ERBB2 and breast cancer: Firstly, the binding of therapeutic anti‐HER‐2 antibodies on HER‐2 ECD neutralizes the biological activity of trastuzumab.25 Furthermore, amounts of HER‐2 ECD shedding leaves a truncated HER‐2 form (p95) with constitutive kinase activity that can provide ligand‐independent growth and survival signals to the cells.26 Therefore, serum HER‐2 ECD could be a biomarker that helps to identify the subgroup of poorer outcome among HER‐2 overexpression breast cancer patients.