The novel antibody–drug conjugate trastuzumab deruxtecan (T-DXd) has been approved by the United States Food and Drug Administration (FDA) for HER2-low (i.e., score 1+ or 2+/ISH-negative) breast cancers [18], and it is currently under investigation in histology-agnostic settings in the DESTINY-PanTumor02 study (NCT04482309). This evidence concerns the gene ERBB2 and breast cancer.