive)/HER2-negative disease,

Patients without radiological and/or pathological confirmation of locally unresectable and/or metastatic breast cancer,

Patients who are NOT candidates for further systemic treatment following a diagnosis of metastatic disease or disease progression,

Patients who have already started a CDK4/6 inhibitor in combination with endocrine therapy (with or without ovarian suppression) for first-line metastatic disease (for Cohort 1);