In the dose expansion part of the study, T-DXd at the recommended doses (i.e., 5.4 mg/kg and 6.4 mg/kg once every 3 weeks) was assessed in five cohorts: advanced metastatic HER2-positive breast cancer progressed on prior TDM-1, HER2-positive gastric, or gastroesophageal junction cancer progressed on prior trastuzumab, HER2-low expressing breast cancer (i.e., immunohistochemistry (IHC) 1+ or 2+, in-situ hybridization (ISH) negative), other HER2-expressing (defined as IHC 3+, 2+, or 1+ or amplified) or HER2-mutated solid tumors [23,24]. This evidence concerns the gene ERBB2 and breast carcinoma.