ERBB2 and breast carcinoma: It should be noted that when a breast cancer displays >10% of tumor cells harboring HER2 overexpression and gene amplification, it is diagnosed as HER2-positive and the patient is treated accordingly without a precise understanding of the clinical significance and the biological implications of this heterogeneity and of having a large proportion of the tumor composed of cells lacking HER2 overexpression/gene amplification.