ERBB2 and breast cancer: Traditionally, HER2 expression levels in BC have been determined using immunohistochemistry (IHC) and fluorescence in situ hybridization (FISH), with a binary classification system: HER2-negative (IHC 0, IHC 1 +, or IHC 2 + without FISH amplification) or HER2-positive (IHC 2 + with FISH amplification, or IHC 3 +) [3].