To determine the indication of T-DXd for metastatic HER2-low breast cancer, immunostaining using a CDx and scoring of the results using guidelines for the ASCO/CAP HER2 test [6] and CDx decision guide are recommended (https://www.accessdata.fda.gov/cdrh_docs/pdf/P990081S047D.pdf). This evidence concerns the gene ERBB2 and breast cancer.