ERBB2 and breast carcinoma: Human epidermal growth factor receptor 2 (HER2) is a transmembrane receptor tyrosine kinase that is amplified in 18%-20% of human breast carcinomas and has been identified as a therapeutic target.1-3 The definition of HER2-positive (HER2+) breast cancer derives from quantification of the HER2 protein via immunohistochemistry (IHC) or fluorescence in situ hybridization (FISH) analyses of biopsy samples.