ERBB2 and cancer: Pioneer studies by Sorlie et al. revealed the molecular properties of human breast cancer and proposed a patient stratification system according to the clinical outcomes of their cancer subtype, classifying breast cancer into five intrinsic subtypes: luminal A, luminal B, HER2 overexpression, basal-like, and normal-like cancers [5]; which have been further supported by several other studies based on gene expression profiling.