ERBB2 and cancer: This cancer arises from the basal reserve cells of terminal ductular lobular unit (TDLU) and is defined by expression of high weight cytokeratins (CK5/6, CK14, CK17) and epidermal growth factor receptor (EGFR) and by the lack of expression of the hormone receptors and of human epidermal growth factor receptor 2 (HER2).