Trastuzumab Deruxtecan (T-DXd) in Patients Who Have Hormone Receptor-negative and Hormone Receptor-positive HER2-low or HER2 IHC 0 Metastatic Breast Cancer

This study will evaluate the safety and efficacy of trastuzumab deruxtecan (T-DXd) in participants with human epidermal growth factor receptor 2 (HER2)-low or HER2 immunohistochemistry (IHC) 0 (who are both hormone receptor \[HR\]-negative and HR-positive) unresectable and/or metastatic breast cancer.

Inclusion Criteria: * Sign and date the main informed consent form * Must agree to provide a newly obtained or archival baseline biopsy from primary and/or metastatic lesion. * Pathologically documented Breast Cancer (BC) tumor * Is unresectable and/or metastatic. * Is hormone receptor-negative or hormone receptor-positive. * Must include percentage of positively stained cells to characterize if hormone receptor-positive or -negative. * Has confirmed HER2 IHC 1+ or IHC 2+/ISH- (HER2-low) status or HER2 IHC 0 status as determined according to ASCO CAP 2018 guidelines1 based on sample collected during Tissue Screening as described above. * Was never previously HER2-positive (IHC 3+ or IHC 2+/ISH+) on prior pathology testing (per ASCO CAP guidelines). * Was never previously treated with anti-HER2 therapy in the metastatic setting. * Has had at least one and up to two prior lines of therapy in the metastatic setting. * In participants with hormone receptor-positive HER2-low metastatic BC (Cohort 3): * Has recurrent disease \