ERBB2 and breast cancer: Although gene expression studies2–6 have revealed an impressive and complex biological diversity among breast tumors, in clinical practice breast cancer classification driving the treatment-decision process is based on the distinction between 3 major breast cancer subsets2,7,8, based on conventional immunohistochemistry (IHC) and in situ hybridization (ISH) analyses: hormone receptor (HR)-positive/HER2-negative, HER2-positive (HR positive or negative), triple-negative.