Estrogen receptor and human epidermal growth factor receptor-2 quantification and efficacy to trastuzumab

Trastuzumab in breast cancer exemplifies the paradigm of successful targeted therapy. This anti-HER2 monoclonal antibody reduces the risk of recurrence by half when combined with standard chemotherapy (1-3). Its target—HER2—is overexpressed in 20% of breast cancers and testing for HER2 positivity is the sine qua non for anti-HER2 therapies. Consensus guidelines have been developed for the definition of HER2-positive (HER2+) tumors. HER2 protein expression has to be 3+ by immunohistochemistry (IHC) or the gene has to be amplified by fluorescence (FISH) or chromogenic (CISH) in situ hybridization (4).