In metastatic breast cancer, T-DXd is currently approved for HER2+ (IHC 3+ or IHC 2+/ISH+), HER2-low (IHC 1+ or IHC 2+/ISH–), and HER2-ultralow (IHC 0 with membrane staining) cancers, with HER2-ultralow and HER2-low breast cancer patients demonstrating similar benefit (60, 61). This evidence concerns the gene ERBB2 and cancer.