Observational Study on the Treatment of Patients With Metastatic Breast Cancer

Inclusion Criteria: * Patient or next of kin/legal representative willing and able to provide written informed consent according to the local regulations unless a waiver is granted by the local IRB/IEC/EC; * Adult female or male patients ≥18 years old; * Unresectable or metastatic breast cancer; Estrogen Receptor ≥ 1% at the moment first-line treatment was initiated, either in a biopsy immediately before starting treatment for metastatic disease or in the biopsy of initial breast cancer diagnosis if a new biopsy was not performed; * HER2-negative (IHC 0, 1+ or 2+/ISH negative) at the moment first-line treatment was initiated, either in a biopsy immediately before starting treatment for metastatic disease or in the biopsy of initial breast cancer diagnosis if a new biopsy was not performed; * Have received treatment with a CDK4/6i in the first-line treatment for unresectable or metastatic breast cancer; * Have discontinued permanently the treatment with CDK4/6 for any reason: progressed disease on first-line treatment, died during first-line treatment due to any cause, or interrupted treatment for any reason; * Patients with recurrent disease are not mandated to have a new biopsy at the time of recurrence Exclusion Criteria: \- Patients with recurrent disease with a primary tumor showing a different pattern of receptors (i.e. initial BC other than ER ≥ 1% and HER2 negative (IHC 0, 1+ or 2+/ISH negative); * Patients with a concomitant cancer at the time of the diagnosis of MBC HR-positive HER2-negative except for the non-metastatic non-melanoma skin cancers, or in situ or benign neoplasms; a cancer is considered concomitant if it occurs within 5 years of HR-positive HER2-negative BC diagnosis; * Patients who at time of data collection for this study are participating or have participated in an interventional study that remains blinded.