ERBB2 and breast cancer: Moreover, a revolution in the field of HER2-targeting has recently happened, with the emergence of the targetability of “HER2-low” (tumors with HER2 immunohistochemical staining of 1+ or 2+ without evidence of ERBB2 gene amplification by in situ hybridization) breast cancer with fam-trastuzumab deruxtecan (an antibody-drug conjugate composed of a humanized anti-HER2 monoclonal antibody and a topoisomerase I inhibitor payload).