ERBB2 and breast neoplasm: Inter‐ and intratumour heterogeneity (ITH)7, 8 act as a critical barrier to improving therapeutics: most of the targets and biomarkers approved by the United States Food and Drug Administration are not expressed uniformly throughout tumour tissue; for example, the human epidermal growth factor receptor 2 (HER2) in gastric adenocarcinoma,9, 10 progesterone and oestrogen receptors in breast tumours (considered positive when detected in 1% of tumour cells)11 and EML4–ALK translocation in lung adenocarcinoma (threshold of 15%),12 among others.13, 14