ERBB2 and breast cancer: Similar findings were observed by Ballard et al., who found 1.4% of tumours fell in this group they called ‘monosomy non-classical’ HER2 amplified BC cases (ratio ≥ 2.0 with an average HER2 copy number <4.0 signals per cell).21 The other two groups in their ‘non classical amplification category’ were designated as the ‘co-amplified group’ (ratio < 2.0, and average HER2 copy number/cell ≥6.0) and a ‘low amplified group’ (ratio ≥ 2.0 and average HER2 copy number/cell 4.0–5.9) and represented 0.8% and 2.1% of cases, respectively.