ERBB2 and breast cancer: There is an agreement that BC with HER2 gene copy number ≥6.0 are categorised as HER2 positive and historically a ratio ≥2.0 alone was used to define HER2 positivity regardless of the gene copy number.10,17,19 However, there are tumours with HER2/CEP17 ratio ≥2.0 and HER2 copy number <6.0 and categorisation of these cases is difficult due to their rarity and consequent lack of evidence on treatment efficacy.