ERBB2 and breast cancer: However, tumor HER2 heterogeneity, defined as the presence of a separate population of cells with different HER2 copy number and/or HER2/CEP17 ratio accounting for at least 10% of the overall tumor cell population [24], is observed in breast cancer in up to 30% of cases, being accountable of less responsiveness to anti-HER2 targeted therapy [25] and failure to achieve pathological complete response in neoadjuvant setting [26].