ERBB2 and breast cancer: Whilst the debate on the most appropriate definition of breast cancer subtypes is still open, it is now accepted that breast cancer consists of at least five different molecular subtypes which include - according to the PAM50 classification scheme1 - basal-like, HER2, Luminal A, Luminal B and the additional category of Normal-like, made of tumours which transcriptionally resemble normal breast tissue samples2.