T-Dxd has demonstrated activity not only in HER2-positive breast cancer (overexpressed by immunohistochemistry (IHC, 3+) or amplified by in situ hybridization) but also in HER2-low tumors (defined as a score of 1+ on IHC analysis or as an IHC score of 2+ and negative results on in situ hybridization) [23]. This evidence concerns the gene ERBB2 and breast cancer.