HER2 is a 185 KDa transmembrane protein encoded by the HER2 gene and over-expressed in approximately 15% of invasive breast cancers. HER2 belongs to the human epidermal growth factor (EGF) receptor family, which comprises HER2, HER3, HER4, and EGFR. https://www.creative-biolabs.com/adc/anti-her2-antibody-development.htm