Inclusion Criteria:  Male or female adult patient (age ≥ 18 years);  Unresectable or metastatic HER2+ breast cancer, previously treated with at least 2 lines of anti-HER2, and:  previously treated by compassionate trastuzumab deruxtecan (ATU group), or previously treated or planned to be treated by trastuzumab deruxtecan (Enhertu®), upon the investigator's decision (Post-Marketing authorization group); Patient who expresses non-opposition to participate in the study, or deceased patient who did not express his/her opposition to the use of his/her personal data while alive.  Exclusion Criteria:  - Previous participation in an interventional clinical trial with trastuzumab deruxtecan