ERBB2 and breast cancer: Breast cancer represents a heterogeneous disease and it is clinically divided into three basic subtypes: (I) based on the level of expression of estrogen and progesterone receptors, (II) based on the human epidermal growth factor 2 (HER2) and (III) a third subtype, when neither estrogen, progesterone or HER2 is expressed (triple negative breast cancer [12].