The molecular classification of breast cancer, based on the seminal work of Perou et al, 2000 (Sorlie et al, 2001), identified five main subtypes of tumours with distinct patterns of gene expression and different disease outcomes: luminal A and luminal B cancers (mainly ER positive and sensitive to endocrine therapy), HER2 positive (ER negative or with low ER expression, HER2 over-expressing and resistant to endocrine therapy), basal-like (ER negative, PR negative and HER2 negative, also called triple negative) and normal-like. Here, ERBB2 is linked to cancer.