ESR1 and triple-negative breast carcinoma: Using immunohistochemistry with p63 isoform‐specific antibodies, we identified a ΔNp63/p40+ tumour cell subpopulation in 100 of 173 (58%) non‐triple negative breast cancers and the presence of this population associated with improved survival in patients with ER−/HER2+ tumours (p = 0.006).