ESR1 and breast cancer: The basal breast cancer subtype is characterized by triple negativity (for ESR1, PgR and HER‐2), expression of EGFR and cytokeratin 5/6 [93, 94, 95] and is a more aggressive subtype; associated with high tumour grade, younger age and poor prognosis, including shorter disease‐free and overall survival [96, 97].