In September 2022, the US Food and Drug Administration (FDA) expanded approval of the legacy Roche monoclonal antibody 4B5-based immunohistochemistry (IHC) assay to identify patients with HER2-low breast cancers predicted to respond to trastuzumab deruxtecan (T-DXd), based on findings from the DESTINY-Breast04 and DESTINY-Breast06 clinical trials. This evidence concerns the gene ERBB2 and breast cancer.