Trastuzumab deruxtecan was approved by FDA for the treatment of adult patients with unresectable or metastatic HER2-low (immunohistochemistry (IHC) 1+ or IHC 2+/in situ hybridization (ISH)-) breast cancer who have received prior chemotherapy in the metastatic setting or developed disease recurrence during or within six months of completing adjuvant chemotherapy [49]. This evidence concerns the gene ERBB2 and breast carcinoma.