DESTINY Breast Respond HER2-(Ultra)Low Europe

Inclusion Criteria: Study Group 1 and Study Group 2 * Adult patient (age ≥ 18 years) with histological or cytological confirmed diagnosis of unresectable and/or mBC * Decision to newly initiate therapy of T-DXd or conventional chemotherapy according to the physicians choice per SmPC * Written and signed Informed Consent to participate in the study Study Group 1 * Documented HER2-low status (IHC1+, IHC2+/ISH-) * Patients who have received prior chemotherapy in the metastatic setting or * Patients who have developed disease recurrence during or within 6 months of completing adjuvant chemotherapy Study Group 2: * Documented HR+ status * Documented HER2-low status (IHC1+ or IHC2+/ISH-) or HER2-ultralow (defined as IHC 0 with membrane staining \[IHC \> 0 to \<1+\]) status * Patients who have received at least one endocrine therapy in the metastatic setting * Patients who have NOT received prior chemotherapy in the metastatic setting Exclusion Criteria: Study Group 1 and Study Group 2 * Pregnancy or breastfeeding * Patients who at time of data collection for this study are participating in or have participated in an interventional study that remains blinded. No other specific exclusion criteria are defined, as patients will be treated according to the proposed indication statements in the SmPC.