RecruitingDrug trialEfficacy of Trastuzumab Deruxtecan in Metastatic Breast Cancer With Different HER2 Expression PatternsThe purpose of this observational study is to learn about the HER2 heterogeneity and its impact on benefit from trastuzumab deruxtecan in metastatic breast cancer. The main question it aims to answer is: \- Does the heterogeneity of HER2 expression level and spatial distribution in different tissues affect the efficacy of trastuzumab deruxtecan in metastatic breast cancer?